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应用程序Activity启动时,是如何对每个Activity中的View进行绘制的了?接下来,我们就来看一看整个绘制的过程。
当activity获得焦点(Focus)时,系统会发送消息要求绘制布局。Android中间层会负责整个绘制过程,但在这之前,Activity需要提供布局结构的根节点(root node)。
获取到布局的根节点后,开始对布局树(所有的View组成一个View Tree)进行测量与绘制。绘制是通过遍历整个布局树以及对每个在非有效区域里的View进行着色(render)完成的。而每个ViewGroup(布局)则负责调用其子View的绘制函数onDraw(),具体的绘制由子View自身来完成。整个绘制的遍历过程是采用中序遍历的方式进行的,因此parent总是在子view之前绘制,而同一个ViewGroup中姊妹view则根据他们在布局中出现的顺序来绘制。
若需要强制一个view进行绘制,可以调用
invalidate()
绘制一个布局主要有两个过程:测量与布局。测量过程通过函数measure(int, int)
实现,其从上而下对整个View tree进行遍历:每个view都将尺寸大小往树的底部传递。当过程结束后,每个view都保存了自己的尺寸数据。第二个布局过程,同样是一个从上之下的遍历过程,通过layout(int,int,int,int)
来实现,在此过程中,每个父节点的View根据已经计算出的尺寸,负责将所有子view放到合适的位置上。
要初始化一个布局,调用
requestLayout()
。若视图认为布局不能够适配当前的边界时,这个方法会被视图自身调用。
当一个view的measure()
函数返回时,它的宽度与高度必须被确定下来,与其相关的子view大小也需要确定下来,同时还需要满足父节点view对其宽度与高度的约束。这能够确保在测量过程结束后,程序中父视图可以接受子视图的尺寸。一个父视图有可能多次调用子视图的measure()
函数。
测量过程通过两个类来交换彼此的尺寸信息:ViewGroup.LayoutParams
被子视图用于通知父视图其自身想要如何被测量以及放置在什么位置。对于每个尺寸,可以指定如下几个参数:
MATCH_PARENT
,保持子视图跟父视图一样的大小(减去padding);WRAP_CONTENT
,视图刚好能够包含它的内容(加上padding)而MeasureSpec
对象用于将尺寸信息从父视图传递给子视图,一个MeasureSpec
有如下三种模式:
UNSEPCIFIED
: 用于一个父视图确定一个子视图的期望的尺寸。例如,一个高度指定为 UNSPECIFIED
而高度指定为EXACTLY 240
的LinearLayout
也许会调用子视图的measure()
函数,以确定在给定的240像素宽度下,子视图想要多大的尺寸;EXACTLY
: 父视图强制给子视图一个确定的尺寸,而子视图必须要保证使用该尺寸,并且确保其子视图的大小也同样在该尺寸范围内;AT_MOST
: 父视图给子视图指定一个最大的尺寸,子视图需要确保其自身以及子视图在该尺寸的范围之内。以下源码分析基于Android MM6.0
在Activity的启动过程中,一个应用程序的UI界面是从什么时候开始绘制的了? 我们知道,在写一个Activity的程序时,重载onCreate(Bundle savedState)
函数后,会调用 setContentView()
函数,将Layout资源文件传递给系统,正是从这里,Android开始了View的绘制。
关于Activity启动流程:
View绘制真正的幕后由三个火枪手组成: View, ViewRootImpl, Choerographer; View就是要绘制的视图了,ViewRootImpl则负责将绘制的View与对应的Window绑定起来,Choerographer则是协调窗口中input,animation以及drawing的魔法师。
ViewRootImpl: the top of a view hierarchy, implementing the needed protocol between View and the WindowManager
直接上图。看一看Activity启动后,UI绘制的流程简图:
final void attach(Context context, ActivityThread aThread,Instrumentation instr, IBinder token, int ident, Application application, Intent intent, ActivityInfo info,CharSequence title, Activity parent, String id,NonConfigurationInstances lastNonConfigurationInstances, Configuration config, String referrer, IVoiceInteractor voiceInteractor) { attachBaseContext(context); mFragments.attachHost(null /*parent*/); /* { Multi-Window */ if (MultiWindowFeatures.MULTIWINDOW_ENABLED && !DUALSCREEN_ENABLED) { final MultiWindowStyle mwStyle = getMultiWindowStyle(); if (parent == null && mwStyle != null && mwStyle.isMultiPhoneWindowNeeded(info, context)) { mWindow = makeNewWindow(this, true); } else { mWindow = new PhoneWindow(this); } } else { mWindow = new PhoneWindow(this); } /* Multi-Window } */ mWindow.setCallback(this); mWindow.setOnWindowDismissedCallback(this); mWindow.getLayoutInflater().setPrivateFactory(this); if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) { mWindow.setSoftInputMode(info.softInputMode); } if (info.uiOptions != 0) { mWindow.setUiOptions(info.uiOptions); } mUiThread = Thread.currentThread(); ... mWindow.setWindowManager( (WindowManager)context.getSystemService(Context.WINDOW_SERVICE), mToken, mComponent.flattenToString(), (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0); if (mParent != null) { mWindow.setContainer(mParent.getWindow()); } mWindowManager = mWindow.getWindowManager(); } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
onCreate()
函数,通过setContentView()
将Layout资源文件传递给PhoneWindowpublic void setContentView(@LayoutRes int layoutResID) { getWindow().setContentView(layoutResID); initWindowDecorActionBar(); } 1 2 3 4 5 6 1 2 3 4 5 6
public void setContentView(View view, ViewGroup.LayoutParams params) { // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window // decor, when theme attributes and the like are crystalized. Do not check the feature // before this happens. if (mContentParent == null) { installDecor(); } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) { mContentParent.removeAllViews(); } if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) { view.setLayoutParams(params); final Scene newScene = new Scene(mContentParent, view); transitionTo(newScene); } else { // 将子View添加到窗口中(DecorView) mContentParent.addView(view, params); } mContentParent.requestApplyInsets(); final Callback cb = getCallback(); if (cb != null && !isDestroyed()) { cb.onContentChanged(); } } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
requestLayout
