Android的Surface的创建

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ViewRootImpl管理着整个view tree。对于ViewRootImpl.setView()，我们可以简单的把它当做一个UI渲染操作的入口。

http://androidxref.com/6.0.1_r10/xref/frameworks/base/core/java/android/view/WindowManagerImpl.java

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
...
//mWindowSession是一个aidl，ViewRootImpl利用它来和WindowManagerService交互
//mWindow是一个aidl，WindowManagerService可以利用这个对象与服务端交互
//mAttachInfo可以理解为是一个data bean，可以跨进程传递
res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
getHostVisibility(), mDisplay.getDisplayId(), mWinFrame,
mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
mAttachInfo.mOutsets, mAttachInfo.mDisplayCutout, mInputChannel);
...
}

ViewRootImpl.setView()方法会向WindowManagerService请求添加一个Window,mWindowSession.addToDisplay()跨进程最终调用到了WindowManagerService.addWindow():

http://androidxref.com/6.0.1_r10/xref/frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java

public int addWindow(Session session, IWindow client...) {
...
//WindowState用来描述一个Window
final WindowState win = new WindowState(this, session, client, token, parentWindow,
appOp[0], seq, attrs, viewVisibility, session.mUid,
session.mCanAddInternalSystemWindow);
...
win.attach(); //会创建一个SurfaceSession mWindowMap.put(client.asBinder(), win); //mWindowMap是WindowManagerService用来保存当前所有Window新的的集合
...
win.mToken.addWindow(win); //一个token下会有多个win state。其实token与PhoneWindow是一一对应的。
...
}

http://androidxref.com/6.0.1_r10/xref/frameworks/base/services/core/java/com/android/server/wm/WindowState.java

void attach() {
if (WindowManagerService.localLOGV) Slog.v(
TAG, "Attaching " + this + " token=" + mToken
+ ", list=" + mToken.windows);
mSession.windowAddedLocked();
}

WindowState是WindowManagerService用来描述应用程序的一个Window的对象。上面注释我标注了win.attach(),这个方法可以说是Window与SurfaceFlinger链接的起点,它最终会调用到Session.windowAddedLocked():

http://androidxref.com/6.0.1_r10/xref/frameworks/base/services/core/java/com/android/server/wm/Session.java

void windowAddedLocked(String packageName) {
...
if (mSurfaceSession == null) {
...
mSurfaceSession = new SurfaceSession();
...
}
}

http://androidxref.com/6.0.1_r10/xref/frameworks/base/core/java/android/view/SurfaceSession.java

//SurfaceSession类的构造方法
public final class SurfaceSession {
private long mNativeClient; // SurfaceComposerClient* public SurfaceSession() {
mNativeClient = nativeCreate();
}

这里调用了native方法nativeCreate()，这个方法其实是返回了一个SurfaceComposerClient指针。那这个对象是怎么创建的呢？

SurfaceComposerClient的创建

http://androidxref.com/6.0.1_r10/xref/frameworks/base/core/jni/android_view_SurfaceSession.cpp

static jlong nativeCreate(JNIEnv* env, jclass clazz) {
SurfaceComposerClient* client = new SurfaceComposerClient(); //构造函数其实并没有做什么
client->incStrong((void*)nativeCreate);
return reinterpret_cast<jlong>(client);
}

即构造了一个SurfaceComposerClient对象。并返回它的指针。这个对象一个应用程序就有一个，它是应用程序与SurfaceFlinger沟通的桥梁，为什么这么说呢？在SurfaceComposerClient指针第一次使用时会调用下面这个方法:

//这个方法在第一次使用SurfaceComposerClient的指针的时候会调用
void SurfaceComposerClient::onFirstRef() {
....
sp<ISurfaceComposerClient> conn;
//sf 就是SurfaceFlinger
conn = (rootProducer != nullptr) ? sf->createScopedConnection(rootProducer) :
sf->createConnection();
...
}

