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FlutterRender分析
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FlutterView分析
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总结（类图，架构图）
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- 类关系图
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架构图
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? 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：
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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。
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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());
}
/**
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当 FlutterSurfaceView 想要开始将 Flutter UI 渲染到此 FlutterSurfaceView 时，由此 FlutterSurfaceView 的所有者调用。
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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。
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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
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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。
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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.stopRenderingToSu
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rface();
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。
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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。
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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。
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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。