【转载】Android 中 View 绘制流程分析

建立Window

在Activity的attach方法中经过调用PolicyManager.makeNewWindo建立Window,将一个View add到WindowManager时，WindowManagerImpl建立一个ViewRoot来管理该窗口的根View。并经过ViewRoot.setView方法把该View传给ViewRoot。

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) { attachBaseContext(context); mFragments.attachActivity(this, mContainer, null); mWindow = PolicyManager.makeNewWindow(this); mWindow.setCallback(this); mWindow.getLayoutInflater().setPrivateFactory(this);

建立DecorView

DecorView为整个Window界面的最顶层View。

Activity中的Window对象帮咱们建立了一个PhoneWindow内部类DecorView（父类为FrameLayout）窗口顶层视图，而后经过LayoutInflater将xml内容布局解析成View树形结构添加到DecorView顶层视图中id为content的FrameLayout父容器上面。Activity的content内容布局最终会添加到DecorView窗口顶层视图上面。

protected boolean initializePanelDecor(PanelFeatureState st) { st.decorView = new DecorView(getContext(), st.featureId); st.gravity = Gravity.CENTER | Gravity.BOTTOM; st.setStyle(getContext()); return true; }

建立ViewRoot并关联View

WindowManagerImpl保存DecorView到mViews，建立对应的ViewRoot；

ViewRoot用于管理窗口的根View，并和global window manger进行交互。ViewRoot中有一个nested class： W，W是一个Binder子类，用于接收global window manager的各类消息， 如按键消息， 触摸消息等。 ViewRoot有一个W类型的成员mWindow，ViewRoot在Constructor中建立一个W的instance并赋值给mWindow。 ViewRoot是Handler的子类， W会经过Looper把消息传递给ViewRoot。 ViewRoot在setView方法中把mWindow传给sWindowSession。

public void addView(View view, ViewGroup.LayoutParams params, Display display, Window parentWindow) { if (view == null) { throw new IllegalArgumentException("view must not be null"); } if (display == null) { throw new IllegalArgumentException("display must not be null"); } if (!(params instanceof WindowManager.LayoutParams)) { throw new IllegalArgumentException("Params must be WindowManager.LayoutParams"); } final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params; if (parentWindow != null) { parentWindow.adjustLayoutParamsForSubWindow(wparams); } ViewRootImpl root; View panelParentView = null; synchronized (mLock) { // Start watching for system property changes. if (mSystemPropertyUpdater == null) { mSystemPropertyUpdater = new Runnable() { @Override public void run() { synchronized (mLock) { for (ViewRootImpl viewRoot : mRoots) { viewRoot.loadSystemProperties(); } } } }; SystemProperties.addChangeCallback(mSystemPropertyUpdater); } int index = findViewLocked(view, false); if (index >= 0) { throw new IllegalStateException("View " + view + " has already been added to the window manager."); } // If this is a panel window, then find the window it is being // attached to for future reference. if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW && wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) { final int count = mViews != null ? mViews.length : 0; for (int i=0; i<count; i++) { if (mRoots[i].mWindow.asBinder() == wparams.token) { panelParentView = mViews[i]; } } } root = new ViewRootImpl(view.getContext(), display); view.setLayoutParams(wparams); if (mViews == null) { index = 1; mViews = new View[1]; mRoots = new ViewRootImpl[1]; mParams = new WindowManager.LayoutParams[1]; } else { index = mViews.length + 1; Object[] old = mViews; mViews = new View[index]; System.arraycopy(old, 0, mViews, 0, index-1); old = mRoots; mRoots = new ViewRootImpl[index]; System.arraycopy(old, 0, mRoots, 0, index-1); old = mParams; mParams = new WindowManager.LayoutParams[index]; System.arraycopy(old, 0, mParams, 0, index-1); } index--; mViews[index] = view; mRoots[index] = root; mParams[index] = wparams; } // do this last because it fires off messages to start doing things try { root.setView(view, wparams, panelParentView); } catch (RuntimeException e) { // BadTokenException or InvalidDisplayException, clean up. synchronized (mLock) { final int index = findViewLocked(view, false); if (index >= 0) { removeViewLocked(index, true); } } throw e; } }

