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深入浅出Android开发之Surface介绍

 2013/12/16 16:09:11  xiaochao1234  博客园  我要评论(0)
  • 摘要:一目的本节的目的就是为了讲清楚Android中的Surface系统,大家耳熟能详的SurfaceFlinger到底是个什么东西,它的工作流程又是怎样的。当然,鉴于SurfaceFlinger的复杂性,我们依然将采用情景分析的办法,找到合适的切入点。一个Activity是怎么在屏幕上显示出来的呢?我将首先把这个说清楚。接着我们把其中的关键调用抽象在Native层,以这些函数调用为切入点来研究SurfaceFlinger。好了,开始我们的征途吧。二Activity是如何显示的最初的想法就是
  • 标签:android 开发 Android开发

一 目的

本节的目的就是为了讲清楚Android中的Surface系统,大家耳熟能详的SurfaceFlinger到底是个什么东西,它的工作流程又是怎样的。当然,鉴于SurfaceFlinger的复杂性,我们依然将采用情景分析的办法,找到合适的切入点

一个Activity是怎么在屏幕上显示出来的呢?我将首先把这个说清楚。

接着我们把其中的关键调用抽象在Native层,以这些函数调用为切入点来研究SurfaceFlinger。好了,开始我们的征途吧。

 Activity是如何显示的

最初的想法就是,Activity获得一块显存,然后在上面绘图,最后交给设备去显示。这个道理是没错,但是AndroidSurfaceFlinger是在System Server进程中创建的,Activity一般另有线程,这之间是如何...如何挂上关系的呢?我可以先提前告诉大家,这个过程还比较复杂。呵呵。

好吧,我们从Activity最初的启动开始。代码在

framework/base/core/java/android/app/ActivityThread.java中,这里有个函数叫handleLaunchActivity

[---->ActivityThread:: handleLaunchActivity()]

private final void handleLaunchActivity(ActivityRecord r, Intent customIntent) {

      Activity a = performLaunchActivity(r, customIntent);

 

        if (a != null) {

            r.createdConfig = new Configuration(mConfiguration);

            Bundle oldState = r.state;

            handleResumeActivity(r.token, false, r.isForward);

---->调用handleResumeActivity

}

handleLaunchActivity中会调用handleResumeActivity

[--->ActivityThread:: handleResumeActivity]

final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward) {

         boolean willBeVisible = !a.mStartedActivity;

          

if (r.window == null && !a.mFinished && willBeVisible) {

                r.window = r.activity.getWindow();

                View decor = r.window.getDecorView();

                decor.setVisibility(View.INVISIBLE);

                ViewManager wm = a.getWindowManager();

                WindowManager.LayoutParams l = r.window.getAttributes();

                a.mDecor = decor;

                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;

                if (a.mVisibleFromClient) {

                    a.mWindowAdded = true;

                    wm.addView(decor, l); //这个很关键。

                }

上面addView那几行非常关键,它关系到咱们在ActivitysetContentView后,整个Window到底都包含了些什么。我先告诉大家。所有你创建的View之上,还有一个DecorView,这是一个FrameLayout,另外还有一个PhoneWindow。上面这些东西的代码在

framework/Policies/Base/Phone/com/android/Internal/policy/impl。这些隐藏的View的创建都是由你在AcitivtyonCreate中调用setContentView导致的。

[---->PhoneWindow:: addContentView]

   public void addContentView(View view, ViewGroup.LayoutParams params) {

        if (mContentParent == null) {  //刚创建的时候mContentParent为空

            installDecor();

        }

        mContentParent.addView(view, params);

        final Callback cb = getCallback();

        if (cb != null) {

            cb.onContentChanged();

        }

}

installDecor将创建mDecor和mContentParent。mDecor是DecorView类型,

mContentParent是ViewGroup类型

private void installDecor() {

        if (mDecor == null) {

            mDecor = generateDecor();

            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);

            mDecor.setIsRootNamespace(true);

        }

        if (mContentParent == null) {

            mContentParent = generateLayout(mDecor);

那么,ViewManager wm = a.getWindowManager()又返回什么呢?

PhoneWindowWindow中派生,Acitivity创建的时候会调用它的setWindowManager。而这个函数由Window类实现。

代码在framework/base/core/java/android/view/Window.java

public void setWindowManager(WindowManager wm,IBinder appToken, String appName) {

        mAppToken = appToken;

        mAppName = appName;

        if (wm == null) {

            wm = WindowManagerImpl.getDefault();

        }

        mWindowManager = new LocalWindowManager(wm);

    }

你看见没,分析JAVA代码这个东西真的很复杂。mWindowManager的实现是LocalWindowManager,但由通过Bridge模式把功能交给WindowManagerImpl去实现了。

真的很复杂!

