Activity界面的加载和绘制

初探窗口

在知道Activity的生命周期调用时机之后，这些都太抽象了，到底我们看到的界面是怎么加载出来的才是最感兴趣的，这里面就主要涉及Activity的Window对象了，具体是什么机制呢？还是回到源码。

加载布局

还是先从Activity的加载过程中来看看到底是怎么加载布局的，来到Activity的attach方法：

Activity.attach

    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*/);//初始化Window对象mWindow = new PhoneWindow(this);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();mMainThread = aThread;mInstrumentation = instr;mToken = token;mIdent = ident;mApplication = application;mIntent = intent;mReferrer = referrer;mComponent = intent.getComponent();mActivityInfo = info;mTitle = title;mParent = parent;mEmbeddedID = id;mLastNonConfigurationInstances = lastNonConfigurationInstances;if (voiceInteractor != null) {if (lastNonConfigurationInstances != null) {mVoiceInteractor = lastNonConfigurationInstances.voiceInteractor;} else {mVoiceInteractor = new VoiceInteractor(voiceInteractor, this, this,Looper.myLooper());}}//设置WindowManagermWindow.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();mCurrentConfig = config;}

在这个方法里对Activity的一些成员变量进行了初始化（上下文，线程，标题。。。），特别是mWindow初始化为PhoneWindow这个类的实例。

好了，在初始化了Window之后，来到我们最熟悉的onCreate方法，我们一般在这个方法中会调用setContentView指定该Activity的布局文件，来看看这个方法：

Activity.setContentView

    public void setContentView(@LayoutRes int layoutResID) {getWindow().setContentView(layoutResID);initWindowDecorActionBar();}public Window getWindow() {return mWindow;}

这里的getWindow获取了mWindow变量，我们知道刚才在attach方法中它初始化为了PhoneWindow的实例，其实就是调用的PhoneWindow的setContentView方法：

PhoneWindow.setContentView

    @Overridepublic void setContentView(int layoutResID) {if (mContentParent == null) {installDecor();} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {mContentParent.removeAllViews();}if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,getContext());transitionTo(newScene);} else {mLayoutInflater.inflate(layoutResID, mContentParent);}mContentParent.requestApplyInsets();final Callback cb = getCallback();if (cb != null && !isDestroyed()) {cb.onContentChanged();}}

第一次的话，mContentParent为空，执行installDecor：

PhoneWindow.installDecor

    private void installDecor() {if (mDecor == null) {mDecor = generateDecor();mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);mDecor.setIsRootNamespace(true);if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);}}if (mContentParent == null) {mContentParent = generateLayout(mDecor);......}

mDecor对象声明如下：

private DecorView mDecor;

这是window最开始的视图，其他布局的父容器，DecorView继承自FrameLayout，然后因为mContentParent为空，调用generateLayout(mDecor)：

PhoneWindow.generateLayout(mDecor)

protected ViewGroup generateLayout(DecorView decor) {......mDecor.startChanging();//关键执行的代码View in = mLayoutInflater.inflate(layoutResource, null);decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));mContentRoot = (ViewGroup) in;//将布局赋值给mContentRootViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);......return contentParent;
}

根据不同的特性（features）来判断相应的布局文件，然后通过mLayoutInflater.inflate方法来解析加载，最后添加到mDecor上面，在这里我们发现mContentParent是mContentRoot的子布局，mContentRoot是mDecor的子布局，再来看看inflate方法是怎么来加载xml文件视图的：

LayoutInflater.inflater

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {return inflate(resource, root, root != null);
}
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {final Resources res = getContext().getResources();if (DEBUG) {Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("+ Integer.toHexString(resource) + ")");}final XmlResourceParser parser = res.getLayout(resource);try {return inflate(parser, root, attachToRoot);} finally {parser.close();}
}