ViewRoot是GUI管理系统与GUI呈现系统之间的桥梁，须要注意它并非一个View类型。

它的主要做用以下：

一、向DecorView分发收到的用户发起的event事件，如按键，触屏，轨迹球等事件；
二、与WindowManagerService交互，完成整个Activity的GUI的绘制。

View绘制基本流程

这里先给出Android系统View的绘制流程：依次执行View类里面的以下三个方法：

measure(int ,int) :测量View的大小
layout(int ,int ,int ,int) ：设置子View的位置
draw(Canvas) ：绘制View内容到Canvas画布上

整个View树的绘图流程是在ViewRoot.Java类的performTraversals()函数展开的，该函数作的执行过程可简单概况为根据以前设置的状态，判断是否须要从新计算视图大小(measure)、是否从新须要安置视图的位置(layout)、以及是否须要重绘 (draw)

 mesarue()测量过程

主要做用：为整个View树计算实际的大小，即设置实际的高(mMeasuredHeight)和宽(mMeasureWidth)，每一个View的控件的实际宽高都是由父视图和自己视图决定的。

具体的调用以下：

ViewRootImpl 的performTraversals方法中，调用measureHierarchy，而后调用performMeasure

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); try { mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } }

ViewRoot根对象地属性mView(其类型通常为ViewGroup类型)调用measure()方法去计算View树的大小，回调View/ViewGroup对象的onMeasure()方法，该方法实现的功能以下：

一、设置本View视图的最终大小，该功能的实现经过调用setMeasuredDimension()方法去设置实际的高(mMeasuredHeight)和宽(mMeasureWidth)

二、若是该View对象是个ViewGroup类型，须要重写onMeasure()方法，对其子视图进行遍历的measure()过程。

对每一个子视图的measure()过程，是经过调用父类ViewGroup.java类里的measureChildWithMargins()方法去实现，该方法内部只是简单地调用了View对象的measure()方法。

整个measure调用流程就是个树形的递归过程measure()方法两个参数都是父View传递过来的，也就是表明了父view的规格。他由两部分组成，高2位表示MODE，定义在MeasureSpec类（View的内部类）中，有三种类型，MeasureSpec.EXACTLY表示肯定大小， MeasureSpec.AT_MOST表示最大大小， MeasureSpec.UNSPECIFIED不肯定。低30位表示size，也就是父View的大小。对于系统Window类的DecorVIew对象Mode通常都为MeasureSpec.EXACTLY ，而size分别对应屏幕宽高。对于子View来讲大小是由父View和子View共同决定的。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); }

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) { boolean optical = isLayoutModeOptical(this); if (optical != isLayoutModeOptical(mParent)) { Insets insets = getOpticalInsets(); int opticalWidth = insets.left + insets.right; int opticalHeight = insets.top + insets.bottom; measuredWidth += optical ? opticalWidth : -opticalWidth; measuredHeight += optical ? opticalHeight : -opticalHeight; } mMeasuredWidth = measuredWidth; mMeasuredHeight = measuredHeight; mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET; }