好了,罗里罗嗦的,我们回到wm.addView(decor, l)。最终会由WindowManagerImpl来完成

addView操作,我们直接看它的实现好了。

代码在framework/base/core/java/android/view/WindowManagerImpl.java

[---->addView]

private void addView(View view, ViewGroup.LayoutParams params, boolean nest)

    {

          ViewRoot root; //ViewRoot,我们的主人公终于登场!

         synchronized (this) {

         root = new ViewRoot(view.getContext());

          root.mAddNesting = 1;

          view.setLayoutParams(wparams);

           

            if (mViews == null) {

                index = 1;

                mViews = new View[1];

                mRoots = new ViewRoot[1];

                mParams = new WindowManager.LayoutParams[1];

            } else {

           }

            index--;

            mViews[index] = view;

            mRoots[index] = root;

            mParams[index] = wparams;

        }

       root.setView(view, wparams, panelParentView);

}

ViewRoot是整个显示系统中最为关键的东西,看起来这个东西好像和View有那么点关系,其实它根本和ViewUI关系不大,它不过是一个Handler罢了,唯一有关系的就是它其中有一个变量为Surface类型。我们看看它的定义。ViewRoot代码在

framework/base/core/java/android/view/ViewRoot.java

public final class ViewRoot extends Handler implements ViewParent,

        View.AttachInfo.Callbacks

{

private final Surface mSurface = new Surface();

}

它竟然从handler派生,而ViewParent不过定义了一些接口函数罢了。

看到Surface直觉上感到它和SurfaceFlinger有点关系。要不先去看看?

Surface代码在framework/base/core/java/android/view/Surface.java中,我们调用的是无参构造函数

public Surface() {

       mCanvas = new CompatibleCanvas(); //就是创建一个Canvas!

}

如果你有兴趣的话,看看Surface其他构造函数,最终都会调用native的实现,而这些native的实现将和SurfaceFlinger建立关系,但我们这里ViewRoot中的mSurface显然还没有到这一步。那它到底是怎么和SurfaceFlinger搞上的呢?这一切待会就会水落石出的。

另外,为什么ViewRoot是主人公呢?因为ViewRoot建立了客户端和SystemServer的关系。我们看看它的构造函数。

public ViewRoot(Context context) {

        super();

       ....

       getWindowSession(context.getMainLooper());

}

getWindowsession将建立和WindowManagerService的关系。

ublic static IWindowSession getWindowSession(Looper mainLooper) {

        synchronized (mStaticInit) {

            if (!mInitialized) {

                try {

                //sWindowSession是通过Binder机制创建的。终于让我们看到点希望了

                    InputMethodManager imm = InputMethodManager.getInstance(mainLooper);

                    sWindowSession = IWindowManager.Stub.asInterface(

                            ServiceManager.getService("window"))

                            .openSession(imm.getClient(), imm.getInputContext());

                    mInitialized = true;

                } catch (RemoteException e) {

                }

            }

            return sWindowSession;

        }

    }

上面跨Binder的进程调用另一端是WindowManagerService,代码在

framework/base/services/java/com/android/server/WindowManagerService.java中。我们先不说这个。

回过头来看看ViewRoot接下来的调用。

[-->ViewRoot::setView()],这个函数很复杂,我们看其中关键几句。

public void setView(View view, WindowManager.LayoutParams attrs,

            View panelParentView) {

        synchronized (this) {

            requestLayout();

                try {

                    res = sWindowSession.add(mWindow, mWindowAttributes,

                            getHostVisibility(), mAttachInfo.mContentInsets);

                }

}

requestLayout实现很简单,就是往handler中发送了一个消息。

public void requestLayout() {

        checkThread();

        mLayoutRequested = true;

        scheduleTraversals(); //发送DO_TRAVERSAL消息

}

public void scheduleTraversals() {

        if (!mTraversalScheduled) {

            mTraversalScheduled = true;

            sendEmptyMessage(DO_TRAVERSAL);

        }

}

我们看看跨进程的那个调用。sWindowSession.add。它的最终实现在WindowManagerService中。

[--->WindowSession::add()]

public int add(IWindow window, WindowManager.LayoutParams attrs,

                int viewVisibility, Rect outContentInsets) {

            return addWindow(this, window, attrs, viewVisibility, outContentInsets);

