Add a java injector for testing

ci/licenses_golden/licenses_flutter

@@ -694,6 +694,7 @@ FILE: ../../../flutter/shell/platform/android/external_view_embedder/surface_poo
 FILE: ../../../flutter/shell/platform/android/external_view_embedder/surface_pool_unittests.cc
 FILE: ../../../flutter/shell/platform/android/flutter_main.cc
 FILE: ../../../flutter/shell/platform/android/flutter_main.h
+FILE: ../../../flutter/shell/platform/android/io/flutter/FlutterInjector.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/Log.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/app/FlutterActivity.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/app/FlutterActivityDelegate.java

shell/platform/android/BUILD.gn

@@ -117,6 +117,7 @@ embedding_sources_jar_filename = "$embedding_artifact_id-sources.jar"
 embedding_source_jar_path = "$root_out_dir/$embedding_sources_jar_filename"
 
 android_java_sources = [
+  "io/flutter/FlutterInjector.java",
   "io/flutter/Log.java",
   "io/flutter/app/FlutterActivity.java",
   "io/flutter/app/FlutterActivityDelegate.java",
@@ -415,6 +416,7 @@ action("robolectric_tests") {
   jar_path = "$root_out_dir/robolectric_tests.jar"
 
   sources = [
+    "test/io/flutter/FlutterInjectorTest.java",
     "test/io/flutter/FlutterTestSuite.java",
     "test/io/flutter/SmokeTest.java",
     "test/io/flutter/embedding/android/AndroidKeyProcessorTest.java",
@@ -434,6 +436,7 @@ action("robolectric_tests") {
     "test/io/flutter/embedding/engine/RenderingComponentTest.java",
     "test/io/flutter/embedding/engine/dart/DartExecutorTest.java",
     "test/io/flutter/embedding/engine/loader/ApplicationInfoLoaderTest.java",
+    "test/io/flutter/embedding/engine/loader/FlutterLoaderTest.java",
     "test/io/flutter/embedding/engine/plugins/shim/ShimPluginRegistryTest.java",
     "test/io/flutter/embedding/engine/renderer/FlutterRendererTest.java",
     "test/io/flutter/embedding/engine/systemchannels/KeyEventChannelTest.java",

shell/platform/android/io/flutter/FlutterInjector.java

@@ -0,0 +1,136 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package io.flutter;
+
+import android.support.annotation.NonNull;
+import android.support.annotation.VisibleForTesting;
+import io.flutter.embedding.engine.loader.FlutterLoader;
+
+/**
+ * This class is a simple dependency injector for the relatively thin Android part of the Flutter
+ * engine.
+ *
+ * <p>This simple solution is used facilitate testability without bringing in heavier
+ * app-development centric dependency injection frameworks such as Guice or Dagger2 or spreading
+ * construction injection everywhere.
+ */
+public final class FlutterInjector {
+
+  private static FlutterInjector instance;
+  private static boolean accessed;
+
+  /**
+   * Use {@link FlutterInjector.Builder} to specify members to be injected via the static {@code
+   * FlutterInjector}.
+   *
+   * <p>This can only be called at the beginning of the program before the {@link #instance()} is
+   * accessed.
+   */
+  public static void setInstance(@NonNull FlutterInjector injector) {
+    if (accessed) {
+      throw new IllegalStateException(
+          "Cannot change the FlutterInjector instance once it's been "
+              + "read. If you're trying to dependency inject, be sure to do so at the beginning of "
+              + "the program");
+    }
+    instance = injector;
+  }
+
+  /**
+   * Retrieve the static instance of the {@code FlutterInjector} to use in your program.
+   *
+   * <p>Once you access it, you can no longer change the values injected.
+   *
+   * <p>If no override is provided for the injector, reasonable defaults are provided.
+   */
+  public static FlutterInjector instance() {
+    accessed = true;
+    if (instance == null) {
+      instance = new Builder().build();
+    }
+    return instance;
+  }
+
+  // This whole class is here to enable testing so to test the thing that lets you test, some degree
+  // of hack is needed.
+  @VisibleForTesting
+  public static void reset() {
+    accessed = false;
+    instance = null;
+  }
+
+  private FlutterInjector(boolean shouldLoadNative, @NonNull FlutterLoader flutterLoader) {
+    this.shouldLoadNative = shouldLoadNative;
+    this.flutterLoader = flutterLoader;
+  }
+
+  private boolean shouldLoadNative;
+  private FlutterLoader flutterLoader;
+
+  /**
+   * Returns whether the Flutter Android engine embedding should load the native C++ engine.
+   *
+   * <p>Useful for testing since JVM tests via Robolectric can't load native libraries.
+   */
+  public boolean shouldLoadNative() {
+    return shouldLoadNative;
+  }
+
+  /** Returns the {@link FlutterLoader} instance to use for the Flutter Android engine embedding. */
+  @NonNull
+  public FlutterLoader flutterLoader() {
+    return flutterLoader;
+  }
+
+  /**
+   * Builder used to supply a custom FlutterInjector instance to {@link
+   * FlutterInjector#setInstance(FlutterInjector)}.
+   *
+   * <p>Non-overriden values have reasonable defaults.
+   */
+  public static final class Builder {
+
+    private boolean shouldLoadNative = true;
+    /**
+     * Sets whether the Flutter Android engine embedding should load the native C++ engine.
+     *
+     * <p>Useful for testing since JVM tests via Robolectric can't load native libraries.
+     *
+     * <p>Defaults to true.
+     */
+    public Builder setShouldLoadNative(boolean shouldLoadNative) {
+      this.shouldLoadNative = shouldLoadNative;
+      return this;
+    }
+
+    private FlutterLoader flutterLoader;
+    /**
+     * Sets a {@link FlutterLoader} override.
+     *
+     * <p>A reasonable default will be used if unspecified.
+     */
+    public Builder setFlutterLoader(@NonNull FlutterLoader flutterLoader) {
+      this.flutterLoader = flutterLoader;
+      return this;
+    }
+
+    private void fillDefaults() {
+      if (flutterLoader == null) {
+        flutterLoader = new FlutterLoader();
+      }
+    }
+
+    /**
+     * Builds a {@link FlutterInjector} from the builder. Unspecified properties will have
+     * reasonable defaults.
+     */
+    public FlutterInjector build() {
+      fillDefaults();
+
+      System.out.println("should load native is " + shouldLoadNative);
+      return new FlutterInjector(shouldLoadNative, flutterLoader);
+    }
+  }
+}

