【Android】VirtualDisplay创建流程及原理

Android VirtualDisplay创建流程及原理

• Android DisplayManager提供了createVirtualDisplay接口，用于创建虚拟屏。虚拟屏可用于录屏（网上很多资料说这个功能），分屏幕（比如一块很长的屏幕，通过虚拟屏分出不同的区域）等等。

  创建VirtualDisplay

  • DisplayManager中的函数原型如下。后两个Hide的API，只有平台的应用才可以使用。

    // frameworks/base/core/java/android/hardware/display/DisplayManager.java
    public VirtualDisplay createVirtualDisplay(@NonNull String name,
    		int width, int height, int densityDpi, @Nullable Surface surface, int flags) {
    }
    public VirtualDisplay createVirtualDisplay(@NonNull String name,
    		int width, int height, int densityDpi, @Nullable Surface surface, int flags,
    		@Nullable VirtualDisplay.Callback callback, @Nullable Handler handler) {
    }
    /** @hide */
    public VirtualDisplay createVirtualDisplay(@Nullable MediaProjection projection,
    		@NonNull String name, int width, int height, int densityDpi, @Nullable Surface surface,
    		int flags, @Nullable VirtualDisplay.Callback callback, @Nullable Handler handler,
    		@Nullable String uniqueId) {
    }
    /** @hide */
    public VirtualDisplay createVirtualDisplay(@Nullable MediaProjection projection,
    		@NonNull VirtualDisplayConfig virtualDisplayConfig,
    		@Nullable VirtualDisplay.Callback callback, @Nullable Handler handler) {
    }
    

    • 补充一点，MediaProjection中也提供了 createVirtualDisplay这个接口，实际上也是通过调用DisplayManager实现的功能。

      // frameworks/base/media/java/android/media/projection/MediaProjection.java
          public VirtualDisplay createVirtualDisplay(@NonNull VirtualDisplayConfig virtualDisplayConfig,
                  @Nullable VirtualDisplay.Callback callback, @Nullable Handler handler) {
              DisplayManager dm = mContext.getSystemService(DisplayManager.class);
              // 调用DisplayManager的接口
              return dm.createVirtualDisplay(this, virtualDisplayConfig, callback, handler);
          }
      

      • 创建VirtualDisplay时，需要传入Surface。**VirtualDisplay上要绘制的内容，实际是通过传入的Surface显示出来的。**比如在主屏（根据物理屏，分配逻辑Display）上创建了一个SurfaceView，通过把这个SurfaceView传给VirtualDisplay。那么VirtualDisplay的 内容，实际上是在主屏的SurfaceView上显示的。下面是一段Android原生的例子。

        // frameworks/base/packages/SystemUI/tests/src/com/android/systemui/navigationbar/NavigationBarButtonTest.java
        private Display createVirtualDisplay() {
        	final String displayName = "NavVirtualDisplay";
        	final DisplayInfo displayInfo = new DisplayInfo();
        	mContext.getDisplay().getDisplayInfo(displayInfo);
        	final DisplayManager displayManager = mContext.getSystemService(DisplayManager.class);
        	// 创建ImageReader，通过它得到一张Surface
        	mReader = ImageReader.newInstance(displayInfo.logicalWidth,
        			displayInfo.logicalHeight, PixelFormat.RGBA_8888, 2);
        	assertNotNull("ImageReader must not be null", mReader);
        	// 创建虚拟屏，传入Surface。
        	mVirtualDisplay = displayManager.createVirtualDisplay(displayName, displayInfo.logicalWidth,
        			displayInfo.logicalHeight, displayInfo.logicalDensityDpi, mReader.getSurface(),
        			0 /*flags*/);
        	assertNotNull("virtual display must not be null", mVirtualDisplay);
        	waitForDisplayReady(mVirtualDisplay.getDisplay().getDisplayId());
        	return mVirtualDisplay.getDisplay();
        }
        

        • 上面的例子中创建虚拟屏，返回Display（实际上是VirtualDislay）对象。有了Display对象，我们就可以将View绑定到这个虚拟的Display上了（绑定网上方法比较多可自行搜索）。关于Surface的创建，有很多种方法，比如通过SurfaceContron+Buffer这种方式也可以。
        • VituralDisplay创建时，需要提供flag。其值定义如下，可通过 “或”将flag组合。

           
              public static final int VIRTUAL_DISPLAY_FLAG_PUBLIC = 1 
                  /**
                   * Called when the virtual display video projection has been
                   * paused by the system or when the surface has been detached
                   * by the application by calling setSurface(null).
                   * The surface will not receive any more buffers while paused.
                   */
                   public void onPaused() { }
                  /**
                   * Called when the virtual display video projection has been
                   * resumed after having been paused.
                   */
                   public void onResumed() { }
                  /**
                   * Called when the virtual display video projection has been
                   * stopped by the system.  It will no longer receive frames
                   * and it will never be resumed.  It is still the responsibility
                   * of the application to release() the virtual display.
                   */
                  public void onStopped() { }
              }
          
          	return createVirtualDisplay(name, width, height, densityDpi, surface, flags, null, null);
          }
          public VirtualDisplay createVirtualDisplay(@NonNull String name,
          		int width, int height, int densityDpi, @Nullable Surface surface, int flags,
          		@Nullable VirtualDisplay.Callback callback, @Nullable Handler handler) {
          	final VirtualDisplayConfig.Builder builder = new VirtualDisplayConfig.Builder(name, width,
          			height, densityDpi);
          	builder.setFlags(flags);
          	if (surface != null) {
          		builder.setSurface(surface);
          	}
          	return createVirtualDisplay(null /* projection */, builder.build(), callback, handler);
          }
          // TODO : Remove this hidden API after remove all callers. (Refer to MultiDisplayService)
          /** @hide */
          public VirtualDisplay createVirtualDisplay(@Nullable MediaProjection projection,
          		@NonNull String name, int width, int height, int densityDpi, @Nullable Surface surface,
          		int flags, @Nullable VirtualDisplay.Callback callback, @Nullable Handler handler,
          		@Nullable String uniqueId) {
          	final VirtualDisplayConfig.Builder builder = new VirtualDisplayConfig.Builder(name, width,
          			height, densityDpi);
          	builder.setFlags(flags);
          	if (uniqueId != null) {
          		builder.setUniqueId(uniqueId);
          	}
          	if (surface != null) {
          		builder.setSurface(surface);
          	}
          	return createVirtualDisplay(projection, builder.build(), callback, handler);
          }
          /** @hide */
          public VirtualDisplay createVirtualDisplay(@Nullable MediaProjection projection,
          		@NonNull VirtualDisplayConfig virtualDisplayConfig,
          		@Nullable VirtualDisplay.Callback callback, @Nullable Handler handler) {
          	// 走的这里，会调用到DisplayManagerGlobal中。
          	return mGlobal.createVirtualDisplay(mContext, projection, virtualDisplayConfig, callback,
          			handler);
          }
          
          	VirtualDisplayCallback callbackWrapper = new VirtualDisplayCallback(callback, handler);
          	// 从MediaProjection过来的调用，这个地方非空。
          	IMediaProjection projectionToken = projection != null ? projection.getProjection() : null;
          	int displayId;
          	try {
          		// 告知DMS创建虚拟屏，并返回DisplayID
          		displayId = mDm.createVirtualDisplay(virtualDisplayConfig, callbackWrapper,
          				projectionToken, context.getPackageName());
          	} catch (RemoteException ex) {
          		throw ex.rethrowFromSystemServer();
          	}
          	if (displayId