再调用重载方法后新建了XML解析器后再调用重载方法：

    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {synchronized (mConstructorArgs) {Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");final Context inflaterContext = mContext;final AttributeSet attrs = Xml.asAttributeSet(parser);Context lastContext = (Context) mConstructorArgs[0];mConstructorArgs[0] = inflaterContext;View result = root;try {// Look for the root node.int type;while ((type = parser.next()) != XmlPullParser.START_TAG &&type != XmlPullParser.END_DOCUMENT) {// Empty}if (type != XmlPullParser.START_TAG) {throw new InflateException(parser.getPositionDescription()+ ": No start tag found!");}final String name = parser.getName();if (DEBUG) {System.out.println("**************************");System.out.println("Creating root view: "+ name);System.out.println("**************************");}if (TAG_MERGE.equals(name)) {if (root == null || !attachToRoot) {throw new InflateException("<merge /> can be used only with a valid "+ "ViewGroup root and attachToRoot=true");}rInflate(parser, root, inflaterContext, attrs, false);} else {// Temp is the root view that was found in the xmlfinal View temp = createViewFromTag(root, name, inflaterContext, attrs);ViewGroup.LayoutParams params = null;if (root != null) {if (DEBUG) {System.out.println("Creating params from root: " +root);}// Create layout params that match root, if suppliedparams = root.generateLayoutParams(attrs);if (!attachToRoot) {// Set the layout params for temp if we are not// attaching. (If we are, we use addView, below)temp.setLayoutParams(params);}}if (DEBUG) {System.out.println("-----> start inflating children");}// Inflate all children under temp against its context.rInflateChildren(parser, temp, attrs, true);if (DEBUG) {System.out.println("-----> done inflating children");}// We are supposed to attach all the views we found (int temp)// to root. Do that now.if (root != null && attachToRoot) {root.addView(temp, params);}// Decide whether to return the root that was passed in or the// top view found in xml.if (root == null || !attachToRoot) {result = temp;}}} catch (XmlPullParserException e) {InflateException ex = new InflateException(e.getMessage());ex.initCause(e);throw ex;} catch (Exception e) {InflateException ex = new InflateException(parser.getPositionDescription()+ ": " + e.getMessage());ex.initCause(e);throw ex;} finally {// Don't retain static reference on context.mConstructorArgs[0] = lastContext;mConstructorArgs[1] = null;}Trace.traceEnd(Trace.TRACE_TAG_VIEW);return result;}}

这里原理是通过解析XML文件依次将组件添加到指定的root容器布局中，最后返回解析出来的布局视图即可。

再回到PhoneWindow.setContentView，看到这行代码：

mLayoutInflater.inflate(layoutResID, mContentParent);

将我们指定的布局添加到mContentParent之上。

最后梳理一下（布局从下到上）：

mDecor->mContentRoot->mContentParent->自己的布局

绘制布局

了解了加载XML布局文件的整个过程，来看看这些组件是怎么绘制的，并展现到Activity上。还是从生命周期方法开始看，在onCreate方法中我们加载了视图，下一个生命周期就是onResume方法，不过在执行onResume之前会先执行ActivityThread.handleResumeActivity:

ActivityThread.handleResumeActivity

            ......if (!r.activity.mFinished && willBeVisible&& r.activity.mDecor != null && !r.hideForNow) {if (r.newConfig != null) {r.tmpConfig.setTo(r.newConfig);if (r.overrideConfig != null) {r.tmpConfig.updateFrom(r.overrideConfig);}if (DEBUG_CONFIGURATION) Slog.v(TAG, "Resuming activity "+ r.activityInfo.name + " with newConfig " + r.tmpConfig);performConfigurationChanged(r.activity, r.tmpConfig);freeTextLayoutCachesIfNeeded(r.activity.mCurrentConfig.diff(r.tmpConfig));r.newConfig = null;}if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward="+ isForward);WindowManager.LayoutParams l = r.window.getAttributes();if ((l.softInputMode& WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)!= forwardBit) {l.softInputMode = (l.softInputMode& (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))| forwardBit;if (r.activity.mVisibleFromClient) {ViewManager wm = a.getWindowManager();View decor = r.window.getDecorView();wm.updateViewLayout(decor, l);}}r.activity.mVisibleFromServer = true;mNumVisibleActivities++;if (r.activity.mVisibleFromClient) {r.activity.makeVisible();//使Activity可见}}......