        }

WindowSession是个内部类,会调用外部类的addWindow

这个函数巨复杂无比,但是我们的核心目标是找到创建显示相关的部分。所以,最后精简的话就简单了。

[--->WindowManagerService:: addWindow]

public int addWindow(Session session, IWindow client,

            WindowManager.LayoutParams attrs, int viewVisibility,

            Rect outContentInsets) {

        //创建一个WindowState,这个又是什么玩意儿呢?

              win = new WindowState(session, client, token,

                    attachedWindow, attrs, viewVisibility);

           win.attach();

           return res;

}

WindowState类中有一个和Surface相关的成员变量,叫SurfaceSession。它会在

attach函数中被创建。SurfaceSession嘛,就和SurfaceFlinger有关系了。我们待会看。

好,我们知道ViewRoot创建及调用add后,我们客户端的View系统就和WindowManagerService建立了牢不可破的关系。

另外,我们知道ViewRoot是一个handler,而且刚才我们调用了requestLayout,所以接下来消息循环下一个将调用的就是ViewRoothandleMessage

public void handleMessage(Message msg) {

        switch (msg.what) {

       case DO_TRAVERSAL:

            performTraversals();

performTraversals更加复杂无比,经过我仔细挑选,目标锁定为下面几个函数。当然,后面我们还会回到performTraversals,不过我们现在更感兴趣的是Surface是如何创建的。

private void performTraversals() {

        // cache mView since it is used so much below...

        final View host = mView;

 

         boolean initialized = false;

            boolean contentInsetsChanged = false;

            boolean visibleInsetsChanged;

            try {

//ViewRoot也有一个Surface成员变量,叫mSurface,这个就是代表SurfaceFlinger的客户端

//ViewRoot在这个Surface上作画,最后将由SurfaceFlinger来合成显示。刚才说了mSurface还没有什么内容。

          relayoutResult = relayoutWindow(params, viewVisibility, insetsPending);

[---->ViewRoot:: relayoutWindow()]

private int relayoutWindow(WindowManager.LayoutParams params, int viewVisibility,

            boolean insetsPending) throws RemoteException {

      

//relayOut是跨进程调用,mSurface做为参数传进去了,看来离真相越来越近了呀!

        int relayoutResult = sWindowSession.relayout(

                mWindow, params,

                (int) (mView.mMeasuredWidth * appScale + 0.5f),

                (int) (mView.mMeasuredHeight * appScale + 0.5f),

                viewVisibility, insetsPending, mWinFrame,

                mPendingContentInsets, mPendingVisibleInsets,

                mPendingConfiguration, mSurface); mSurface做为参数传进去了。

       }

我们赶紧转到WindowManagerService去看看吧。、

public int relayoutWindow(Session session, IWindow client,

            WindowManager.LayoutParams attrs, int requestedWidth,

            int requestedHeight, int viewVisibility, boolean insetsPending,

            Rect outFrame, Rect outContentInsets, Rect outVisibleInsets,

            Configuration outConfig, Surface outSurface){

               .....

         try {

           //看到这里,我内心一阵狂喜,有戏,太有戏了!

         //其中win是我们最初创建的WindowState!

                    Surface surface = win.createSurfaceLocked();

                    if (surface != null) {

                  //先创建一个本地surface,然后把传入的参数outSurface copyFrom一下

                        outSurface.copyFrom(surface);

                        win.mReportDestroySurface = false;

                        win.mSurfacePendingDestroy = false;

                       } else {

                       outSurface.release();

                    }

                }

}

[--->WindowState::createSurfaceLocked]

Surface createSurfaceLocked() {

         

                try {

                    mSurface = new Surface(

                            mSession.mSurfaceSession, mSession.mPid,

                            mAttrs.getTitle().toString(),

                            0, w, h, mAttrs.format, flags);

                  }

                Surface.openTransaction();

这里使用了Surface的另外一个构造函数。

    public Surface(SurfaceSession s,

            int pid, String name, int display, int w, int h, int format, int flags)

        throws OutOfResourcesException {

        mCanvas = new CompatibleCanvas();

        init(s,pid,name,display,w,h,format,flags); ---->调用了native的init函数。

        mName = name;

