Split out common code into multiplatform Kotlin library

21 files changed, 1074 insertions, 0 deletions

diff --git a/taler-kotlin-android/.gitignore b/taler-kotlin-android/.gitignore
new file mode 100644
index 0000000..796b96d
--- /dev/null
+++ b/taler-kotlin-android/.gitignore
@@ -0,0 +1 @@
+/build
diff --git a/taler-kotlin-android/.gitlab-ci.yml b/taler-kotlin-android/.gitlab-ci.yml
new file mode 100644
index 0000000..bb5af21
--- /dev/null
+++ b/taler-kotlin-android/.gitlab-ci.yml
@@ -0,0 +1,12 @@
+taler_kotlin_android_test:
+  stage: test
+  only:
+    changes:
+      - taler-kotlin-android/**/*
+      - taler-kotlin-common/**/*
+      - build.gradle
+  script: ./gradlew :taler-kotlin-android:check
+  artifacts:
+    paths:
+      - taler-kotlin-android/build/reports/lint-results.html
+    expire_in: 1 week
diff --git a/taler-kotlin-android/build.gradle b/taler-kotlin-android/build.gradle
new file mode 100644
index 0000000..d6d6003
--- /dev/null
+++ b/taler-kotlin-android/build.gradle
@@ -0,0 +1,78 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+plugins {
+    id 'com.android.library'
+    id 'kotlin-android'
+    id 'kotlin-android-extensions'
+    id 'kotlinx-serialization'
+}
+
+android {
+    compileSdkVersion 29
+    //noinspection GradleDependency
+    buildToolsVersion "$build_tools_version"
+
+    defaultConfig {
+        minSdkVersion 24
+        targetSdkVersion 29
+        versionCode 1
+        versionName "0.1"
+
+        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+        consumerProguardFiles 'consumer-rules.pro'
+    }
+
+    buildTypes {
+        release {
+            minifyEnabled false
+            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+        }
+    }
+
+    packagingOptions {
+        exclude("META-INF/*.kotlin_module")
+    }
+
+}
+
+dependencies {
+    api project(":taler-kotlin-common")
+
+    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
+    implementation 'androidx.appcompat:appcompat:1.1.0'
+    implementation 'androidx.core:core-ktx:1.3.0'
+
+    // Navigation
+    implementation "androidx.navigation:navigation-ui-ktx:$nav_version"
+    implementation "androidx.navigation:navigation-fragment-ktx:$nav_version"
+
+    // ViewModel and LiveData
+    def lifecycle_version = "2.2.0"
+    implementation "androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version"
+
+    // QR codes
+    implementation 'com.google.zxing:core:3.4.0'  // needs minSdkVersion 24+
+
+    // JSON parsing and serialization
+    api "org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.20.0"
+    implementation "com.fasterxml.jackson.module:jackson-module-kotlin:2.10.2"
+
+    lintChecks 'com.github.thirdegg:lint-rules:0.0.4-alpha'
+
+    testImplementation 'junit:junit:4.13'
+    testImplementation 'org.json:json:20190722'
+}
diff --git a/taler-kotlin-android/consumer-rules.pro b/taler-kotlin-android/consumer-rules.pro
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/taler-kotlin-android/consumer-rules.pro
diff --git a/taler-kotlin-android/proguard-rules.pro b/taler-kotlin-android/proguard-rules.pro
new file mode 100644
index 0000000..f1b4245
--- /dev/null
+++ b/taler-kotlin-android/proguard-rules.pro
@@ -0,0 +1,21 @@
+# Add project specific ProGuard rules here.
+# You can control the set of applied configuration files using the
+# proguardFiles setting in build.gradle.
+#
+# For more details, see
+#   http://developer.android.com/guide/developing/tools/proguard.html
+
+# If your project uses WebView with JS, uncomment the following
+# and specify the fully qualified class name to the JavaScript interface
+# class:
+#-keepclassmembers class fqcn.of.javascript.interface.for.webview {
+#   public *;
+#}
+
+# Uncomment this to preserve the line number information for
+# debugging stack traces.
+#-keepattributes SourceFile,LineNumberTable
+
+# If you keep the line number information, uncomment this to
+# hide the original source file name.
+#-renamesourcefileattribute SourceFile
diff --git a/taler-kotlin-android/src/main/AndroidManifest.xml b/taler-kotlin-android/src/main/AndroidManifest.xml
new file mode 100644
index 0000000..902ddc1
--- /dev/null
+++ b/taler-kotlin-android/src/main/AndroidManifest.xml
@@ -0,0 +1,24 @@
+<!--
+  ~ This file is part of GNU Taler
+  ~ (C) 2020 Taler Systems S.A.
