【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.javapublic 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SurfacemReader = 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 << 0;public static final int VIRTUAL_DISPLAY_FLAG_PRESENTATION = 1 << 1;public static final int VIRTUAL_DISPLAY_FLAG_SECURE = 1 << 2;public static final int VIRTUAL_DISPLAY_FLAG_OWN_CONTENT_ONLY = 1 << 3;public static final int VIRTUAL_DISPLAY_FLAG_AUTO_MIRROR = 1 << 4;public static final int VIRTUAL_DISPLAY_FLAG_CAN_SHOW_WITH_INSECURE_KEYGUARD = 1 << 5;public static final int VIRTUAL_DISPLAY_FLAG_SUPPORTS_TOUCH = 1 << 6;public static final int VIRTUAL_DISPLAY_FLAG_ROTATES_WITH_CONTENT = 1 << 7;public static final int VIRTUAL_DISPLAY_FLAG_DESTROY_CONTENT_ON_REMOVAL = 1 << 8;public static final int VIRTUAL_DISPLAY_FLAG_SHOULD_SHOW_SYSTEM_DECORATIONS = 1 << 9;public static final int VIRTUAL_DISPLAY_FLAG_TRUSTED = 1 << 10;public static final int VIRTUAL_DISPLAY_FLAG_OWN_DISPLAY_GROUP = 1 << 11;

• DisplayManager公开的接口中，有VirtualDisplay.Callback ，提供了其状态的回调。

    public static abstract class Callback {/*** 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() { }}

VirtualDisplay原理

• 关于VirtualDisplay的实现原理，主要从AndroidFramework角度进行分析。
  

// /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) {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);
}

• DisplayManagerGlobal调用DMS（DisplayManagerService）服务创建虚拟屏，得到DMS返回的DisplayID后，通过DisplayID在Client端创建了VirtualDisplay对象。

// /frameworks/base/core/java/android/hardware/display/DisplayManager.java
public VirtualDisplay createVirtualDisplay(@NonNull Context context, MediaProjection projection,@NonNull VirtualDisplayConfig virtualDisplayConfig, VirtualDisplay.Callback callback,Handler 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DisplayIDdisplayId = mDm.createVirtualDisplay(virtualDisplayConfig, callbackWrapper,projectionToken, context.getPackageName());} catch (RemoteException ex) {throw ex.rethrowFromSystemServer();}if (displayId < 0) {Log.e(TAG, "Could not create virtual display: " + virtualDisplayConfig.getName());return null;}// 通过DisplayID，取得Display对象信息（也是调用DMS得到的）Display display = getRealDisplay(displayId);if (display == null) {Log.wtf(TAG, "Could not obtain display info for newly created "+ "virtual display: " + virtualDisplayConfig.getName());try {// 创建失败，需要释放mDm.releaseVirtualDisplay(callbackWrapper);} catch (RemoteException ex) {throw ex.rethrowFromSystemServer();}return null;}// 创建VirtualDisplayreturn new VirtualDisplay(this, display, callbackWrapper,virtualDisplayConfig.getSurface());
}

• DisplayManagerService(DMS)中创建DisplayDevice并添加到Device列表中管理

// /frameworks/base/services/core/java/com/android/server/display/DisplayManagerService.java
@Override // Binder call
public int createVirtualDisplay(VirtualDisplayConfig virtualDisplayConfig,IVirtualDisplayCallback callback, IMediaProjection projection, String packageName) {// 检查uid与包名，是否相符。final int callingUid = Binder.getCallingUid();if (!validatePackageName(callingUid, packageName)) {throw new SecurityException("packageName must match the calling uid");}if (callback == null) {throw new IllegalArgumentException("appToken must not be null");}if (virtualDisplayConfig == null) {throw new IllegalArgumentException("virtualDisplayConfig must not be null");}//  拿到client端传过来的surface对象final Surface surface = virtualDisplayConfig.getSurface();int flags = virtualDisplayConfig.getFlags();if (surface != null && surface.isSingleBuffered()) {throw new IllegalArgumentException("Surface can't be single-buffered");}// 下面开始针对Flag，做一些逻辑判断。if ((flags & VIRTUAL_DISPLAY_FLAG_PUBLIC) != 0) {flags |= VIRTUAL_DISPLAY_FLAG_AUTO_MIRROR;// Public displays can't be allowed to show content when locked.if ((flags & VIRTUAL_DISPLAY_FLAG_CAN_SHOW_WITH_INSECURE_KEYGUARD) != 0) {throw new IllegalArgumentException("Public display must not be marked as SHOW_WHEN_LOCKED_INSECURE");}}if ((flags & VIRTUAL_DISPLAY_FLAG_OWN_CONTENT_ONLY) != 0) {flags &= ~VIRTUAL_DISPLAY_FLAG_AUTO_MIRROR;}if ((flags & VIRTUAL_DISPLAY_FLAG_AUTO_MIRROR) != 0) {flags &= ~VIRTUAL_DISPLAY_FLAG_OWN_DISPLAY_GROUP;}if (projection != null) {try {if (!getProjectionService().isValidMediaProjection(projection)) {throw new SecurityException("Invalid media projection");}flags = projection.applyVirtualDisplayFlags(flags);} catch (RemoteException e) {throw new SecurityException("unable to validate media projection or flags");}}if (callingUid != Process.SYSTEM_UID &&(flags & VIRTUAL_DISPLAY_FLAG_AUTO_MIRROR) != 0) {if (!canProjectVideo(projection)) {throw new SecurityException("Requires CAPTURE_VIDEO_OUTPUT or "+ "CAPTURE_SECURE_VIDEO_OUTPUT permission, or an appropriate "+ "MediaProjection token in order to create a screen sharing virtual "+ "display.");}}if (callingUid != Process.SYSTEM_UID && (flags & VIRTUAL_DISPLAY_FLAG_SECURE) != 0) {if (!canProjectSecureVideo(projection)) {throw new SecurityException("Requires CAPTURE_SECURE_VIDEO_OUTPUT "+ "or an appropriate MediaProjection token to create a "+ "secure virtual display.");}}if (callingUid != Process.SYSTEM_UID && (flags & VIRTUAL_DISPLAY_FLAG_TRUSTED) != 0) {if (!checkCallingPermission(ADD_TRUSTED_DISPLAY, "createVirtualDisplay()")) {EventLog.writeEvent(0x534e4554, "162627132", callingUid,"Attempt to create a trusted display without holding permission!");throw new SecurityException("Requires ADD_TRUSTED_DISPLAY permission to "+ "create a trusted virtual display.");}}if (callingUid != Process.SYSTEM_UID&& (flags & VIRTUAL_DISPLAY_FLAG_OWN_DISPLAY_GROUP) != 0) {if (!checkCallingPermission(ADD_TRUSTED_DISPLAY, "createVirtualDisplay()")) {throw new SecurityException("Requires ADD_TRUSTED_DISPLAY permission to "+ "create a virtual display which is not in the default DisplayGroup.");}}if ((flags & VIRTUAL_DISPLAY_FLAG_TRUSTED) == 0) {flags &= ~VIRTUAL_DISPLAY_FLAG_SHOULD_SHOW_SYSTEM_DECORATIONS;}// Sometimes users can have sensitive information in system decoration windows. An app// could create a virtual display with system decorations support and read the user info// from the surface.// We should only allow adding flag VIRTUAL_DISPLAY_FLAG_SHOULD_SHOW_SYSTEM_DECORATIONS// to trusted virtual displays.final int trustedDisplayWithSysDecorFlag =(VIRTUAL_DISPLAY_FLAG_SHOULD_SHOW_SYSTEM_DECORATIONS| VIRTUAL_DISPLAY_FLAG_TRUSTED);if ((flags & trustedDisplayWithSysDecorFlag)== VIRTUAL_DISPLAY_FLAG_SHOULD_SHOW_SYSTEM_DECORATIONS&& !checkCallingPermission(INTERNAL_SYSTEM_WINDOW, "createVirtualDisplay()")) {throw new SecurityException("Requires INTERNAL_SYSTEM_WINDOW permission");}final long token = Binder.clearCallingIdentity();try {// 调用内部实现return createVirtualDisplayInternal(callback, projection, callingUid, packageName,surface, flags, virtualDisplayConfig);} finally {Binder.restoreCallingIdentity(token);}
}// /frameworks/base/services/core/java/com/android/server/display/DisplayManagerService.java
private int createVirtualDisplayInternal(IVirtualDisplayCallback callback,IMediaProjection projection, int callingUid, String packageName, Surface surface,int flags, VirtualDisplayConfig virtualDisplayConfig) {synchronized (mSyncRoot) {if (mVirtualDisplayAdapter == null) {Slog.w(TAG, "Rejecting request to create private virtual display "+ "because the virtual display adapter is not available.");return -1;}// 为虚拟屏创建Device（告知surfaceflinger创建Display）DisplayDevice device = mVirtualDisplayAdapter.createVirtualDisplayLocked(callback, projection, callingUid, packageName, surface, flags,virtualDisplayConfig);if (device == null) {return -1;}// 发送添加Device通知，这里比较重要mDisplayDeviceRepo.onDisplayDeviceEvent(device,DisplayAdapter.DISPLAY_DEVICE_EVENT_ADDED);// 检查Display是否创建成功final LogicalDisplay display = mLogicalDisplayMapper.getDisplayLocked(device);if (display != null) {return display.getDisplayIdLocked();}// Something weird happened and the logical display was not created.Slog.w(TAG, "Rejecting request to create virtual display "+ "because the logical display was not created.");mVirtualDisplayAdapter.releaseVirtualDisplayLocked(callback.asBinder());mDisplayDeviceRepo.onDisplayDeviceEvent(device,DisplayAdapter.DISPLAY_DEVICE_EVENT_REMOVED);}return -1;
}