}

到这里,不进入JNI是不可能说清楚了。不过我们要先回顾下之前的关键步骤。

l         add中,new了一个SurfaceSession

l         创建new了一个Surface

l         调用copyFrom,把本地Surface信息传到outSurface

JNI

上面两个类的JNI实现都在framework/base/core/jni/android_view_Surface.cpp中。

 [---->SurfaceSession:: SurfaceSession()]

public class SurfaceSession {

    /** Create a new connection with the surface flinger. */

    public SurfaceSession() {

        init();

}

它的init函数对应为:

[--->SurfaceSession_init]

static void SurfaceSession_init(JNIEnv* env, jobject clazz)

{

   //SurfaceSession对应为SurfaceComposerClient

    sp<SurfaceComposerClient> client = new SurfaceComposerClient;

client->incStrong(clazz);

//Google常用做法,在JAVA对象中保存C++对象的指针。

    env->SetIntField(clazz, sso.client, (int)client.get());

}

Surfaceinit对应为:

[--->Surface_init]

static void Surface_init(

        JNIEnv* env, jobject clazz,

        jobject session,

        jint pid, jstring jname, jint dpy, jint w, jint h, jint format, jint flags)

{

   SurfaceComposerClient* client =

            (SurfaceComposerClient*)env->GetIntField(session, sso.client);

 

    sp<SurfaceControl> surface;

if (jname == NULL) {

   //client是SurfaceComposerClient,返回的surface是一个SurfaceControl

  //真得很复杂!

        surface = client->createSurface(pid, dpy, w, h, format, flags);

    } else {

        const jchar* str = env->GetStringCritical(jname, 0);

        const String8 name(str, env->GetStringLength(jname));

        env->ReleaseStringCritical(jname, str);

        surface = client->createSurface(pid, name, dpy, w, h, format, flags);

}

  //把surfaceControl信息设置到Surface对象中

    setSurfaceControl(env, clazz, surface);

}

 

static void setSurfaceControl(JNIEnv* env, jobject clazz,

        const sp<SurfaceControl>& surface)

{

    SurfaceControl* const p =

        (SurfaceControl*)env->GetIntField(clazz, so.surfaceControl);

    if (surface.get()) {

        surface->incStrong(clazz);

    }

    if (p) {

        p->decStrong(clazz);

    }

    env->SetIntField(clazz, so.surfaceControl, (int)surface.get());

}

[--->Surface_copyFrom]

static void Surface_copyFrom(

        JNIEnv* env, jobject clazz, jobject other)

{

   const sp<SurfaceControl>& surface = getSurfaceControl(env, clazz);

    const sp<SurfaceControl>& rhs = getSurfaceControl(env, other);

    if (!SurfaceControl::isSameSurface(surface, rhs)) {

        setSurfaceControl(env, clazz, rhs);

//把本地那个surface的surfaceControl对象转移到outSurface上

    }

}

这里仅仅是surfaceControl的转移,但是并没有看到Surface相关的信息。

那么Surface在哪里创建的呢?为了解释这个问题,我使用了终极武器,aidl

终极武器AIDL

aidl可以把XXX.aidl文件转换成对应的java文件。我们刚才调用的是WindowSession

relayOut函数。如下:

sWindowSession.relayout(

                mWindow, params,

                (int) (mView.mMeasuredWidth * appScale + 0.5f),

                (int) (mView.mMeasuredHeight * appScale + 0.5f),

                viewVisibility, insetsPending, mWinFrame,

                mPendingContentInsets, mPendingVisibleInsets,

                mPendingConfiguration, mSurface);

它的aidl文件在framework/base/core/java/android/view/IWindowSession.aidl

interface IWindowSession {

    int add(IWindow window, in WindowManager.LayoutParams attrs,

            in int viewVisibility, out Rect outContentInsets);

    void remove(IWindow window);

   //注意喔,这个outSurface前面的是out,表示输出参数,这个类似于C++的引用。

 int relayout(IWindow window, in WindowManager.LayoutParams attrs,

            int requestedWidth, int requestedHeight, int viewVisibility,

            boolean insetsPending, out Rect outFrame, out Rect outContentInsets,

            out Rect outVisibleInsets, out Configuration outConfig,

            out Surface outSurface);

刚才说了,JNI及其JAVA调用只是copyFromSurfaceControl对象到outSurface中,但是没看到哪里创建Surface。这其中的奥秘就在aidl文件编译后生成的java文件中。

你在命令行下可以输入:

aidl -Id:\android-2.2-froyo-20100625-source\source\frameworks\base\core\java\ -Id:\android-2.2-froyo-20100625-source\source\frameworks\base\Graphics\java d:\android-2.2-froyo-20100625-source\source\frameworks\base\core\java\android\view\IWindowSession.aidl test.java