+  ~
+  ~ GNU Taler is free software; you can redistribute it and/or modify it under the
+  ~ terms of the GNU General Public License as published by the Free Software
+  ~ Foundation; either version 3, or (at your option) any later version.
+  ~
+  ~ GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+  ~ WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+  ~ A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+  ~
+  ~ You should have received a copy of the GNU General Public License along with
+  ~ GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+  -->
+
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="net.taler.common">
+
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+    <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.NFC" />
+
+</manifest>
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/AmountMixin.kt b/taler-kotlin-android/src/main/java/net/taler/common/AmountMixin.kt
new file mode 100644
index 0000000..f9b1330
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/AmountMixin.kt
@@ -0,0 +1,51 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import com.fasterxml.jackson.core.JsonGenerator
+import com.fasterxml.jackson.core.JsonParser
+import com.fasterxml.jackson.databind.DeserializationContext
+import com.fasterxml.jackson.databind.JsonMappingException
+import com.fasterxml.jackson.databind.SerializerProvider
+import com.fasterxml.jackson.databind.annotation.JsonDeserialize
+import com.fasterxml.jackson.databind.annotation.JsonSerialize
+import com.fasterxml.jackson.databind.deser.std.StdDeserializer
+import com.fasterxml.jackson.databind.ser.std.StdSerializer
+
+/**
+ * Used to support Jackson serialization along with KotlinX.
+ */
+@JsonSerialize(using = AmountSerializer::class)
+@JsonDeserialize(using = AmountDeserializer::class)
+abstract class AmountMixin
+
+class AmountSerializer : StdSerializer<Amount>(Amount::class.java) {
+    override fun serialize(value: Amount, gen: JsonGenerator, provider: SerializerProvider) {
+        gen.writeString(value.toJSONString())
+    }
+}
+
+class AmountDeserializer : StdDeserializer<Amount>(Amount::class.java) {
+    override fun deserialize(p: JsonParser, ctxt: DeserializationContext): Amount {
+        val node = p.codec.readValue(p, String::class.java)
+        try {
+            return Amount.fromJSONString(node)
+        } catch (e: AmountParserException) {
+            throw JsonMappingException(p, "Error parsing Amount", e)
+        }
+    }
+}
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/AndroidUtils.kt b/taler-kotlin-android/src/main/java/net/taler/common/AndroidUtils.kt
new file mode 100644
index 0000000..b46f306
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/AndroidUtils.kt
@@ -0,0 +1,123 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import android.content.Context
+import android.content.Context.CONNECTIVITY_SERVICE
+import android.content.Intent
+import android.content.pm.PackageManager.MATCH_DEFAULT_ONLY
+import android.net.ConnectivityManager
+import android.net.NetworkCapabilities.NET_CAPABILITY_INTERNET
+import android.os.Build.VERSION.SDK_INT
+import android.os.Looper
+import android.text.format.DateUtils.DAY_IN_MILLIS
+import android.text.format.DateUtils.FORMAT_ABBREV_ALL
+import android.text.format.DateUtils.FORMAT_ABBREV_MONTH
+import android.text.format.DateUtils.FORMAT_ABBREV_RELATIVE
+import android.text.format.DateUtils.FORMAT_NO_YEAR
+import android.text.format.DateUtils.FORMAT_SHOW_DATE
+import android.text.format.DateUtils.FORMAT_SHOW_TIME
+import android.text.format.DateUtils.FORMAT_SHOW_YEAR
+import android.text.format.DateUtils.MINUTE_IN_MILLIS
+import android.text.format.DateUtils.formatDateTime
+import android.text.format.DateUtils.getRelativeTimeSpanString
+import android.view.View
+import android.view.View.INVISIBLE
+import android.view.View.VISIBLE
+import android.view.inputmethod.InputMethodManager
+import androidx.core.content.ContextCompat.getSystemService
+import androidx.fragment.app.Fragment
+import androidx.navigation.NavDirections
+import androidx.navigation.fragment.findNavController
+
+fun View.fadeIn(endAction: () -> Unit = {}) {
+    if (visibility == VISIBLE && alpha == 1f) return
+    alpha = 0f
+    visibility = VISIBLE
+    animate().alpha(1f).withEndAction {
+        if (context != null) endAction.invoke()
+    }.start()
+}
+
+fun View.fadeOut(endAction: () -> Unit = {}) {
+    if (visibility == INVISIBLE) return
+    animate().alpha(0f).withEndAction {
+        if (context == null) return@withEndAction
+        visibility = INVISIBLE
+        alpha = 1f
+        endAction.invoke()
+    }.start()
+}
+
+fun View.hideKeyboard() {
+    getSystemService(context, InputMethodManager::class.java)
+        ?.hideSoftInputFromWindow(windowToken, 0)
+}
+
+fun assertUiThread() {
+    check(Looper.getMainLooper().thread == Thread.currentThread())
+}
+
+/**
+ * Use this with 'when' expressions when you need it to handle all possibilities/branches.