shell/platform/android/io/flutter/app/FlutterActivityDelegate.java

@@ -35,7 +35,8 @@
 import java.util.ArrayList;
 
 /**
- * Class that performs the actual work of tying Android {@link Activity} instances to Flutter.
+ * Deprecated class that performs the actual work of tying Android {@link Activity} instances to
+ * Flutter.
  *
  * <p>This exists as a dedicated class (as opposed to being integrated directly into {@link
  * FlutterActivity}) to facilitate applications that don't wish to subclass {@code FlutterActivity}.
@@ -48,6 +49,10 @@
  * FlutterActivityEvents} from your activity to an instance of this class. Optionally, you can make
  * your activity implement {@link PluginRegistry} and/or {@link
  * io.flutter.view.FlutterView.Provider} and forward those methods to this class as well.
+ *
+ * <p>Deprecation: {@link io.flutter.embedding.android.FlutterActivity} is the new API that now
+ * replaces this class and {@link io.flutter.app.FlutterActivity}. See
+ * https://flutter.dev/go/android-project-migration for more migration details.
  */
 public final class FlutterActivityDelegate
     implements FlutterActivityEvents, FlutterView.Provider, PluginRegistry {

shell/platform/android/io/flutter/app/FlutterApplication.java

@@ -7,7 +7,7 @@
 import android.app.Activity;
 import android.app.Application;
 import androidx.annotation.CallSuper;
-import io.flutter.view.FlutterMain;
+import io.flutter.FlutterInjector;
 
 /**
  * Flutter implementation of {@link android.app.Application}, managing application-level global
@@ -21,7 +21,7 @@ public class FlutterApplication extends Application {
   @CallSuper
   public void onCreate() {
     super.onCreate();
-    FlutterMain.startInitialization(this);
+    FlutterInjector.instance().flutterLoader().startInitialization(this);
   }
 
   private Activity mCurrentActivity = null;

shell/platform/android/io/flutter/embedding/engine/FlutterEngine.java

@@ -7,6 +7,7 @@
 import android.content.Context;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
+import io.flutter.FlutterInjector;
 import io.flutter.Log;
 import io.flutter.embedding.engine.dart.DartExecutor;
 import io.flutter.embedding.engine.loader.FlutterLoader;
@@ -131,7 +132,8 @@ public void onPreEngineRestart() {
    * <p>In order to pass Dart VM initialization arguments (see {@link
    * io.flutter.embedding.engine.FlutterShellArgs}) when creating the VM, manually set the
    * initialization arguments by calling {@link FlutterLoader#startInitialization(Context)} and
-   * {@link FlutterLoader#ensureInitializationComplete(Context, String[])}.
+   * {@link FlutterLoader#ensureInitializationComplete(Context, String[])} before constructing the
+   * engine.
    */
   public FlutterEngine(@NonNull Context context) {
     this(context, null);
@@ -143,7 +145,7 @@ public FlutterEngine(@NonNull Context context) {
    * <p>If the Dart VM has already started, the given arguments will have no effect.
    */
   public FlutterEngine(@NonNull Context context, @Nullable String[] dartVmArgs) {
-    this(context, FlutterLoader.getInstance(), new FlutterJNI(), dartVmArgs, true);
+    this(context, /* flutterLoader */ null, new FlutterJNI(), dartVmArgs, true);
   }
 