以生成test.java文件。-I参数指定include目录,例如aidl有些参数是在别的java文件中指定的,那么这个-I就需要把这些目录包含进来。

先看看ViewRoot这个客户端生成的代码是什么。

public int relayout(

android.view.IWindow window,

android.view.WindowManager.LayoutParams attrs,

 int requestedWidth, int requestedHeight,

int viewVisibility, boolean insetsPending,

android.graphics.Rect outFrame,

android.graphics.Rect outContentInsets,

    android.graphics.Rect outVisibleInsets,

android.content.res.Configuration outConfig,

android.view.Surface outSurface) ---->outSurface是第11个参数

throws android.os.RemoteException

{

android.os.Parcel _data = android.os.Parcel.obtain();

android.os.Parcel _reply = android.os.Parcel.obtain();

int _result;

try {

_data.writeInterfaceToken(DESCRIPTOR);

_data.writeStrongBinder((((window!=null))?(window.asBinder()):(null)));

if ((attrs!=null)) {

_data.writeInt(1);

attrs.writeToParcel(_data, 0);

}

else {

_data.writeInt(0);

}

_data.writeInt(requestedWidth);

_data.writeInt(requestedHeight);

_data.writeInt(viewVisibility);

_data.writeInt(((insetsPending)?(1):(0)));

 //奇怪,outSurface的信息没有写到_data中。那.....

mRemote.transact(Stub.TRANSACTION_relayout, _data, _reply, 0);

_reply.readException();

_result = _reply.readInt();

if ((0!=_reply.readInt())) {

outFrame.readFromParcel(_reply);

}

....

if ((0!=_reply.readInt())) {

outSurface.readFromParcel(_reply); //从Parcel中读取信息来填充outSurface

}

}

finally {

_reply.recycle();

_data.recycle();

}

return _result;

}

真奇怪啊,Binder客户端这头竟然没有把outSurface的信息发过去。我们赶紧看看服务端

服务端这边处理是在onTranscat函数中。

@Override public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException

{

switch (code)

{

case TRANSACTION_relayout:

{

data.enforceInterface(DESCRIPTOR);

android.view.IWindow _arg0;

android.view.Surface _arg10;

//刚才说了,Surface信息并没有传过来,那么我们在relayOut中看到的outSurface是怎么

//出来的呢?看下面这句,原来在服务端这边竟然new了一个新的Surface!!!

_arg10 = new android.view.Surface();

int _result = this.relayout(_arg0, _arg1, _arg2, _arg3, _arg4, _arg5, _arg6, _arg7, _arg8, _arg9, _arg10);

reply.writeNoException();

reply.writeInt(_result);

//_arg10是copyFrom了,那怎么传到客户端呢?

if ((_arg10!=null)) {

reply.writeInt(1);//调用Surface的writeToParcel,把信息加入reply

_arg10.writeToParcel(reply, android.os.Parcelable.PARCELABLE_WRITE_RETURN_VALUE);

}

return true;

}

太诡异了!竟然有这么多花花肠子。我相信如果没有aidl的帮助,我无论如何也不会知道这其中的奥妙。

那好,我们的流程明白了。

l         客户端虽然传了一个surface,但其实没传递给服务端

l         服务端调用writeToParcel,把信息写到Parcel中,然后数据传回客户端

l         客户端调用SurfacereadFromParcel,获得surface信息。

那就去看看writeToParcel吧。

[---->Surface_writeToParcel]

static void Surface_writeToParcel(

        JNIEnv* env, jobject clazz, jobject argParcel, jint flags)

{

    Parcel* parcel = (Parcel*)env->GetIntField(

            argParcel, no.native_parcel);

  

const sp<SurfaceControl>& control(getSurfaceControl(env, clazz));

//还好,只是把数据序列化到Parcel中

    SurfaceControl::writeSurfaceToParcel(control, parcel);

    if (flags & PARCELABLE_WRITE_RETURN_VALUE) {

        setSurfaceControl(env, clazz, 0);

    }

}

那看看客户端的Surface_readFromParcel吧。

[----->Surface_readFromParcel]

static void Surface_readFromParcel(

        JNIEnv* env, jobject clazz, jobject argParcel)

{

    Parcel* parcel = (Parcel*)env->GetIntField( argParcel, no.native_parcel);

  

   //客户端这边还没有surface呢

 const sp<Surface>& control(getSurface(env, clazz));

 //不过我们看到希望了,根据服务端那边Parcel信息来构造一个新的surface

    sp<Surface> rhs = new Surface(*parcel);

    if (!Surface::isSameSurface(control, rhs)) {

       setSurface(env, clazz, rhs); //把这个新surface赋给客户端。终于我们有了surface!