主要关注上面的几行有关WindowManager和Activity显示有关的方法，在初始化了相关对象后调用r.activity.makeVisible()：

Activity.makeVisible()

boolean mWindowAdded = false;
void makeVisible() {if (!mWindowAdded) {ViewManager wm = getWindowManager();wm.addView(mDecor, getWindow().getAttributes());mWindowAdded = true;}mDecor.setVisibility(View.VISIBLE);
}

这里的循环只执行一次，在执行完后会将mWindowAdded这个全局变量置为true，ViewManager的通过getWindowManager初始化：

    public WindowManager getWindowManager() {return mWindowManager;}

其实就是返回Activity的WindowManager，这个变量在一开始的Activity.attach方法中赋值：

mWindowManager=mWindow.getWindowManager();

Window.getWindowManager

public WindowManager getWindowManager() {return mWindowManager;
}

还是没有具体的赋值，找了一下源码后发现有setWindowManager这个方法，这个方法也是在Activity的attach方法中调用

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();

Window.setWindowManager

    public void setWindowManager(WindowManager wm, IBinder appToken, String appName,boolean hardwareAccelerated) {mAppToken = appToken;mAppName = appName;mHardwareAccelerated = hardwareAccelerated|| SystemProperties.getBoolean(PROPERTY_HARDWARE_UI, false);if (wm == null) {wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);}mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);}

所以每个Activity都会对应一个Window，Window会对应一个WindowManager，最后返回的是WindowManagerImpl实例，该类是WindowManager接口的具体实现,再回到

Activity.makeVisible()

boolean mWindowAdded = false;
void makeVisible() {if (!mWindowAdded) {ViewManager wm = getWindowManager();wm.addView(mDecor, getWindow().getAttributes());mWindowAdded = true;}mDecor.setVisibility(View.VISIBLE);
}

在wm赋值后调用wm.addView(mDecor, getWindow().getAttributes())，具体的实现在WindowManagerImpl中：

WindowManagerImpl.addView

    private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();@Overridepublic void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {applyDefaultToken(params);mGlobal.addView(view, params, mDisplay, mParentWindow);}

调用WindowManagerGlobal的addView方法，并且将mDecor传入，此类为单例：

WindowManagerGlobal.addView

    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");}//获取窗口参数final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;if (parentWindow != null) {parentWindow.adjustLayoutParamsForSubWindow(wparams);} else {//获取上下文final Context context = view.getContext();if (context != null&& (context.getApplicationInfo().flags& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;}}ViewRootImpl root;View panelParentView = null;synchronized (mLock) {if (mSystemPropertyUpdater == null) {mSystemPropertyUpdater = new Runnable() {@Override public void run() {synchronized (mLock) {for (int i = mRoots.size() - 1; i >= 0; --i) {mRoots.get(i).loadSystemProperties();}}}};SystemProperties.addChangeCallback(mSystemPropertyUpdater);}int index = findViewLocked(view, false);if (index >= 0) {if (mDyingViews.contains(view)) {mRoots.get(index).doDie();} else {throw new IllegalStateException("View " + view+ " has already been added to the window manager.");}}if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {final int count = mViews.size();for (int i = 0; i < count; i++) {if (mRoots.get(i).mWindow.asBinder() == wparams.token) {panelParentView = mViews.get(i);}}}root = new ViewRootImpl(view.getContext(), display);//设置属性view.setLayoutParams(wparams);//添加布局mViews.add(view);mRoots.add(root);mParams.add(wparams);}try {root.setView(view, wparams, panelParentView);} catch (RuntimeException e) {synchronized (mLock) {final int index = findViewLocked(view, false);if (index >= 0) {removeViewLocked(index, true);}}throw e;}}

这里面主要涉及三个数组的操作：

private final ArrayList<View> mViews = new ArrayList<View>();
private final ArrayList<ViewRootImpl> mRoots = new ArrayList<ViewRootImpl>();
private final ArrayList<WindowManager.LayoutParams> mParams =new ArrayList<WindowManager.LayoutParams>();

mViews用来保存根布局（mDecor），mRoots用来保存ViewRootImpl（此类用来关联View和ViewManager），mParams用来保存布局的属性