即通过SurfaceFlinger(它本身具有跨进程通信的能力)创建了一个ISurfaceComposerClient对象:

http://androidxref.com/6.0.1_r10/xref/frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp

sp<ISurfaceComposerClient> SurfaceFlinger::createConnection() {
return initClient(new Client(this)); //initClient这个方法其实并没有做什么，
}

即构造了一个Client对象，Client实现了ISurfaceComposerClient接口。是一个可以跨进程通信的aidl对象。即SurfaceComposerClient可以通过它来和SurfaceFlinger通信。除此之外它还可以创建Surface，并且维护一个应用程序的所有Layer(下文会分析到它是什么)。它是一个十分重要的对象，我们先来看一下它的组成,它所涉及的其他东西在下文分析中都会讲到:

http://androidxref.com/6.0.1_r10/xref/frameworks/native/services/surfaceflinger/Client.h

class Client : public BnSurfaceComposerClient
{
public:
...
void attachLayer(const sp<IBinder>& handle, const sp<Layer>& layer);
void detachLayer(const Layer* layer);
...
private:
// ISurfaceComposerClient interface。 gbp很重要，它维护这一个应用程序的渲染 Buffer队列
virtual status_t createSurface(...sp<IBinder>* handle, sp<IGraphicBufferProducer>* gbp); virtual status_t destroySurface(const sp<IBinder>& handle); //跨进程通信方法
virtual status_t onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags);
... // constant
sp<SurfaceFlinger> mFlinger; // protected by mLock
DefaultKeyedVector< wp<IBinder>, wp<Layer> > mLayers; // 一个应用程序的所有Layer
...
};

经过上面这一顿源码分析，我们大概知道了ViewRootImpl.setView()所引发的主要操作:

WindowManagerService创建了一个WindowState。用来表示客户端的一个Window
WindowManagerService创建了一个SurfaceSession,SurfaceSession会与SurfaceFlinger构建链接，创建了一个SurfaceComposerClient对象，一个应用程序只具有一个这个对象。
SurfaceComposerClient创建了一个Client, 这个对象十分重要，它维护这应用程序的渲染核心数据，并负责与SurfaceFlinger通信。

经过上面的步骤，应用程序的ViewRootImpl已经被WindowManagerService识别，并且应用程序已经与SurfaceFlinger建立连接。即创建了SurfaceComposerClient和Client对象

文章开始就已经说了Surface是Window(ViewRootImpl)的UI载体，那Surface是在哪里创建的呢？

Surface的创建

其实一个ViewRootImpl就对应一个Surface。

http://androidxref.com/6.0.1_r10/xref/frameworks/base/core/java/android/view/Surface.java

这点可以通过ViewRootImpl的源码看出:

即ViewRootImpl在构造的时候就new 了一个 Surface。但其实这个新new的Surface并没有什么逻辑，它的构造函数是空的。

http://androidxref.com/6.0.1_r10/xref/frameworks/base/core/java/android/view/ViewRootImpl.java

public final class ViewRootImpl implements ViewParent,
View.AttachInfo.Callbacks, HardwareRenderer.HardwareDrawCallbacks {
//...
public final Surface mSurface = new Surface();
//...
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
...
requestLayout(); //susion 请求layout。先添加到待渲染队列中
...
res = mWindowSession.addToDisplay(mWindow, ...); //WindowManagerService会创建mWindow对应的WindowState
...
}
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}}

即在向WindowManagerService请求创建WindowState之前，调用了requestLayout(),这个方法会引起ViewRootImpl所管理的整个view tree的重新渲染。它最终会调用到scheduleTraversals():

http://androidxref.com/6.0.1_r10/xref/frameworks/base/core/java/android/view/ViewRootImpl.java

void scheduleTraversals() {
...
mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
...
}

scheduleTraversals()会通过Choreographer来post一个mTraversalRunnable，Choreographer接收显示系统的时间脉冲(垂直同步信号-VSync信号，16ms发出一次)，在下一个frame渲染时控制执行这个mTraversalRunnable。