+ */
+val <T> T.exhaustive: T
+    get() = this
+
+fun Context.isOnline(): Boolean {
+    val cm = getSystemService(CONNECTIVITY_SERVICE) as ConnectivityManager
+    return if (SDK_INT < 29) {
+        @Suppress("DEPRECATION")
+        cm.activeNetworkInfo?.isConnected == true
+    } else {
+        val capabilities = cm.getNetworkCapabilities(cm.activeNetwork) ?: return false
+        capabilities.hasCapability(NET_CAPABILITY_INTERNET)
+    }
+}
+
+fun Intent.isSafe(context: Context): Boolean {
+    return context.packageManager.queryIntentActivities(this, MATCH_DEFAULT_ONLY).isNotEmpty()
+}
+
+fun Fragment.navigate(directions: NavDirections) = findNavController().navigate(directions)
+
+fun Long.toRelativeTime(context: Context): CharSequence {
+    val now = System.currentTimeMillis()
+    return if (now - this > DAY_IN_MILLIS * 2) {
+        val flags = FORMAT_SHOW_TIME or FORMAT_SHOW_DATE or FORMAT_ABBREV_MONTH or FORMAT_NO_YEAR
+        formatDateTime(context, this, flags)
+    } else getRelativeTimeSpanString(this, now, MINUTE_IN_MILLIS, FORMAT_ABBREV_RELATIVE)
+}
+
+fun Long.toAbsoluteTime(context: Context): CharSequence {
+    val flags = FORMAT_SHOW_TIME or FORMAT_SHOW_DATE or FORMAT_SHOW_YEAR
+    return formatDateTime(context, this, flags)
+}
+
+fun Long.toShortDate(context: Context): CharSequence {
+    val flags = FORMAT_SHOW_DATE or FORMAT_SHOW_YEAR or FORMAT_ABBREV_ALL
+    return formatDateTime(context, this, flags)
+}
+
+fun Version.getIncompatibleStringOrNull(context: Context, otherVersion: String): String? {
+    val other = Version.parse(otherVersion) ?: return context.getString(R.string.version_invalid)
+    val match = compare(other) ?: return context.getString(R.string.version_invalid)
+    if (match.compatible) return null
+    if (match.currentCmp < 0) return context.getString(R.string.version_too_old)
+    if (match.currentCmp > 0) return context.getString(R.string.version_too_new)
+    throw AssertionError("$this == $other")
+}
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/ByteArrayUtils.kt b/taler-kotlin-android/src/main/java/net/taler/common/ByteArrayUtils.kt
new file mode 100644
index 0000000..fba0d07
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/ByteArrayUtils.kt
@@ -0,0 +1,53 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+object ByteArrayUtils {
+
+    private const val HEX_CHARS = "0123456789ABCDEF"
+
+    fun hexStringToByteArray(data: String): ByteArray {
+        val result = ByteArray(data.length / 2)
+
+        for (i in data.indices step 2) {
+            val firstIndex = HEX_CHARS.indexOf(data[i])
+            val secondIndex = HEX_CHARS.indexOf(data[i + 1])
+
+            val octet = firstIndex.shl(4).or(secondIndex)
+            result[i.shr(1)] = octet.toByte()
+        }
+        return result
+    }
+
+
+    private val HEX_CHARS_ARRAY = HEX_CHARS.toCharArray()
+
+    @Suppress("unused")
+    fun toHex(byteArray: ByteArray): String {
+        val result = StringBuffer()
+
+        byteArray.forEach {
+            val octet = it.toInt()
+            val firstIndex = (octet and 0xF0).ushr(4)
+            val secondIndex = octet and 0x0F
+            result.append(HEX_CHARS_ARRAY[firstIndex])
+            result.append(HEX_CHARS_ARRAY[secondIndex])
+        }
+        return result.toString()
+    }
+
+}
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/CombinedLiveData.kt b/taler-kotlin-android/src/main/java/net/taler/common/CombinedLiveData.kt
new file mode 100644
index 0000000..4e7016b
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/CombinedLiveData.kt
@@ -0,0 +1,51 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import androidx.lifecycle.LiveData
+import androidx.lifecycle.MediatorLiveData
+import androidx.lifecycle.Observer
+
+class CombinedLiveData<T, K, S>(
+    source1: LiveData<T>,
+    source2: LiveData<K>,
+    private val combine: (data1: T?, data2: K?) -> S
+) : MediatorLiveData<S>() {
+
+    private var data1: T? = null
+    private var data2: K? = null
+
+    init {
+        super.addSource(source1) { t ->
+            data1 = t
+            value = combine(data1, data2)
+        }
+        super.addSource(source2) { k ->
+            data2 = k
+            value = combine(data1, data2)
+        }
+    }
+
+    override fun <S : Any?> addSource(source: LiveData<S>, onChanged: Observer<in S>) {
+        throw UnsupportedOperationException()
+    }
+
+    override fun <T : Any?> removeSource(toRemote: LiveData<T>) {
+        throw UnsupportedOperationException()
+    }
+
+}
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/ContractTerms.kt b/taler-kotlin-android/src/main/java/net/taler/common/ContractTerms.kt
new file mode 100644
index 0000000..0d5fe5b
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/ContractTerms.kt
@@ -0,0 +1,91 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import androidx.annotation.RequiresApi
+import com.fasterxml.jackson.annotation.JsonIgnore
+import com.fasterxml.jackson.annotation.JsonInclude
+import com.fasterxml.jackson.annotation.JsonInclude.Include.NON_EMPTY
+import com.fasterxml.jackson.annotation.JsonInclude.Include.NON_NULL
+import com.fasterxml.jackson.annotation.JsonProperty
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import net.taler.common.TalerUtils.getLocalizedString
+
+@Serializable
+data class ContractTerms(
+    val summary: String,
+    @SerialName("summary_i18n")
+    @get:JsonProperty("summary_i18n")
+    val summaryI18n: Map<String, String>? = null,
+    val amount: Amount,
+    @SerialName("fulfillment_url")
+    @get:JsonProperty("fulfillment_url")
+    val fulfillmentUrl: String,
+    val products: List<ContractProduct>
+)
+
+@JsonInclude(NON_NULL)
+abstract class Product {
+    @get:JsonProperty("product_id")
+    abstract val productId: String?
+    abstract val description: String
+
+    @get:JsonProperty("description_i18n")
+    abstract val descriptionI18n: Map<String, String>?
+    abstract val price: Amount
+
+    @get:JsonProperty("delivery_location")
+    abstract val location: String?
+    abstract val image: String?
+
+    @get:JsonIgnore
+    val localizedDescription: String
+        @RequiresApi(26)
+        get() = getLocalizedString(descriptionI18n, description)
+}
+
+@Serializable
+data class ContractProduct(
+    @SerialName("product_id")
+    override val productId: String? = null,
+    override val description: String,
+    @SerialName("description_i18n")
+    override val descriptionI18n: Map<String, String>? = null,
+    override val price: Amount,
+    @SerialName("delivery_location")
+    override val location: String? = null,
+    override val image: String? = null,
+    val quantity: Int
+) : Product() {
+    @get:JsonIgnore
+    val totalPrice: Amount by lazy {
+        price * quantity
+    }
+}
+
+data class ContractMerchant(
+    val name: String
+)
+
+@Serializable
+@JsonInclude(NON_EMPTY)
+class Timestamp(
+    @SerialName("t_ms")
+    @JsonProperty("t_ms")
+    val ms: Long
+)
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/Event.kt b/taler-kotlin-android/src/main/java/net/taler/common/Event.kt
new file mode 100644
index 0000000..779247f
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/Event.kt
@@ -0,0 +1,51 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import androidx.lifecycle.LiveData
+import androidx.lifecycle.Observer
+import java.util.concurrent.atomic.AtomicBoolean
+
+/**
+ * Used as a wrapper for data that is exposed via a [LiveData] that represents an one-time event.
+ */
+open class Event<out T>(private val content: T) {
+
+    private val isConsumed = AtomicBoolean(false)
+
+    /**
+     * Returns the content and prevents its use again.
+     */
+    fun getIfNotConsumed(): T? {
+        return if (isConsumed.compareAndSet(false, true)) content else null
+    }
+
+}
+
+fun <T> T.toEvent() = Event(this)
+
+/**
+ * An [Observer] for [Event]s, simplifying the pattern of checking if the [Event]'s content has
+ * already been consumed.
+ *
+ * [onEvent] is *only* called if the [Event]'s contents has not been consumed.
+ */
+class EventObserver<T>(private val onEvent: (T) -> Unit) : Observer<Event<T>> {
+    override fun onChanged(event: Event<T>?) {
+        event?.getIfNotConsumed()?.let { onEvent(it) }
+    }
+}
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/NfcManager.kt b/taler-kotlin-android/src/main/java/net/taler/common/NfcManager.kt
new file mode 100644
index 0000000..11e1e1e
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/NfcManager.kt
@@ -0,0 +1,234 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import android.app.Activity
+import android.content.Context
+import android.nfc.NfcAdapter
+import android.nfc.NfcAdapter.FLAG_READER_NFC_A
+import android.nfc.NfcAdapter.FLAG_READER_SKIP_NDEF_CHECK
+import android.nfc.NfcAdapter.getDefaultAdapter
+import android.nfc.Tag
+import android.nfc.tech.IsoDep
+import android.util.Log
+import net.taler.common.ByteArrayUtils.hexStringToByteArray
+import org.json.JSONObject
+import java.io.ByteArrayOutputStream
+import java.net.URL
+import javax.net.ssl.HttpsURLConnection
+
+@Suppress("unused")
+private const val TALER_AID = "A0000002471001"
+
+class NfcManager : NfcAdapter.ReaderCallback {
+
+    companion object {
+        const val TAG = "taler-merchant"
+
+        /**
+         * Returns true if NFC is supported and false otherwise.
+         */
+        fun hasNfc(context: Context): Boolean {
+            return getNfcAdapter(context) != null
+        }
+
+        /**
+         * Enables NFC reader mode. Don't forget to call [stop] afterwards.
+         */
+        fun start(activity: Activity, nfcManager: NfcManager) {
+            getNfcAdapter(activity)?.enableReaderMode(activity, nfcManager, nfcManager.flags, null)
+        }
+
+        /**
+         * Disables NFC reader mode. Call after [start].
+         */
+        fun stop(activity: Activity) {
+            getNfcAdapter(activity)?.disableReaderMode(activity)
+        }
+
+        private fun getNfcAdapter(context: Context): NfcAdapter? {
+            return getDefaultAdapter(context)
+        }
+    }
+
+    private val flags = FLAG_READER_NFC_A or FLAG_READER_SKIP_NDEF_CHECK
+
+    private var tagString: String? = null
+    private var currentTag: IsoDep? = null
+
+    fun setTagString(tagString: String) {
+        this.tagString = tagString
+    }
+
+    override fun onTagDiscovered(tag: Tag?) {
+
+        Log.v(TAG, "tag discovered")
+
+        val isoDep = IsoDep.get(tag)
+        isoDep.connect()
+
+        currentTag = isoDep
+
+        isoDep.transceive(apduSelectFile())
+
+        val tagString: String? = tagString
+        if (tagString != null) {
+            isoDep.transceive(apduPutTalerData(1, tagString.toByteArray()))
+        }
+
+        // FIXME: use better pattern for sleeps in between requests
+        // -> start with fast polling, poll more slowly if no requests are coming
+
+        while (true) {
+            try {
+                val reqFrame = isoDep.transceive(apduGetData())
+                if (reqFrame.size < 2) {
+                    Log.v(TAG, "request frame too small")
+                    break
+                }
+                val req = ByteArray(reqFrame.size - 2)
+                if (req.isEmpty()) {
+                    continue
+                }
+                reqFrame.copyInto(req, 0, 0, reqFrame.size - 2)
+                val jsonReq = JSONObject(req.toString(Charsets.UTF_8))
+                val reqId = jsonReq.getInt("id")
+                Log.v(TAG, "got request $jsonReq")
+                val jsonInnerReq = jsonReq.getJSONObject("request")
+                val method = jsonInnerReq.getString("method")
+                val urlStr = jsonInnerReq.getString("url")
+                Log.v(TAG, "url '$urlStr'")
+                Log.v(TAG, "method '$method'")
+                val url = URL(urlStr)
+                val conn: HttpsURLConnection = url.openConnection() as HttpsURLConnection
+                conn.setRequestProperty("Accept", "application/json")
+                conn.connectTimeout = 5000
+                conn.doInput = true
+                when (method) {
+                    "get" -> {
+                        conn.requestMethod = "GET"
+                    }
+                    "postJson" -> {
+                        conn.requestMethod = "POST"
+                        conn.doOutput = true
+                        conn.setRequestProperty("Content-Type", "application/json; utf-8")
+                        val body = jsonInnerReq.getString("body")
+                        conn.outputStream.write(body.toByteArray(Charsets.UTF_8))
+                    }
+                    else -> {
+                        throw Exception("method not supported")
+                    }
+                }
+                Log.v(TAG, "connecting")
+                conn.connect()
+                Log.v(TAG, "connected")
+
+                val statusCode = conn.responseCode
+                val tunnelResp = JSONObject()
+                tunnelResp.put("id", reqId)
+                tunnelResp.put("status", conn.responseCode)
+
+                if (statusCode == 200) {
+                    val stream = conn.inputStream
+                    val httpResp = stream.buffered().readBytes()
+                    tunnelResp.put("responseJson", JSONObject(httpResp.toString(Charsets.UTF_8)))
+                }
+
+                Log.v(TAG, "sending: $tunnelResp")
+
+                isoDep.transceive(apduPutTalerData(2, tunnelResp.toString().toByteArray()))
+            } catch (e: Exception) {
+                Log.v(TAG, "exception during NFC loop: $e")
+                break
+            }
+        }
+
+        isoDep.close()
+    }
+
+    private fun writeApduLength(stream: ByteArrayOutputStream, size: Int) {
+        when {
+            size == 0 -> {
+                // No size field needed!
