前言

通过前面文章的分析我们了解到FlutterActivITy的显示最终是通过FlutterView进行渲染。本文我们深入FlutterView的源码来具体分析下Flutter UI是如何一步步显示到Activity，又是如何跟Flutter Engine关联，下面来看一下FlutterView的大致相关类F1a;

Flutter实践深入分析之——FlutterView相关源码分析

Flutter渲染模式

要了解FlutterView我们首先来了解一下Flutter的RenderMode，在Flutter里面渲染模式分为三种：RenderMode.surface、RenderMode.texture、RenderMode.image，前面两种模式是在接口FlutterActivityAnDFragmentDelegate.Host接口getRenderMode()方法中返回，其内部的判断条件在FlutterActivity和FlutterFragment中是不一样的：

FlutterActivity
```
  public RenderMode getRenderMode() {
    return getBackgroundMode() == BackgroundMode.opaque ? RenderMode.surface : RenderMode.texture;
  }
```
说白了在FlutterActivity中RenderMode就是根据是否透明来决定，如果是透明窗体那么就使用RenderMode.texture，如果是不透明的就是用RenderMode.surface

FlutterFragment

  public RenderMode getRenderMode() {
    String renderModeName =
        getarguments().getString(arg_FLUTTERVIEW_RENDER_MODE, RenderMode.surface.name());
    return RenderMode.valueOf(renderModeName);
  }

而在FlutterFragment中RenderMode是通过ARG_FLUTTERVIEW_RENDER_MODE参数来决定。

思考：为什么透明模式下使用RenderMode.texture，而不透明模式下使用RenderMode.surface

显而易见三种渲染模式对应的在FlutterView创建时以来不同的类，分别是：FlutterSurfaceView，FlutterTextureView，FlutterImageView。下面我们一起来看看这三个类。

FlutterSurfaceView分析

从文中开头大致的类图关系我们知道FlutterSurfaceView、FlutterTextureView和FlutterImageView都是FlutterView的一个成员变量，通过构造函数根据不同的渲染模式传递对应的参数对FlutterView进行初始化。

//继承自SurfaceView，实现RenderSurface接口
public class FlutterSurfaceView extends SurfaceView implements RenderSurface {
    PRivate final SurfaceHolder.Callback surfaceCallback =
      new SurfaceHolder.Callback() {
        @override
        public void surfaceCreated(@NonNull SurfaceHolder holder) {
            //......
            connectSurfaceToRenderer();
        }

        @Override
        public void surfaceChanged(
            @NonNull SurfaceHolder holder, int format, int width, int height) {
          	//....
          	changeSurfaceSize(width, height);
        }

        @Override
        public void surfaceDestroyed(@NonNull SurfaceHolder holder) {
						//...
          	disconnectSurfaceFromRenderer();
        }
      };
 			
  	private void connectSurfaceToRenderer() {
      if (flutterRenderer == null || getHolder() == null) {
        throw new IllegalstateException(
            "connectSurfaceToRenderer() should only be called when flutterRenderer and getHolder() are non-null.");
      }

      flutterRenderer.startRenderingToSurface(getHolder().getSurface());
  	}
  /**
  * 当 FlutterSurfaceView 想要开始将 Flutter UI 渲染到此 FlutterSurfaceView 时，由此 FlutterSurfaceView 的所有者调用。
	*	如果 AndROId android.view.Surface 可用，则此方法会将 android.view.Surface 提供给给定的 FlutterRenderer 以开始将 Flutter 的 UI 渲染到此 	 *FlutterSurfaceView。
	*	如果还没有 Android android.view.Surface 可用，则此 FlutterSurfaceView 将等到 android.view.Surface 可用，然后将该 				     
	* android.view.Surface 提供给给定的 FlutterRenderer 以开始将 Flutter 的 UI 渲染到此 FlutterSurfaceView。
  */
  public void attachToRenderer(@NonNull FlutterRenderer flutterRenderer) {
    connectSurfaceToRenderer();
  }

 	/***
 	当 FlutterSurfaceView 不再想将 Flutter UI 渲染到此 FlutterSurfaceView 时，由此 FlutterSurfaceView 的所有者调用。此方法将停止从 Flutter 到此 FlutterSurfaceView 的任何正在进行的渲染。
	***/
  public void detachFromRenderer() {
    disconnectSurfaceFromRenderer();
  }

  // FlutterRenderer and getSurfaceTexture() must both be non-null.
  private void connectSurfaceToRenderer() {
    flutterRenderer.startRenderingToSurface(getHolder().getSurface());
  }

  // FlutterRenderer must be non-null.
  private void changeSurfaceSize(int width, int height) {
    flutterRenderer.surfaceChanged(width, height);
  }

  // FlutterRenderer must be non-null.
  private void disconnectSurfaceFromRenderer() {
    flutterRenderer.stopRenderingToSurface();
  }
}

上面的代码逻辑相当简介易懂，可以看到，FlutterSurfaceView实际上是一个SurfaceView，用法也是跟SurfaceView一样，只是渲染数据是最终是通过flutterJNI作为桥梁实现Flutter Engine与Android View层进行数据传递实现界面绘制的。

FlutterTextureView分析

与上面的FlutterSurfaceView一样，逻辑实现简洁易懂，先看看主要逻辑：

//继承自TextureView，实现RenderSurface接口
public class FlutterTextureView extends TextureView implements RenderSurface {
  private final SurfaceTextureListener surfaceTextureListener =
      new SurfaceTextureListener() {
        @Override
        public void onSurfaceTextureAvailable(
            SurfaceTexture surfaceTexture, int width, int height) {
            connectSurfaceToRenderer();
        }

        @Override
        public void onSurfaceTextureSizeChanged(
            @NonNull SurfaceTexture surface, int width, int height) {
            changeSurfaceSize(width, height);
        }

        @Override
        public void onSurfaceTextureUpdated(@NonNull SurfaceTexture surface) {
          // Invoked every time a new frame is available. We don't care.
        }

        @Override
        public boolean onSurfaceTexturedestroyed(@NonNull SurfaceTexture surface) {
          disconnectSurfaceFromRenderer();
          return true;
        }
      };


  public void attachToRenderer(@NonNull FlutterRenderer flutterRenderer) {
      connectSurfaceToRenderer();
  }

  public void detachFromRenderer() {
      disconnectSurfaceFromRenderer();
  }

  private void connectSurfaceToRenderer() {
    renderSurface = new Surface(getSurfaceTexture());
    flutterRenderer.startRenderingToSurface(renderSurface);
  }

  private void changeSurfaceSize(int width, int height) {
    flutterRenderer.surfaceChanged(width, height);
  }

  private void disconnectSurfaceFromRenderer() {
    flutterRenderer.stopRenderingToSurface();
    if (renderSurface != null) {
      renderSurface.release();
      renderSurface = null;
    }
  }
}

从上面的代码可以看到FlutterTextureView和FlutterSurfaceView的逻辑基本一致，唯一不同的就是提供的Surface来源不一样，逻辑很简单这里不做多余介绍。

FlutterImageView分析

FlutterImageView并不是一个真正意义上的ImageView。它的工作流程和FlutterTextureView，FlutterSurfaceView工作流程极度相似，但是它的使用场景却完全不一样，想要了解FlutterImageView的工作场景，必须要先了解Flutter的Hybrid Composition工作模式。由于Hybrid Composition 模式涉及到的内容关联比较多，下次单独讲解。这里简单理解来讲Hybrid Composition模式就是解决原生控件和Flutter混合显示问题。

我们来看下FlutterImageView源码：

@TargetApi(19)
public class FlutterImageView extends View implements RenderSurface {
  
  // 原生控件的绘制操作
  @Override
  protected void onDraw(Canvas canvas) {
    suPEr.onDraw(canvas);

    if (!imageQueue.iSEMpty()) {
      if (currentImage != null) {
        currentImage.close();
      }
      currentImage = imageQueue.poll();
      updateCurrentBitmap();
    }
    if (currentBitmap != null) {
      canvas.drawBitmap(currentBitmap, 0, 0, null);
    }
  }
}

我们可以看到FlutterImageView实际上就是一个原生的View，但是它又实现了RenderSurface接口，不难判断FlutterImageView既可以显示渲染我们的FlutterUI，又可以显示渲染我们的原生View。它的使用场景已经原理分享后面再花费专题文章讲解。

FlutterRender分析

通过上面的分析，我们知道FlutterRenderer 的主要职责最终是通过 FlutterJNI 进行渲染关联处理，与原生平台提供的 FlutterSurfaceView、FlutterTextureView 、FlutterImageView进行纯 UI 渲染，将 Flutter 像素绘制到 Android 视图层次结构。

public class FlutterRenderer implements TextureRegistry {
  //......
  @NonNull private final FlutterJNI flutterJNI;
  @Nullable private Surface surface;
  //......
}

通过上面源码的两个属性成员就能看出来他的职责。

FlutterView分析

通俗来说FlutterView 的作用是在 Android 设备上显示一个 Flutter UI，绘制内容来自于 FlutterEngine 的flutterJNI提供。FlutterView常用模式下有两种渲染模式：

RenderMode.surface

这种模式性能最好，但是这种模式下的一个 FlutterView 不能定位在 z-index 中其他 2 个 Android Views 之间，也不能进行动画/转换。除非需要 android.graphics.SurfaceTexture 的特殊功能，否则强烈建议开发人员使用这种渲染模式。
RenderMode.texture

它将 Flutter UI 绘制到 android.graphics.SurfaceTexture。这种模式的性能不如 io.flutter.embedding.android.RenderMode.surface，但这种模式下的 FlutterView 可以进行动画和变换，也可以定位在 2+ 其他 Android 视图之间的 z-index 中。除非需要 android.graphics.SurfaceTexture 的特殊功能，否则开发人员应该强烈推荐

FlutterView 的默认构造器就是 surface 模式

下面我们再来分析下FlutterView，首先我们来看看它的变量和构造函数声明

public class FlutterView extends FrameLayout implements MouseCursorPlugin.MouseCursorViewDelegate {
  private static final String TAG = "FlutterView";

  // 用来真正渲染绘制视图的相关类
  @Nullable private FlutterSurfaceView flutterSurfaceView;
  @Nullable private FlutterTextureView flutterTextureView;
  @Nullable private FlutterImageView flutterImageView;
  @Nullable private RenderSurface renderSurface;
  @Nullable private RenderSurface previousRenderSurface;

  //与 FlutterEngine连接
  @Nullable private FlutterEngine flutterEngine;

  // 处理各种类型的 Android View 输入和事件的组件
  @Nullable private MouseCursorPlugin mouseCursorPlugin;
  @Nullable private TextInputPlugin textInputPlugin;
  @Nullable private LocalizationPlugin localizationPlugin;
  @Nullable private AndroidKeyProcessor androidKeyProcessor;
  @Nullable private AndroidTouchProcessor androidTouchProcessor;
  @Nullable private AccessibilityBridge accessibilityBridge;

  //构造函数，默认模式为surface
  private FlutterView(
      @NonNull Context context,
      @Nullable AttributeSet attrs,
      @NonNull FlutterSurfaceView flutterSurfaceView) {
    super(context, attrs);

    this.flutterSurfaceView = flutterSurfaceView;
    this.renderSurface = flutterSurfaceView;

    init();
  }

  //构造函数，模式为texture
  private FlutterView(
      @NonNull Context context,
      @Nullable AttributeSet attrs,
      @NonNull FlutterTextureView flutterTextureView) {
    super(context, attrs);

    this.flutterTextureView = flutterTextureView;
    this.renderSurface = flutterTextureView;

    init();
  }

  //构造函数 模式为image
  @TargetApi(19)
  private FlutterView(
      @NonNull Context context,
      @Nullable AttributeSet attrs,
      @NonNull FlutterImageView flutterImageView) {
    super(context, attrs);

    this.flutterImageView = flutterImageView;
    this.renderSurface = flutterImageView;

    init();
  }

  //根据不同的模式指定RenderSurface
  private void init() {
    if (flutterSurfaceView != null) {
      addView(flutterSurfaceView);
    } else if (flutterTextureView != null) {
      addView(flutterTextureView);
    } else {
      addView(flutterImageView);
    }
  }

通过上面的代码我们知道FlutterView其实是一个原生的继承自FrameLayout的ViewGroup。在init中根据不同的模式添加不同的View从而指定surface，也就是 FlutterSurfaceView、FlutterTextureView、FlutterImageView 之一。而最终FlutterEngine渲染数据到不同的surface。