    }

}

到此,我们终于七拐八绕的得到了surface,这其中经历太多曲折了。下一节,我们将精简这其中复杂的操作,统一归到Native层,以这样为切入点来了解Surface的工作流程和原理。

好,反正你知道ViewRoot调用了relayout后,Surface就真正从WindowManagerService那得到了。继续回到ViewRoot,其中还有一个重要地方是我们知道却不了解的。

private void performTraversals() {

        // cache mView since it is used so much below...

        final View host = mView;

 

         boolean initialized = false;

            boolean contentInsetsChanged = false;

            boolean visibleInsetsChanged;

            try {

         relayoutResult = relayoutWindow(params, viewVisibility, insetsPending);

      // relayoutWindow完后,我们得到了一个无比宝贵的Surface

     //那我们画界面的地方在哪里?就在这个函数中,离relayoutWindow不远处。

....

  boolean cancelDraw = attachInfo.mTreeObserver.dispatchOnPreDraw();

 

        if (!cancelDraw && !newSurface) {

            mFullRedrawNeeded = false;

            draw(fullRedrawNeeded); //draw?draw什么呀?

       }

    [--->ViewRoot::draw()]

private void draw(boolean fullRedrawNeeded) {

        Surface surface = mSurface; //嘿嘿,不担心了,surface资源都齐全了

        if (surface == null || !surface.isValid()) {

            return;

        }

        if (mAttachInfo.mViewScrollChanged) {

            mAttachInfo.mViewScrollChanged = false;

            mAttachInfo.mTreeObserver.dispatchOnScrollChanged();

        }

 

        int yoff;

        final boolean scrolling = mScroller != null && mScroller.computeScrollOffset();

        if (scrolling) {

            yoff = mScroller.getCurrY();

        } else {

            yoff = mScrollY;

        }

        if (mCurScrollY != yoff) {

            mCurScrollY = yoff;

            fullRedrawNeeded = true;

        }

        float appScale = mAttachInfo.mApplicationScale;

        boolean scalingRequired = mAttachInfo.mScalingRequired;

 

        Rect dirty = mDirty;

        if (mUseGL) { //我们不用OPENGL

           ...

        }

 

         Canvas canvas;

        try {

            int left = dirty.left;

            int top = dirty.top;

            int right = dirty.right;

            int bottom = dirty.bottom;

          //从Surface中锁定一块区域,这块区域是我们认为的需要重绘的区域

            canvas = surface.lockCanvas(dirty);

            // TODO: Do this in native

            canvas.setDensity(mDensity);

        }

 

        try {

            if (!dirty.isEmpty() || mIsAnimating) {

                long startTime = 0L;

                try {

                    canvas.translate(0, -yoff);

                    if (mTranslator != null) {

                        mTranslator.translateCanvas(canvas);

                    }

                    canvas.setScreenDensity(scalingRequired

                            ? DisplayMetrics.DENSITY_DEVICE : 0);

                 //mView就是之前的decoreView,

                    mView.draw(canvas);

                }

            } finally {

             //我们的图画完了,告诉surface释放这块区域

            surface.unlockCanvasAndPost(canvas);

        }

       if (scrolling) {

            mFullRedrawNeeded = true;

            scheduleTraversals();

        }

}

看起来,这个surface的用法很简单嘛:

l         lockSurface,得到一个画布Canvas

l         调用Viewdraw,让他们在这个Canvas上尽情绘图才。另外,这个View会调用所有它的子View画图,最终会进入到ViewonDraw函数中,在这里我们可以做定制化的界面美化工作。当然,如果你想定制化整个系统画图的话,完全可以把performTranvsal看懂,然后再修改。

l         unlockCanvasAndPost,告诉Surface释放这块画布

当然,这几个重要函数调用干了具体的活。这些重要函数,我们最终会精简到Native层的。

总结

到这里,你应该知道了一个Activity中,调用setContentView后它如何从系统中获取一块Surface,以及它是如何使用这个Surface的了。不得不说,关于UI这块,Android绝对是够复杂的。难怪2.3UI这块代码基本重写一遍,希望能够简单精炼点。

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