函数;public void addView(View child, LayoutParams params) { addView(child, -1, params); } public void addView(View child, int index, LayoutParams params) { if (DBG) { System.out.println(this + " addView"); } if (child == null) { throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup"); } // addViewInner() will call child.requestLayout() when setting the new LayoutParams // therefore, we call requestLayout() on ourselves before, so that the child's request // will be blocked at our level // 开始布局的工作了 requestLayout(); invalidate(true); addViewInner(child, index, params, false); } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
scheduleTraversal()
将TraverasalRunnable
放置到Choreographer
消息队列里,等待调用:void scheduleTraversals() { if (!mTraversalScheduled) { mTraversalScheduled = true; mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier(); // mTraversalRunnable --> 就是进行布局遍历的一把手了 mChoreographer.postCallback( Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null); if (!mUnbufferedInputDispatch) { scheduleConsumeBatchedInput(); } notifyRendererOfFramePending(); pokeDrawLockIfNeeded(); } } // mTraveralRunnable final class TraversalRunnable implements Runnable { @Override public void run() { doTraversal(); } } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Choreographer
接着着手准备Frame绘制,private void scheduleFrameLocked(long now) { // 当前没有Frame请求 if (!mFrameScheduled) { mFrameScheduled = true; //判断是否支持VSYNC帧同步 if (USE_VSYNC) { if (DEBUG_FRAMES) { Log.d(TAG, "Scheduling next frame on vsync."); } // If running on the Looper thread, then schedule the vsync immediately, // otherwise post a message to schedule the vsync from the UI thread // as soon as possible. if (isRunningOnLooperThreadLocked()) { // 触发一个Vsync同步信号 scheduleVsyncLocked(); } else { Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC); msg.setAsynchronous(true); mHandler.sendMessageAtFrontOfQueue(msg); } } else { final long nextFrameTime = Math.max( mLastFrameTimeNanos / TimeUtils.NANOS_PER_MS + sFrameDelay, now); if (DEBUG_FRAMES) { Log.d(TAG, "Scheduling next frame in " + (nextFrameTime - now) + " ms."); } Message msg = mHandler.obtainMessage(MSG_DO_FRAME); msg.setAsynchronous(true); mHandler.sendMessageAtTime(msg, nextFrameTime); } } } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
在接收到Kernel发送过来的Vsync信号后,Choreographer
会开始真正的绘制了:
@Override public void onVsync(long timestampNanos, int builtInDisplayId, int frame) { // Post the vsync event to the Handler. // The idea is to prevent incoming vsync events from completely starving // the message queue. If there are no messages in the queue with timestamps // earlier than the frame time, then the vsync event will be processed immediately. // Otherwise, messages that predate the vsync event will be handled first. long now = System.nanoTime(); if (timestampNanos > now) { Log.w(TAG, "Frame time is " + ((timestampNanos - now) * 0.000001f) + " ms in the future! Check that graphics HAL is generating vsync " + "timestamps using the correct timebase."); timestampNanos = now; } ... mTimestampNanos = timestampNanos; mFrame = frame; Message msg = Message.obtain(mHandler, this); msg.setAsynchronous(true); mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS); } @Override public void run() { mHavePendingVsync = false; // Frame 绘制的工作就是在这里 doFrame(mTimestampNanos, mFrame); } } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
好了,准备就绪后,doFrame()
函数将Cheorographer
消息队列的Callbacks取出来,分别运行:
void doFrame(long frameTimeNanos, int frame) { final long startNanos; synchronized (mLock) { if (!mFrameScheduled) { return; // no work to do } ... try { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame"); // 处理InputEvent mFrameInfo.markInputHandlingStart(); doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos); // 处理动画 mFrameInfo.markAnimationsStart(); doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos); // View 布局遍历,绘制,这里运行的就是那个TranversalRunnable了 mFrameInfo.markPerformTraversalsStart(); doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos); doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos); } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
看官方的定义是说,Vsync是一个显示的同步时钟信号,应用程序总是在同步信号开始后进行视图的绘制。Android将屏幕的刷新率(refresh rate)设定在60FPS(Frame Per Second),就是说一帧图像的绘制时间在1000/60 ~ 16.67ms,也就是UI线程必须在这个时间内将Frame绘制完成,不然可能出现由于帧丢失而卡顿。这也是为什么很多耗时的工作不要放在UI线程,而是启动一个新的线程的原因。
VSYNC: synchronizes certain events to the refresh cycle of the display. Applications always start drawing on a VSYNC boundary, and SurfaceFlinger always composites on a VSYNC boundary. This eliminates stutters and improves visual performance of graphics.
这里有一篇Vsync的文章:
Vsync介绍:
附一张View绘制的流程简图:
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