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

// 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.
   */
  @VisibleForTesting
  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(@NonNull FlutterLoader flutterLoader) {
    this.flutterLoader = flutterLoader;
  }

  private FlutterLoader flutterLoader;

  /** 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 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();

      return new FlutterInjector(flutterLoader);
    }
  }
}