+            }
+            size <= 255 -> // One byte size field
+                stream.write(size)
+            size <= 65535 -> {
+                stream.write(0)
+                // FIXME: is this supposed to be little or big endian?
+                stream.write(size and 0xFF)
+                stream.write((size ushr 8) and 0xFF)
+            }
+            else -> throw Error("payload too big")
+        }
+    }
+
+    private fun apduSelectFile(): ByteArray {
+        return hexStringToByteArray("00A4040007A0000002471001")
+    }
+
+    private fun apduPutData(payload: ByteArray): ByteArray {
+        val stream = ByteArrayOutputStream()
+
+        // Class
+        stream.write(0x00)
+
+        // Instruction 0xDA = put data
+        stream.write(0xDA)
+
+        // Instruction parameters
+        // (proprietary encoding)
+        stream.write(0x01)
+        stream.write(0x00)
+
+        writeApduLength(stream, payload.size)
+
+        stream.write(payload)
+
+        return stream.toByteArray()
+    }
+
+    private fun apduPutTalerData(talerInst: Int, payload: ByteArray): ByteArray {
+        val realPayload = ByteArrayOutputStream()
+        realPayload.write(talerInst)
+        realPayload.write(payload)
+        return apduPutData(realPayload.toByteArray())
+    }
+
+    private fun apduGetData(): ByteArray {
+        val stream = ByteArrayOutputStream()
+
+        // Class
+        stream.write(0x00)
+
+        // Instruction 0xCA = get data
+        stream.write(0xCA)
+
+        // Instruction parameters
+        // (proprietary encoding)
+        stream.write(0x01)
+        stream.write(0x00)
+
+        // Max expected response size, two
+        // zero bytes denotes 65536
+        stream.write(0x0)
+        stream.write(0x0)
+
+        return stream.toByteArray()
+    }
+
+}
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/QrCodeManager.kt b/taler-kotlin-android/src/main/java/net/taler/common/QrCodeManager.kt
new file mode 100644
index 0000000..e2a9a55
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/QrCodeManager.kt
@@ -0,0 +1,42 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import android.graphics.Bitmap
+import android.graphics.Bitmap.Config.RGB_565
+import android.graphics.Color.BLACK
+import android.graphics.Color.WHITE
+import com.google.zxing.BarcodeFormat.QR_CODE
+import com.google.zxing.qrcode.QRCodeWriter
+
+object QrCodeManager {
+
+    fun makeQrCode(text: String, size: Int = 256): Bitmap {
+        val qrCodeWriter = QRCodeWriter()
+        val bitMatrix = qrCodeWriter.encode(text, QR_CODE, size, size)
+        val height = bitMatrix.height
+        val width = bitMatrix.width
+        val bmp = Bitmap.createBitmap(width, height, RGB_565)
+        for (x in 0 until width) {
+            for (y in 0 until height) {
+                bmp.setPixel(x, y, if (bitMatrix.get(x, y)) BLACK else WHITE)
+            }
+        }
+        return bmp
+    }
+
+}
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/SignedAmount.kt b/taler-kotlin-android/src/main/java/net/taler/common/SignedAmount.kt
new file mode 100644
index 0000000..03a0d6e
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/SignedAmount.kt
@@ -0,0 +1,40 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import android.annotation.SuppressLint
+
+data class SignedAmount(
+    val positive: Boolean,
+    val amount: Amount
+) {
+
+    companion object {
+        @Throws(AmountParserException::class)
+        @SuppressLint("CheckedExceptions")
+        fun fromJSONString(str: String): SignedAmount = when (str.substring(0, 1)) {
+            "-" -> SignedAmount(false, Amount.fromJSONString(str.substring(1)))
+            "+" -> SignedAmount(true, Amount.fromJSONString(str.substring(1)))
+            else -> SignedAmount(true, Amount.fromJSONString(str))
+        }
+    }
+
+    override fun toString(): String {
+        return if (positive) "$amount" else "-$amount"
+    }
+
+}
\ No newline at end of file