但是mTraversalRunnable的执行至少要在应用程序与SurfaceFlinger建立连接之后。这是因为渲染操作是由SurfaceFlinger负责调度了，如果应用程序还没有与SurfaceFlinger创建连接，那SurfaceFlinger当然不会渲染这个应用程序。所以在执行完mWindowSession.addToDisplay(mWindow, ...)之后，才会执行mTraversalRunnable:

final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}

doTraversal()会调用到ViewRootImpl.performTraversals(),大部分同学可能知道这个方法是一个view tree的measure/layout/draw的控制方法:

private void performTraversals() {
finalView host = mView; //mView是一个Window的根View，对于Activity来说就是DecorView
...
relayoutWindow(params, viewVisibility, insetsPending);
...
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
performLayout(lp, mWidth, mHeight);
...
performDraw();
...
}

Surface的具体创建就由relayoutWindow(params, viewVisibility, insetsPending)这个方法来完成的。这个方法会通过IPC调用到WindowManagerService.relayoutWindow():

http://androidxref.com/6.0.1_r10/xref/frameworks/base/core/java/android/view/ViewRootImpl.java

private int relayoutWindow(WindowManager.LayoutParams params, ...) throws RemoteException {
...
int relayoutResult = mWindowSession.relayout(mWindow,..., mSurface);
...
}

mWindowSession.relayout()方法的很多参数，不过有一个十分重要的参数我没有省略，就是mSurface。前面已经分析了它就是一个空的Surface对象。其实:

真正的Surface创建是由SurfaceControl完成的，应用程序ViewRootImpl的Surface只是一个指针，指向这个Surface

下面就来看一下SurfaceControl是如何创建Surface的:

mWindowSession.relayout()会调用到WindowManagerService.relayoutWindow():

http://androidxref.com/6.0.1_r10/xref/frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java

//这里的outSurface其实就是ViewRootImpl中的那个Surface
public int relayoutWindow(Session session, IWindow client....Surface outSurface){
...
result = createSurfaceControl(outSurface, result, win, winAnimator);
...
}private int createSurfaceControl(Surface outSurface, int result, WindowState win,WindowStateAnimator winAnimator) {
...
surfaceController = winAnimator.createSurfaceLocked(win.mAttrs.type, win.mOwnerUid);
...
surfaceController.getSurface(outSurface);
}

winAnimator.createSurfaceLocked实际上是创建了一个SurfaceControl。即上面是先构造SurfaceControl，然后在构造Surface。

http://androidxref.com/6.0.1_r10/xref/frameworks/base/services/core/java/com/android/server/wm/WindowStateAnimator.java

SurfaceControl createSurfaceLocked() {
final WindowState w = mWin;
if (mSurfaceControl == null) {
//...
mSurfaceFormat = format;
if (DEBUG_SURFACE_TRACE) {
mSurfaceControl = new SurfaceTrace(
mSession.mSurfaceSession,
attrs.getTitle().toString(),
width, height, format, flags);
} else {
mSurfaceControl = new SurfaceControl(
mSession.mSurfaceSession,
attrs.getTitle().toString(),
width, height, format, flags);
}
//...
}
return mSurfaceControl;
}

SurfaceControl的创建

winAnimator.createSurfaceLocked其实是通过SurfaceControl的构造函数创建了一个SurfaceControl对象,这个对象的作用其实就是负责维护Surface,Surface其实也是由这个对象负责创建的，我们看一下这个对象的构造方法:

http://androidxref.com/6.0.1_r10/xref/frameworks/base/core/java/android/view/SurfaceControl.java

long mNativeObject; //成员指针变量，指向native创建的SurfaceControlprivate SurfaceControl(SurfaceSession session, String name, int w, int h, int format, int flags,
SurfaceControl parent, int windowType, int ownerUid){
...
mNativeObject = nativeCreate(session, name, w, h, format, flags,
parent != null ? parent.mNativeObject : 0, windowType, ownerUid);
...
}