• 接下来DMS开始调用SurfaceFlinger的接口，创建Display。并将Display放入自身的List中管理。

// /frameworks/base/services/core/java/com/android/server/display/VirtualDisplayAdapter.java
public DisplayDevice createVirtualDisplayLocked(IVirtualDisplayCallback callback,IMediaProjection projection, int ownerUid, String ownerPackageName, Surface surface,int flags, VirtualDisplayConfig virtualDisplayConfig) {String name = virtualDisplayConfig.getName();// VIRTUAL_DISPLAY_FLAG_SECURE 的用途，是判断是否为安全的Display，这个参数会告知SurfaceFlingerboolean secure = (flags & VIRTUAL_DISPLAY_FLAG_SECURE) != 0;IBinder appToken = callback.asBinder();// 调用SurfaceFligner创建Display（Display的type是virtual）IBinder displayToken = mSurfaceControlDisplayFactory.createDisplay(name, secure);final String baseUniqueId =UNIQUE_ID_PREFIX + ownerPackageName + "," + ownerUid + "," + name + ",";final int uniqueIndex = getNextUniqueIndex(baseUniqueId);String uniqueId = virtualDisplayConfig.getUniqueId();if (uniqueId == null) {uniqueId = baseUniqueId + uniqueIndex;} else {uniqueId = UNIQUE_ID_PREFIX + ownerPackageName + ":" + uniqueId;}// 通过SurfaceFligner返回的displayToken，创建Device对象VirtualDisplayDevice device = new VirtualDisplayDevice(displayToken, appToken,ownerUid, ownerPackageName, surface, flags, new Callback(callback, mHandler),uniqueId, uniqueIndex, virtualDisplayConfig);//  放到虚拟屏的List中管理。mVirtualDisplayDevices.put(appToken, device);try {if (projection != null) {projection.registerCallback(new MediaProjectionCallback(appToken));}appToken.linkToDeath(device, 0);} catch (RemoteException ex) {mVirtualDisplayDevices.remove(appToken);device.destroyLocked(false);return null;}// Return the display device without actually sending the event indicating// that it was added.  The caller will handle it.return device;
}// /frameworks/base/services/core/java/com/android/server/display/DisplayDeviceRepository.java
//  mDisplayDeviceRepo.onDisplayDeviceEvent(device,
//				DisplayAdapter.DISPLAY_DEVICE_EVENT_ADDED); 
// 这段代码，会调用到下面的函数中。
private void handleDisplayDeviceAdded(DisplayDevice device) {synchronized (mSyncRoot) {DisplayDeviceInfo info = device.getDisplayDeviceInfoLocked();if (mDisplayDevices.contains(device)) {Slog.w(TAG, "Attempted to add already added display device: " + info);return;}Slog.i(TAG, "Display device added: " + info);device.mDebugLastLoggedDeviceInfo = info;// 需要是将Device（就是上面创建的虚拟屏幕Device）放入到DMS的管理listmDisplayDevices.add(device);// 通知Device添加，会调用到LogicalDisplayMappe的handleDisplayDeviceAddedLocked中。sendEventLocked(device, DISPLAY_DEVICE_EVENT_ADDED);}
}
// /frameworks/base/services/core/java/com/android/server/display/LogicalDisplayMapper.java
private void handleDisplayDeviceAddedLocked(DisplayDevice device) {DisplayDeviceInfo deviceInfo = device.getDisplayDeviceInfoLocked();// Internal Displays need to have additional initialization.// This initializes a default dynamic display layout for INTERNAL// devices, which is used as a fallback in case no static layout definitions// exist or cannot be loaded.if (deviceInfo.type == Display.TYPE_INTERNAL) {initializeInternalDisplayDeviceLocked(device);}// Create a logical display for the new display deviceLogicalDisplay display = createNewLogicalDisplayLocked(device, Layout.assignDisplayIdLocked(false /*isDefault*/));// 刷新布局和display配置applyLayoutLocked();updateLogicalDisplaysLocked();
}

• 虚拟屏幕的创建，Client端通过Surface告知的DisplayID，创建VirtualDisplay对象。通过DisplayID，与DMS打交道。DMS服务端，通过SurfaceFlinger创建虚拟屏，拿到SurfaceFligner的DisplayToken，然后通过它创建VirtualDisplayDevice + LogicalDisplay来管理虚拟屏幕。
  

如何上屏？

• 创建虚拟屏幕的时候，会传入了一张Surface（比如绑定主屏的一张Buffer）。虚拟屏通过这张Surface拿到Surface对应的Buffer，将上屏内容绘制到这个Buffer上，然后提交到画面流水线上（SurfaceFlinger）。通过SurfaceFlinger将这个这个Buffer最终由SurfaceFlinger描画并显示到Surface所在那张Display上（根据VirtualDisplay位置去显示。）