最后调用ViewRootImpl.setView

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {......requestLayout();......
}

这个方法里面主要执行了这一句用来请求绘制布局：

ViewRootImpl.requestLayout

    @Overridepublic void requestLayout() {if (!mHandlingLayoutInLayoutRequest) {checkThread();mLayoutRequested = true;scheduleTraversals();}}

首先是检查线程，如果不是UI线程则报错：

    void checkThread() {if (mThread != Thread.currentThread()) {throw new CalledFromWrongThreadException("Only the original thread that created a view hierarchy can touch its views.");}}

然后调用scheduleTraversals

    void scheduleTraversals() {if (!mTraversalScheduled) {mTraversalScheduled = true;mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();//通过Handler发送异步消息mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);if (!mUnbufferedInputDispatch) {scheduleConsumeBatchedInput();}notifyRendererOfFramePending();pokeDrawLockIfNeeded();}}

mChoreographer.postCallback这一句内部是通过handler发送了一个异步消息，执行mTraversalRunnable的run方法：

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

ViewRootImpl.doTraversal

    void doTraversal() {if (mTraversalScheduled) {mTraversalScheduled = false;mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);if (mProfile) {Debug.startMethodTracing("ViewAncestor");}performTraversals();if (mProfile) {Debug.stopMethodTracing();mProfile = false;}}}

然后调用performTraversals,这个法就是绘制布局的主要执行方法，在这里面会测量组件的大小，计算位置，绘制组件：

ViewRootImpl.performTraversals

    private void performTraversals() {......//测量组件if (!mStopped || mReportNextDraw) {boolean focusChangedDueToTouchMode = ensureTouchModeLocally((relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()|| mHeight != host.getMeasuredHeight() || contentInsetsChanged) {int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);if (DEBUG_LAYOUT) Log.v(TAG, "Ooops, something changed!  mWidth="+ mWidth + " measuredWidth=" + host.getMeasuredWidth()+ " mHeight=" + mHeight+ " measuredHeight=" + host.getMeasuredHeight()+ " coveredInsetsChanged=" + contentInsetsChanged);// Ask host how big it wants to beperformMeasure(childWidthMeasureSpec, childHeightMeasureSpec);int width = host.getMeasuredWidth();int height = host.getMeasuredHeight();boolean measureAgain = false;if (lp.horizontalWeight > 0.0f) {width += (int) ((mWidth - width) * lp.horizontalWeight);childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width,MeasureSpec.EXACTLY);measureAgain = true;}if (lp.verticalWeight > 0.0f) {height += (int) ((mHeight - height) * lp.verticalWeight);childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height,MeasureSpec.EXACTLY);measureAgain = true;}if (measureAgain) {if (DEBUG_LAYOUT) Log.v(TAG,"And hey let's measure once more: width=" + width+ " height=" + height);//主要调用的方法performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);}layoutRequested = true;}}}......//计算组件的位置final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);boolean triggerGlobalLayoutListener = didLayout|| mAttachInfo.mRecomputeGlobalAttributes;if (didLayout) {//主要调用的方法performLayout(lp, desiredWindowWidth, desiredWindowHeight);if ((host.mPrivateFlags & View.PFLAG_REQUEST_TRANSPARENT_REGIONS) != 0) {host.getLocationInWindow(mTmpLocation);mTransparentRegion.set(mTmpLocation[0], mTmpLocation[1],mTmpLocation[0] + host.mRight - host.mLeft,mTmpLocation[1] + host.mBottom - host.mTop);host.gatherTransparentRegion(mTransparentRegion);if (mTranslator != null) {mTranslator.translateRegionInWindowToScreen(mTransparentRegion);}if (!mTransparentRegion.equals(mPreviousTransparentRegion)) {mPreviousTransparentRegion.set(mTransparentRegion);mFullRedrawNeeded = true;try {mWindowSession.setTransparentRegion(mWindow, mTransparentRegion);} catch (RemoteException e) {}}}if (DBG) {System.out.println("======================================");System.out.println("performTraversals -- after setFrame");host.debug();}} ......//开始绘制if (!cancelDraw && !newSurface) {if (!skipDraw || mReportNextDraw) {if (mPendingTransitions != null && mPendingTransitions.size() > 0) {for (int i = 0; i < mPendingTransitions.size(); ++i) {mPendingTransitions.get(i).startChangingAnimations();}mPendingTransitions.clear();}//主要方法performDraw();}} else {if (viewVisibility == View.VISIBLE) {// 再次请求scheduleTraversals();} else if (mPendingTransitions != null && mPendingTransitions.size() > 0) {for (int i = 0; i < mPendingTransitions.size(); ++i) {mPendingTransitions.get(i).endChangingAnimations();}mPendingTransitions.clear();}}        }