即调用了nativeCreate()并返回一个SurfaceControl指针:

http://androidxref.com/6.0.1_r10/xref/frameworks/native/libs/gui/SurfaceControl.cpp

static jlong nativeCreate(JNIEnv* env, ...) {
//这个client其实就是前面创建的SurfaceComposerClinent
sp<SurfaceComposerClient> client(android_view_SurfaceSession_getClient(env, sessionObj));
sp<SurfaceControl> surface; //创建成功之后，这个指针会指向新创建的SurfaceControl
status_t err = client->createSurfaceChecked(String8(name.c_str()), w, h, format, &surface, flags, parent, windowType, ownerUid);
...
return reinterpret_cast<jlong>(surface.get()); //返回这个SurfaceControl的地址
}

即调用到SurfaceComposerClient.createSurfaceChecked():

http://androidxref.com/6.0.1_r10/xref/frameworks/native/libs/gui/SurfaceComposerClient.cpp

//outSurface会指向新创建的SurfaceControl
status_t SurfaceComposerClient::createSurfaceChecked(...sp<SurfaceControl>* outSurface..)
{
sp<IGraphicBufferProducer> gbp; //这个对象很重要
...
err = mClient->createSurface(name, w, h, format, flags, parentHandle, windowType, ownerUid, &handle, &gbp);
if (err == NO_ERROR) {
//SurfaceControl创建成功, 指针赋值
*outSurface = new SurfaceControl(this, handle, gbp, true);
}
return err;
}

上面这个方法实际上是调用Client.createSurface()来创建一个Surface。在创建时有一个很重要的参数sp<IGraphicBufferProducer> gbp,在下面源码分析中我们也要重点注意它。这是因为应用所渲染的每一帧，实际上都会添加到IGraphicBufferProducer中，来等待SurfaceFlinger的渲染。

http://androidxref.com/6.0.1_r10/xref/frameworks/native/services/surfaceflinger/Client.cpp

status_t Client::createSurface(...)
{
...
//gbp 直接透传到了SurfaceFlinger
return mFlinger->createLayer(name, this, w, h, format, flags, windowType, ownerUid, handle, gbp, &parent);
}

Surface在SurfaceFlinger中对应的实体其实是Layer

我们继续看一下mFlinger->createLayer()

http://androidxref.com/6.0.1_r10/xref/frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp

status_t SurfaceFlinger::createLayer(const String8& name,const sp<Client>& client...)
{
status_t result = NO_ERROR;
sp<Layer> layer; //将要创建的layer
switch (flags & ISurfaceComposerClient::eFXSurfaceMask) {
case ISurfaceComposerClient::eFXSurfaceNormal:
result = createBufferLayer(client,
uniqueName, w, h, flags, format,
handle, gbp, &layer); // 注意gbp，这时候还没有构造呢！
break;
... //Layer 分为好几种，这里不全部列出
}
...
result = addClientLayer(client, *handle, *gbp, layer, *parent); //这个layer和client相关联, 添加到Client的mLayers集合中。
...
return result;
}

从SurfaceFlinger.createLayer()方法可以看出Layer分为好几种。我们这里只对普通的BufferLayer的创建做一下分析，看createBufferLayer():

http://androidxref.com/6.0.1_r10/xref/frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp

status_t SurfaceFlinger::createBufferLayer(const sp<Client>& client... sp<Layer>* outLayer)
{
...
sp<BufferLayer> layer = new BufferLayer(this, client, name, w, h, flags);
status_t err = layer->setBuffers(w, h, format, flags); //设置layer的宽高
if (err == NO_ERROR) {
*handle = layer->getHandle(); //创建handle
*gbp = layer->getProducer(); //创建 gbp IGraphicBufferProducer
*outLayer = layer; //把新建的layer的指针拷贝给outLayer,这样outLayer就指向了新建的BufferLayer
}
return err;
}

前面我说过IGraphicBufferProducer(gbp)是一个很重要的对象，它涉及到SurfaceFlinger的渲染逻辑，下面我们就看一下这个对象的创建逻辑:

IGraphicBufferProducer(gbp)的创建

sp<IGraphicBufferProducer> BufferLayer::getProducer() const {
return mProducer;
}

即mProducer其实是Layer的成员变量，它的创建时机是Layer第一次被使用时:

void BufferLayer::onFirstRef() {
...
BufferQueue::createBufferQueue(&producer, &consumer, true);
mProducer = new MonitoredProducer(producer, mFlinger, this);
...
}

所以mProducer的实例是MonitoredProducer,但其实它只是一个装饰类，它实际功能都委托给构造它的参数producer:

BufferQueue.cpp

void BufferQueue::createBufferQueue(sp<IGraphicBufferProducer>* outProducer,
...
sp<IGraphicBufferProducer> producer(new BufferQueueProducer(core, consumerIsSurfaceFlinger));
sp<IGraphicBufferConsumer> consumer(new BufferQueueConsumer(core)); //注意这个consumer
...
*outProducer = producer;
*outConsumer = consumer;
}

所以实际实现mProducer的工作的queueProducer是BufferQueueProducer。

所以构造一个SurfaceControl所做的工作就是创建了一个SurfaceControl,并让SurfaceFlinger创建了一个对应的Layer，Layer中有一个IGraphicBufferProducer,它的实例是BufferQueueProducer。

可以用下面这个图来描述SurfaceControl的创建过程:

从SurfaceControl中获取Surface

我们回看WindowManagerService.createSurfaceControl()，来看一下java层的Surface对象到底是个什么:

http://androidxref.com/6.0.1_r10/xref/frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java

private int createSurfaceControl(Surface outSurface, int result, WindowState win,WindowStateAnimator winAnimator) {
...
surfaceController = winAnimator.createSurfaceLocked(win.mAttrs.type, win.mOwnerUid);
...
surfaceController.getSurface(outSurface);
}

上面我们已经了解了winAnimator.createSurfaceLocked的整个过程，我们看一下surfaceController.getSurface(outSurface), surfaceController是WindowSurfaceController的实例:

//WindowSurfaceController.java
void getSurface(Surface outSurface) {
outSurface.copyFrom(mSurfaceControl);
}//Surface.java
public void copyFrom(SurfaceControl other) {
...
long surfaceControlPtr = other.mNativeObject;
...
long newNativeObject = nativeGetFromSurfaceControl(surfaceControlPtr);
...
mNativeObject = ptr; // mNativeObject指向native创建的Surface
}

即Surface.copyFrom()方法调用nativeGetFromSurfaceControl()来获取一个指针，这个指针是根据前面创建的SurfaceControl的指针来寻找的，即传入的参数surfaceControlPtr:

android_view_Surface.cpp

static jlong nativeGetFromSurfaceControl(JNIEnv* env, jclass clazz, jlong surfaceControlNativeObj) {
sp<SurfaceControl> ctrl(reinterpret_cast<SurfaceControl *>(surfaceControlNativeObj)); //把java指针转化内native指针
sp<Surface> surface(ctrl->getSurface()); //直接构造一个Surface，指向 ctrl->getSurface()
if (surface != NULL) {
surface->incStrong(&sRefBaseOwner); //强引用
}
return reinterpret_cast<jlong>(surface.get());
}

这里的ctrl指向前面创建的SurfaceControl，继续追溯ctrl->getSurface():

sp<Surface> SurfaceControl::getSurface() const
{
Mutex::Autolock _l(mLock);
if (mSurfaceData == 0) {
return generateSurfaceLocked();
}
return mSurfaceData;
}sp<Surface> SurfaceControl::generateSurfaceLocked() const
{
//这个mGraphicBufferProducer其实就是上面分析的BufferQueueProducer
mSurfaceData = new Surface(mGraphicBufferProducer, false);
return mSurfaceData;
}

即直接new了一个nativie的Surface返回给java层，java层的Surface指向的就是native层的Surface。

所以Surface的实际创建可以用下图表示: