Added sample for multiple flutters for android

add_to_app/multiple_flutters/multiple_flutters_android/.gitignore

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+*.iml
+.gradle
+/local.properties
+/.idea/caches
+/.idea/libraries
+/.idea/modules.xml
+/.idea/workspace.xml
+/.idea/navEditor.xml
+/.idea/assetWizardSettings.xml
+.DS_Store
+/build
+/captures
+.externalNativeBuild
+.cxx
+local.properties

add_to_app/multiple_flutters/multiple_flutters_android/README.md

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+# multiple_flutters_android
+
+This is an add-to-app sample that uses the Flutter engine group API to host
+multiple instances of Flutter in the app.
+
+## Getting Started
+
+```sh
+cd ../multiple_flutters_module
+flutter pub get
+cd -
+open -a "Android Studio" multiple_flutters_android/ # macOS command
+# (build and run)
+```
+
+For more information see
+[multiple_flutters_module](../multiple_flutters_module/README.md).

add_to_app/multiple_flutters/multiple_flutters_android/app/.gitignore

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+/build

add_to_app/multiple_flutters/multiple_flutters_android/app/build.gradle

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+plugins {
+    id 'com.android.application'
+    id 'kotlin-android'
+}
+
+android {
+    signingConfigs {
+        self {
+        }
+    }
+    compileSdkVersion 30
+
+    defaultConfig {
+        applicationId "dev.flutter.multipleflutters"
+        minSdkVersion 24
+        targetSdkVersion 30
+        versionCode 1
+        versionName "1.0"
+
+        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+    }
+
+    buildTypes {
+        release {
+            minifyEnabled false
+            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+            signingConfig debug.signingConfig
+        }
+    }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+    kotlinOptions {
+        jvmTarget = '1.8'
+    }
+}
+
+dependencies {
+
+    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
+    implementation 'androidx.core:core-ktx:1.2.0'
+    implementation 'androidx.appcompat:appcompat:1.1.0'
+    implementation 'com.google.android.material:material:1.1.0'
+    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
+    testImplementation 'junit:junit:4.+'
+    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
+    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
+    implementation project(':flutter')
+}

add_to_app/multiple_flutters/multiple_flutters_android/app/proguard-rules.pro

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+# 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

add_to_app/multiple_flutters/multiple_flutters_android/app/src/main/AndroidManifest.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="dev.flutter.multipleflutters">
+
+    <application
+        android:name=".App"
+        android:allowBackup="true"
+        android:icon="@mipmap/ic_launcher"
+        android:label="@string/app_name"
+        android:roundIcon="@mipmap/ic_launcher_round"
+        android:supportsRtl="true"
+        android:theme="@style/Theme.MultipleFlutters">
+        <activity android:name=".MainActivity">
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+        <activity
+            android:name=".SingleFlutterActivity"
+            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
+            android:hardwareAccelerated="true"
+            android:windowSoftInputMode="adjustResize"
+            />
+        <activity
+            android:name=".DoubleFlutterActivity"
+            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
+            android:hardwareAccelerated="true"
+            android:windowSoftInputMode="adjustResize"
+            />
+    </application>
+
+</manifest>

add_to_app/multiple_flutters/multiple_flutters_android/app/src/main/java/dev/flutter/multipleflutters/App.kt

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+package dev.flutter.multipleflutters
+
+import android.app.Application
+import io.flutter.embedding.engine.FlutterEngineGroup
+
+/**
+ * Application class for this app.
+ *
+ * This holds onto our engine group.
+ */
+class App : Application() {
+    lateinit var engines: FlutterEngineGroup
+
+    override fun onCreate() {
+        super.onCreate()
+        engines = FlutterEngineGroup(this)
+    }
+}

add_to_app/multiple_flutters/multiple_flutters_android/app/src/main/java/dev/flutter/multipleflutters/DataModel.kt

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+package dev.flutter.multipleflutters
+
+import java.lang.ref.WeakReference
+
+/**
+ * Interface for getting notifications when the DataModel is updated.
+ */
+interface DataModelObserver {
+    fun onCountUpdate(newCount: Int)
+}
+
+/**
+ * A singleton/observable data model for the data shared between Flutter and the host platform.
+ *
+ * This is the definitive source of truth for all data.
+ */
+class DataModel {
+    companion object {
+        val instance = DataModel()
+    }
+
+    private val observers = mutableListOf<WeakReference<DataModelObserver>>()
+
+    public var counter = 0
+        set(value) {
+            field = value
+            for (observer in observers) {
+                observer.get()?.onCountUpdate(value)
+            }
+        }
+
+    fun addObserver(observer: DataModelObserver) {
+        observers.add(WeakReference(observer))
+    }
+
+    fun removeObserver(observer: DataModelObserver) {
+        observers.removeIf {
+            if (it.get() != null) it.get() == observer else true
+        }
+    }
+}

add_to_app/multiple_flutters/multiple_flutters_android/app/src/main/java/dev/flutter/multipleflutters/DoubleFlutterActivity.kt