diff --git a/taler-kotlin-android/src/main/java/net/taler/common/TalerUtils.kt b/taler-kotlin-android/src/main/java/net/taler/common/TalerUtils.kt
new file mode 100644
index 0000000..bb2e78a
--- /dev/null
+++ b/taler-kotlin-android/src/main/java/net/taler/common/TalerUtils.kt
@@ -0,0 +1,57 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import androidx.annotation.RequiresApi
+import androidx.core.os.LocaleListCompat
+import java.util.Locale
+
+object TalerUtils {
+
+    @RequiresApi(26)
+    fun getLocalizedString(map: Map<String, String>?, default: String): String {
+        // just return the default, if it is the only element
+        if (map == null) return default
+        // create a priority list of language ranges from system locales
+        val locales = LocaleListCompat.getDefault()
+        val priorityList = ArrayList<Locale.LanguageRange>(locales.size())
+        for (i in 0 until locales.size()) {
+            priorityList.add(Locale.LanguageRange(locales[i].toLanguageTag()))
+        }
+        // create a list of locales available in the given map
+        val availableLocales = map.keys.mapNotNull {
+            if (it == "_") return@mapNotNull null
+            val list = it.split("_")
+            when (list.size) {
+                1 -> Locale(list[0])
+                2 -> Locale(list[0], list[1])
+                3 -> Locale(list[0], list[1], list[2])
+                else -> null
+            }
+        }
+        val match = Locale.lookup(priorityList, availableLocales)
+        return match?.toString()?.let { map[it] } ?: default
+    }
+
+}
+
+/**
+ * Returns the current time in milliseconds epoch rounded to nearest seconds.
+ */
+fun now(): Long {
+    return ((System.currentTimeMillis() + 500) / 1000) * 1000
+}
diff --git a/taler-kotlin-android/src/main/res/drawable/selectable_background.xml b/taler-kotlin-android/src/main/res/drawable/selectable_background.xml
new file mode 100644
index 0000000..3c383a8
--- /dev/null
+++ b/taler-kotlin-android/src/main/res/drawable/selectable_background.xml
@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  ~ This file is part of GNU Taler
+  ~ (C) 2020 Taler Systems S.A.
+  ~
+  ~ GNU Taler is free software; you can redistribute it and/or modify it under the
+  ~ terms of the GNU General Public License as published by the Free Software
+  ~ Foundation; either version 3, or (at your option) any later version.
+  ~
+  ~ GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+  ~ WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+  ~ A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+  ~
+  ~ You should have received a copy of the GNU General Public License along with
+  ~ GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+  -->
+
+<selector xmlns:android="http://schemas.android.com/apk/res/android">
+    <item android:drawable="@color/selectedBackground" android:state_activated="true" />
+    <item android:drawable="@android:color/transparent" />
+</selector>
diff --git a/taler-kotlin-android/src/main/res/values-night/colors.xml b/taler-kotlin-android/src/main/res/values-night/colors.xml
new file mode 100644
index 0000000..10bdbb9
--- /dev/null
+++ b/taler-kotlin-android/src/main/res/values-night/colors.xml
@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <color name="highlightedBackground">#2E2E2E</color>
+    <color name="selectedBackground">#363636</color>
+</resources>
diff --git a/taler-kotlin-android/src/main/res/values/colors.xml b/taler-kotlin-android/src/main/res/values/colors.xml
new file mode 100644
index 0000000..c916442
--- /dev/null
+++ b/taler-kotlin-android/src/main/res/values/colors.xml
@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  ~ This file is part of GNU Taler
+  ~ (C) 2020 Taler Systems S.A.
+  ~
+  ~ GNU Taler is free software; you can redistribute it and/or modify it under the
+  ~ terms of the GNU General Public License as published by the Free Software
+  ~ Foundation; either version 3, or (at your option) any later version.