layout布局过程

主要做用 ：为将整个根据子视图的大小以及布局参数将View树放到合适的位置上。

具体的调用以下：

ViewRootImpl 的performTraversals方法中，调用performLayout

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { mLayoutRequested = false; mScrollMayChange = true; mInLayout = true; final View host = mView; if (DEBUG_ORIENTATION || DEBUG_LAYOUT) { Log.v(TAG, "Laying out " + host + " to (" + host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")"); } Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout"); try { host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mInLayout = false; int numViewsRequestingLayout = mLayoutRequesters.size(); if (numViewsRequestingLayout > 0) { // requestLayout() was called during layout. // If no layout-request flags are set on the requesting views, there is no problem. // If some requests are still pending, then we need to clear those flags and do // a full request/measure/layout pass to handle this situation. ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, false); if (validLayoutRequesters != null) { // Set this flag to indicate that any further requests are happening during // the second pass, which may result in posting those requests to the next // frame instead mHandlingLayoutInLayoutRequest = true; // Process fresh layout requests, then measure and layout int numValidRequests = validLayoutRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = validLayoutRequesters.get(i); Log.w("View", "requestLayout() improperly called by " + view + " during layout: running second layout pass"); view.requestLayout(); } measureHierarchy(host, lp, mView.getContext().getResources(), desiredWindowWidth, desiredWindowHeight); mInLayout = true; host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mHandlingLayoutInLayoutRequest = false; // Check the valid requests again, this time without checking/clearing the // layout flags, since requests happening during the second pass get noop'd validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true); if (validLayoutRequesters != null) { final ArrayList<View> finalRequesters = validLayoutRequesters; // Post second-pass requests to the next frame getRunQueue().post(new Runnable() { @Override public void run() { int numValidRequests = finalRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = finalRequesters.get(i); Log.w("View", "requestLayout() improperly called by " + view + " during second layout pass: posting in next frame"); view.requestLayout(); } } }); } } } } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } mInLayout = false; }

host.layout()开始View树的布局，继而回调给View/ViewGroup类中的layout()方法。具体流程以下

1 、layout方法会设置该View视图位于父视图的坐标轴，即mLeft，mTop，mLeft，mBottom(调用setFrame()函数去实现)，接下来回调onLayout()方法(若是该View是ViewGroup对象，须要实现该方法，对每一个子视图进行布局)。

二、若是该View是个ViewGroup类型，须要遍历每一个子视图chiildView，调用该子视图的layout()方法去设置它的坐标值。

protected void onLayout(boolean changed, int left, int top, int right, int bottom) { }

public void layout(int l, int t, int r, int b) { int oldL = mLeft; int oldT = mTop; int oldB = mBottom; int oldR = mRight; boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) { onLayout(changed, l, t, r, b); mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED; ListenerInfo li = mListenerInfo; if (li != null && li.mOnLayoutChangeListeners != null) { ArrayList<OnLayoutChangeListener> listenersCopy = (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone(); int numListeners = listenersCopy.size(); for (int i = 0; i < numListeners; ++i) { listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB); } } } mPrivateFlags &= ~PFLAG_FORCE_LAYOUT; }

draw()绘图过程

ViewRootImpl 的performTraversals方法中，调用了mView的draw方法

mView.draw()开始绘制，draw()方法实现的功能以下：

1 、绘制该View的背景
2 、为显示渐变框作一些准备操做
三、调用onDraw()方法绘制视图自己   (每一个View都须要重载该方法，ViewGroup不须要实现该方法)
四、调用dispatchDraw ()方法绘制子视图(若是该View类型不为ViewGroup，即不包含子视图，不须要重载该方法)

值得说明的是，ViewGroup类已经为咱们重写了dispatchDraw ()的功能实现，应用程序通常不须要重写该方法，但能够重载父类函数实现具体的功能。

dispatchDraw()方法内部会遍历每一个子视图，调用drawChild()去从新回调每一个子视图的draw()方法。

五、绘制滚动条

刷新视图

Android中实现view的更新有两个方法，一个是invalidate，另外一个是postInvalidate，其中前者是在UI线程自身中使用，然后者在非UI线程中使用。

requestLayout()方法 ：会致使调用measure()过程 和 layout()过程 。

说明：只是对View树从新布局layout过程包括measure()和layout()过程，不会调用draw()过程，但不会从新绘制
任何视图包括该调用者自己。
通常引发invalidate()操做的函数以下：

一、直接调用invalidate()方法，请求从新draw()，但只会绘制调用者自己。

二、setSelection()方法 ：请求从新draw()，但只会绘制调用者自己。

三、setVisibility()方法 ： 当View可视状态在INVISIBLE转换VISIBLE时，会间接调用invalidate()方法，继而绘制该View。

4 、setEnabled()方法 ： 请求从新draw()，但不会从新绘制任何视图包括该调用者自己。

内容参考源码，借鉴了网上的一些分析。