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+package dev.flutter.multipleflutters
+
+import android.content.Intent
+import android.os.Bundle
+import android.widget.FrameLayout
+import android.widget.LinearLayout
+import androidx.fragment.app.FragmentActivity
+import androidx.fragment.app.FragmentManager
+import io.flutter.embedding.android.FlutterFragment
+import io.flutter.embedding.engine.FlutterEngineCache
+
+/**
+ * An activity that displays 2 FlutterFragments vertically.
+ */
+class DoubleFlutterActivity : FragmentActivity(), EngineBindingsDelegate {
+    private val topBindings: EngineBindings by lazy {
+        EngineBindings(activity = this, delegate = this, entrypoint = "topMain")
+    }
+    private val bottomBindings: EngineBindings by lazy {
+        EngineBindings(activity = this, delegate = this, entrypoint = "bottomMain")
+    }
+    private val numberOfFlutters = 2
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        val root = LinearLayout(this)
+        root.layoutParams = LinearLayout.LayoutParams(
+            LinearLayout.LayoutParams.MATCH_PARENT,
+            LinearLayout.LayoutParams.MATCH_PARENT
+        )
+        root.orientation = LinearLayout.VERTICAL
+        root.weightSum = numberOfFlutters.toFloat()
+
+        val fragmentManager: FragmentManager = supportFragmentManager
+
+        setContentView(root)
+
+        val app = applicationContext as App
+
+        for (i in 0 until numberOfFlutters) {
+            val flutterContainer = FrameLayout(this)
+            root.addView(flutterContainer)
+            flutterContainer.id = 12345 + i
+            flutterContainer.layoutParams = LinearLayout.LayoutParams(
+                FrameLayout.LayoutParams.MATCH_PARENT,
+                FrameLayout.LayoutParams.MATCH_PARENT,
+                1.0f
+            )
+            val engine = if (i == 0) topBindings.engine else bottomBindings.engine
+            FlutterEngineCache.getInstance().put(i.toString(), engine)
+            val flutterFragment =
+                FlutterFragment.withCachedEngine(i.toString()).build<FlutterFragment>()
+            fragmentManager
+                .beginTransaction()
+                .add(
+                    12345 + i,
+                    flutterFragment
+                )
+                .commit()
+        }
+
+        topBindings.attach()
+        bottomBindings.attach()
+    }
+
+    override fun onDestroy() {
+        topBindings.detach()
+        topBindings.detach()
+
+        for (i in 0 until numberOfFlutters) {
+            FlutterEngineCache.getInstance().remove(i.toString())
+        }
+
+        super.onDestroy()
+    }
+
+    override fun onNext() {
+        val flutterIntent = Intent(this, MainActivity::class.java)
+        startActivity(flutterIntent)
+    }
+}

add_to_app/multiple_flutters/multiple_flutters_android/app/src/main/java/dev/flutter/multipleflutters/EngineBindings.kt

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+package dev.flutter.multipleflutters
+
+import android.app.Activity
+import io.flutter.FlutterInjector
+import io.flutter.embedding.engine.FlutterEngine
+import io.flutter.embedding.engine.dart.DartExecutor
+import io.flutter.plugin.common.MethodChannel
+
+/**
+ * This interface represents the notifications an EngineBindings may be receiving from the Flutter
+ * instance.
+ *
+ * What methods this interface has depends on the messages that are sent over the EngineBinding's
+ * channel in `main.dart`.  Messages that interact with the DataModel are handled automatically
+ * by the EngineBindings.
+ *
+ * @see main.dart for what messages are getting sent from Flutter.
+ */
+interface EngineBindingsDelegate {
+    fun onNext()
+}
+
+/**
+ * This binds a FlutterEngine instance with the DataModel and a channel for communicating with that
+ * engine.
+ *
+ * Messages involving the DataModel are handled by the EngineBindings, other messages are forwarded
+ * to the EngineBindingsDelegate.
+ *
+ * @see main.dart for what messages are getting sent from Flutter.
+ */
+class EngineBindings(activity: Activity, delegate: EngineBindingsDelegate, entrypoint: String) :
+    DataModelObserver {
+    val channel: MethodChannel
+    val engine: FlutterEngine
+    val delegate: EngineBindingsDelegate
+
+    init {
+        val app = activity.applicationContext as App
+        // This has to be lazy to avoid creation before the FlutterEngineGroup.
+        val dartEntrypoint =
+            DartExecutor.DartEntrypoint(
+                FlutterInjector.instance().flutterLoader().findAppBundlePath(), entrypoint
+            )
+        engine = app.engines.createAndRunEngine(activity, dartEntrypoint)
+        this.delegate = delegate
+        channel = MethodChannel(engine.dartExecutor.binaryMessenger, "multiple-flutters")
+    }
+
+    /**
+     * This setups the messaging connections on the platform channel and the DataModel.
+     */
+    fun attach() {
+        DataModel.instance.addObserver(this)
+        channel.invokeMethod("setCount", DataModel.instance.counter)
+        channel.setMethodCallHandler { call, result ->
+            when (call.method) {
+                "incrementCount" -> {
+                    DataModel.instance.counter = DataModel.instance.counter + 1
+                    result.success(null)
+                }
+                "next" -> {
+                    this.delegate.onNext()
+                    result.success(null)
+                }
+                else -> {
+                    result.notImplemented()
+                }
+            }
+        }
+    }
+
+    /**
+     * This tears down the messaging connections on the platform channel and the DataModel.
+     */
+    fun detach() {
+        DataModel.instance.removeObserver(this)
+        channel.setMethodCallHandler(null)
+    }
+
+    override fun onCountUpdate(newCount: Int) {
+        channel.invokeMethod("setCount", newCount)
+    }
+}