+  ~
+  ~ GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+  ~ WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+  ~ A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+  ~
+  ~ You should have received a copy of the GNU General Public License along with
+  ~ GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+  -->
+
+<resources>
+    <color name="highlightedBackground">#E4E4E4</color>
+    <color name="selectedBackground">#DADADA</color>
+
+    <color name="green">#388E3C</color>
+    <color name="red">#C62828</color>
+</resources>
diff --git a/taler-kotlin-android/src/main/res/values/strings.xml b/taler-kotlin-android/src/main/res/values/strings.xml
new file mode 100644
index 0000000..a5b1df1
--- /dev/null
+++ b/taler-kotlin-android/src/main/res/values/strings.xml
@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="utf-8"?><!--
+  ~ This file is part of GNU Taler
+  ~ (C) 2020 Taler Systems S.A.
+  ~
+  ~ GNU Taler is free software; you can redistribute it and/or modify it under the
+  ~ terms of the GNU General Public License as published by the Free Software
+  ~ Foundation; either version 3, or (at your option) any later version.
+  ~
+  ~ GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+  ~ WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+  ~ A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+  ~
+  ~ You should have received a copy of the GNU General Public License along with
+  ~ GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+  -->
+
+<resources>
+    <string name="version_invalid">Invalid version. Please try again later!</string>
+    <string name="version_too_old">App too old. Please upgrade!</string>
+    <string name="version_too_new">Service outdated. Please wait until it gets upgraded.</string>
+</resources>
diff --git a/taler-kotlin-android/src/test/java/net/taler/common/ContractTermsTest.kt b/taler-kotlin-android/src/test/java/net/taler/common/ContractTermsTest.kt
new file mode 100644
index 0000000..79a7598
--- /dev/null
+++ b/taler-kotlin-android/src/test/java/net/taler/common/ContractTermsTest.kt
@@ -0,0 +1,74 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import com.fasterxml.jackson.databind.DeserializationFeature
+import com.fasterxml.jackson.databind.ObjectMapper
+import com.fasterxml.jackson.module.kotlin.KotlinModule
+import com.fasterxml.jackson.module.kotlin.readValue
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class ContractTermsTest {
+
+    private val mapper = ObjectMapper()
+        .registerModule(KotlinModule())
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .addMixIn(Amount::class.java, AmountMixin::class.java)
+
+    @Test
+    fun test() {
+        val json = """
+            {
+              "amount":"TESTKUDOS:0.5",
+              "extra":{
+                "article_name":"1._The_Free_Software_Definition"
+              },
+              "fulfillment_url":"https://shop.test.taler.net/essay/1._The_Free_Software_Definition",
+              "summary":"Essay: 1. The Free Software Definition",
+              "refund_deadline":{"t_ms":1596128414000},
+              "wire_transfer_deadline":{"t_ms":1596128564000},
+              "products":[],
+              "h_wire":"KV40K023N8EC1F5100TYNS23C4XN68Y1Z3PTJSWFGTMCNYD54KT4S791V2VQ91SZANN86VDAA369M4VEZ0KR6DN71EVRRZA71K681M0",
+              "wire_method":"x-taler-bank",
+              "order_id":"2020.212-01M9VKEAPF76C",
+              "timestamp":{"t_ms":1596128114000},
+              "pay_deadline":{"t_ms":"never"},
+              "max_wire_fee":"TESTKUDOS:1",
+              "max_fee":"TESTKUDOS:1",
+              "wire_fee_amortization":3,
+              "merchant_base_url":"https://backend.test.taler.net/instances/blog/",
+              "merchant":{"name":"Blog","instance":"blog"},
+              "exchanges":[
+                {
+                    "url":"https://exchange.test.taler.net/",
+                    "master_pub":"DY95EXAHQ2BKM2WK9YHZHYG1R7PPMMJPY14FNGP662DAKE35AKQG"
+                },
+                {
+                    "url":"https://exchange.test.taler.net/",
+                    "master_pub":"DY95EXAHQ2BKM2WK9YHZHYG1R7PPMMJPY14FNGP662DAKE35AKQG"}
+                ],
+                "auditors":[],
+                "merchant_pub":"8DR9NKSZY1CXFRE47NEYXM0K85C4ZGAYH7Y7VZ22GPNF0BRFNYNG",
+                "nonce":"FK8ZKJRV6VX6YFAG4CDSC6W0DWD084Q09DP81ANF30GRFQYM2KPG"
+              }
+        """.trimIndent()
+        val contractTerms: ContractTerms = mapper.readValue(json)
+        assertEquals("Essay: 1. The Free Software Definition", contractTerms.summary)
+    }
+
+}