Flutter Engine源码粗看

io.flutter.embedding.android.FlutterActivity（如无特殊说明，下文中的FlutterActivity单指此包下的类。）重写了android.app.Activity中的onCreate方法：

  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
   
      //...
      setContentView(createFlutterView());
      //...
  }

再来看一下方法createFlutterView()的源码：

 @NonNull
  private View createFlutterView() {
   
    return delegate.onCreateView(null,null,null,FLUTTER_VIEW_ID,getRenderMode() == RenderMode.surface);
  }

可以看到，这里调用了一个delegate的onCreateView(…)方法。这个变量delegate是什么东西？在哪里被设值了？可以变更吗？它是FlutterActivity中的一个变量：

protected FlutterActivityAndFragmentDelegate delegate;

它在onCreate方法中被初始化了：

 @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
   
    //...
    delegate = new FlutterActivityAndFragmentDelegate(this);
    //...
    setContentView(createFlutterView());
    //...
  }

接着看一下这个delegate的onCreateView(…)方法的源码：

@NonNull
  View onCreateView(
      LayoutInflater inflater,
      @Nullable ViewGroup container,
      @Nullable Bundle savedInstanceState,
      int flutterViewId,
      boolean shouldDelayFirstAndroidViewDraw) {
   
    //...
    flutterView.attachToFlutterEngine(flutterEngine);
    //...
  }

大致意思就是把flutterView attach到FlutterEngine（Flutter引擎）上去，并且提供了一个引擎对象flutterEngine。FlutterEngine是什么？简单地说，它使得在Android Application中执行Dart成为可能。再简单点说，它就是一个黑盒子，里面在执行Dart代码。至于它到底还干了什么别的，具体怎么干的？我也不知道。
接下来找找看这个引擎对象是如何被初始化的。

class FlutterActivityAndFragmentDelegate implements ... {
   
    @Nullable private FlutterEngine flutterEngine;

    void setupFlutterEngine() {
   
        //...
        // First, check if the host wants to use a cached FlutterEngine.
        //...
        // Second, defer to subclasses for a custom FlutterEngine.
        //...
        // Our host did not provide a custom FlutterEngine. Create a FlutterEngine to back our
        // FlutterView.
        //...
  }
}

源码注释的大致意思应该是：第一步，缓存的引擎用不用？用就返回。第二步，子类有没有提供自己的引擎？有就返回。第三步，创建一个，并返回。这里暂且不管它的引擎缓存机制是如何实现的，也不管它的子类提供引擎的机制是什么，先看一下它是如何被创建出来的。

void setupFlutterEngine() {
   
    //...
    flutterEngine = new FlutterEngine(
            host.getContext(),
            host.getFlutterShellArgs().toArray(),
            false,
            host.shouldRestoreAndSaveState());
    //...
  }

只是调了一个构造而已嘛！没什么大不了的。当我们仔细地观察会发现，所有构造器最后都指向同一个构造器，即

public FlutterEngine(
      @NonNull Context context,
      @Nullable FlutterLoader flutterLoader,
      @NonNull FlutterJNI flutterJNI,
      @NonNull PlatformViewsController platformViewsController,
      @Nullable String[] dartVmArgs,
      boolean automaticallyRegisterPlugins,
      boolean waitForRestorationData)

此处，我们暂且不关注该构造器的具体实现，只看该方法的最后几行代码，如下所示：

//...
if (automaticallyRegisterPlugins && flutterLoader.automaticallyRegisterPlugins()) {
   
      GeneratedPluginRegister.registerGeneratedPlugins(this);
    }

public static void registerGeneratedPlugins(@NonNull FlutterEngine flutterEngine) {
   
    try {
   
      Class<?> generatedPluginRegistrant =
          Class.forName("io.flutter.plugins.GeneratedPluginRegistrant");
      Method registrationMethod =
          generatedPluginRegistrant.getDeclaredMethod("registerWith", FlutterEngine.class);
      registrationMethod.invoke(null, flutterEngine);
    } catch (Exception e) {
   
      Log.e(
          TAG,
          "Tried to automatically register plugins with FlutterEngine ("
              + flutterEngine
              + ") but could not find or invoke the GeneratedPluginRegistrant.");
      Log.e(TAG, "Received exception while registering", e);
    }
  }

这是一段极为普通的反射代码，它执行了类io.flutter.plugins.GeneratedPluginRegistrant的方法registerWith，是不是非常眼熟了？在我们的Flutter APP中android文件夹下就有一个自动生成的类与之同名，即

public final class GeneratedPluginRegistrant {
   
  private static final String TAG = "GeneratedPluginRegistrant";
  public static void registerWith(@NonNull FlutterEngine flutterEngine) {
   
    //...
  }
}

现在我们知道了，当引擎被构造时，就会去注册插件。那么除了这一时机之外，还会有其它时机注册插件吗？
FlutterActivity类有代码如下所示：

@Override
  public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
   
    //...
    GeneratedPluginRegister.registerGeneratedPlugins(flutterEngine);
  }

@Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
   
    //...
    delegate = new FlutterActivityAndFragmentDelegate(this);
    delegate.onAttach(this);
    //...
    setContentView(createFlutterView());
    //...
  }

FlutterFragmentActivity类有代码如下所示：

@Override
  public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
   
    //...
    GeneratedPluginRegister.registerGeneratedPlugins(flutterEngine);
  }

FlutterActivityAndFragmentDelegate类有代码如下所示：

void onAttach(@NonNull Context context) {
   
    //...
    if