   /**
@@ -158,7 +160,7 @@ public FlutterEngine(
       boolean automaticallyRegisterPlugins) {
     this(
         context,
-        FlutterLoader.getInstance(),
+        /* flutterLoader */ null,
         new FlutterJNI(),
         dartVmArgs,
         automaticallyRegisterPlugins);
@@ -189,7 +191,7 @@ public FlutterEngine(
       boolean waitForRestorationData) {
     this(
         context,
-        FlutterLoader.getInstance(),
+        /* flutterLoader */ null,
         new FlutterJNI(),
         new PlatformViewsController(),
         dartVmArgs,
@@ -206,7 +208,7 @@ public FlutterEngine(
    */
   public FlutterEngine(
       @NonNull Context context,
-      @NonNull FlutterLoader flutterLoader,
+      @Nullable FlutterLoader flutterLoader,
       @NonNull FlutterJNI flutterJNI) {
     this(context, flutterLoader, flutterJNI, null, true);
   }
@@ -219,7 +221,7 @@ public FlutterEngine(
    */
   public FlutterEngine(
       @NonNull Context context,
-      @NonNull FlutterLoader flutterLoader,
+      @Nullable FlutterLoader flutterLoader,
       @NonNull FlutterJNI flutterJNI,
       @Nullable String[] dartVmArgs,
       boolean automaticallyRegisterPlugins) {
@@ -238,7 +240,7 @@ public FlutterEngine(
    */
   public FlutterEngine(
       @NonNull Context context,
-      @NonNull FlutterLoader flutterLoader,
+      @Nullable FlutterLoader flutterLoader,
       @NonNull FlutterJNI flutterJNI,
       @NonNull PlatformViewsController platformViewsController,
       @Nullable String[] dartVmArgs,
@@ -256,7 +258,7 @@ public FlutterEngine(
   /** Fully configurable {@code FlutterEngine} constructor. */
   public FlutterEngine(
       @NonNull Context context,
-      @NonNull FlutterLoader flutterLoader,
+      @Nullable FlutterLoader flutterLoader,
       @NonNull FlutterJNI flutterJNI,
       @NonNull PlatformViewsController platformViewsController,
       @Nullable String[] dartVmArgs,
@@ -280,6 +282,9 @@ public FlutterEngine(
     this.localizationPlugin = new LocalizationPlugin(context, localizationChannel);
 
     this.flutterJNI = flutterJNI;
+    if (flutterLoader == null) {
+      flutterLoader = FlutterInjector.instance().flutterLoader();
+    }
     flutterLoader.startInitialization(context.getApplicationContext());
     flutterLoader.ensureInitializationComplete(context, dartVmArgs);
 

shell/platform/android/io/flutter/embedding/engine/FlutterShellArgs.java

@@ -15,8 +15,8 @@
  * <p>The term "shell" refers to the native code that adapts Flutter to different platforms.
  * Flutter's Android Java code initializes a native "shell" and passes these arguments to that
  * native shell when it is initialized. See {@link
- * io.flutter.view.FlutterMain#ensureInitializationComplete(Context, String[])} for more
- * information.
+ * io.flutter.embedding.engine.loader.FlutterLoader#ensureInitializationComplete(Context, String[])}
+ * for more information.
  */
 @SuppressWarnings({"WeakerAccess", "unused"})
 public class FlutterShellArgs {

shell/platform/android/io/flutter/embedding/engine/dart/DartExecutor.java

@@ -8,12 +8,13 @@
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.annotation.UiThread;
+import io.flutter.FlutterInjector;
 import io.flutter.Log;
 import io.flutter.embedding.engine.FlutterJNI;
+import io.flutter.embedding.engine.loader.FlutterLoader;
 import io.flutter.plugin.common.BinaryMessenger;
 import io.flutter.plugin.common.StringCodec;
 import io.flutter.view.FlutterCallbackInformation;
-import io.flutter.view.FlutterMain;
 import java.nio.ByteBuffer;
 
 /**
@@ -250,9 +251,19 @@ public void notifyLowMemoryWarning() {
    * that entrypoint and other assets required for Dart execution.
    */
   public static class DartEntrypoint {
+    /**
+     * Create a DartEntrypoint pointing to the default Flutter assets location with a default Dart
+     * entrypoint.
+     */
     @NonNull
     public static DartEntrypoint createDefault() {
-      return new DartEntrypoint(FlutterMain.findAppBundlePath(), "main");
+      FlutterLoader flutterLoader = FlutterInjector.instance().flutterLoader();
+
+      if (!flutterLoader.initialized()) {
+        throw new AssertionError(
+            "DartEntrypoints can only be created once a FlutterEngine is created.");
+      }
+      return new DartEntrypoint(flutterLoader.findAppBundlePath(), "main");
     }
 
     /** The path within the AssetManager where the app will look for assets. */