主要就是三个方法，负责大小，位置，绘制：

performMeasure
performLayout
performDraw

先来看看performMeasure，测量组件的大小

ViewRootImpl.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);}}

这里调用mView的measure

View.measure

    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {......onMeasure(widthMeasureSpec, heightMeasureSpec);......}

其实mView也就是我们开始说到的mDecor,所以onMeasure具体实现应该来到DecorView ，也就是FrameLayout：

FrameLayout.onMeasure

    @Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {int count = getChildCount();final boolean measureMatchParentChildren =MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;mMatchParentChildren.clear();int maxHeight = 0;int maxWidth = 0;int childState = 0;for (int i = 0; i < count; i++) {//循环遍历子控件final View child = getChildAt(i);if (mMeasureAllChildren || child.getVisibility() != GONE) {measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);//测量子空间间距final LayoutParams lp = (LayoutParams) child.getLayoutParams();maxWidth = Math.max(maxWidth,child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);maxHeight = Math.max(maxHeight,child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);childState = combineMeasuredStates(childState, child.getMeasuredState());if (measureMatchParentChildren) {if (lp.width == LayoutParams.MATCH_PARENT ||lp.height == LayoutParams.MATCH_PARENT) {mMatchParentChildren.add(child);}}}}// 计算内边距(padding)maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();// 检查最小的高度和宽度maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());// 检查前景的最小宽度和高度final Drawable drawable = getForeground();if (drawable != null) {maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());}setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),resolveSizeAndState(maxHeight, heightMeasureSpec,childState << MEASURED_HEIGHT_STATE_SHIFT));count = mMatchParentChildren.size();if (count > 1) {//子控件大于1for (int i = 0; i < count; i++) {final View child = mMatchParentChildren.get(i);final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();final int childWidthMeasureSpec;if (lp.width == LayoutParams.MATCH_PARENT) {final int width = Math.max(0, getMeasuredWidth()- getPaddingLeftWithForeground() - getPaddingRightWithForeground()- lp.leftMargin - lp.rightMargin);childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width, MeasureSpec.EXACTLY);} else {childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,getPaddingLeftWithForeground() + getPaddingRightWithForeground() +lp.leftMargin + lp.rightMargin,lp.width);}final int childHeightMeasureSpec;if (lp.height == LayoutParams.MATCH_PARENT) {final int height = Math.max(0, getMeasuredHeight()- getPaddingTopWithForeground() - getPaddingBottomWithForeground()- lp.topMargin - lp.bottomMargin);childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);} else {childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,getPaddingTopWithForeground() + getPaddingBottomWithForeground() +lp.topMargin + lp.bottomMargin,lp.height);}//调用子控件的测量方法child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}}}

先测量自己的高度和宽度，然后调用setMeasuredDimension设置测量的尺寸，这是父类View的方法：

View.setMeasuredDimension

    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;}setMeasuredDimensionRaw(measuredWidth, measuredHeight);}private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {mMeasuredWidth = measuredWidth;mMeasuredHeight = measuredHeight;mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;}

将测量出来的高度和宽度保存到mMeasuredWidth和mMeasuredHeight中，如果有子控件就循环测量子控件的尺寸。

测量完毕，来到performLayout计算位置:

ViewRootImpl.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 {//主要执行的方法host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());mInLayout = false;int numViewsRequestingLayout = mLayoutRequesters.size();if (numViewsRequestingLayout > 0) {ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,false);if (validLayoutRequesters != null) {mHandlingLayoutInLayoutRequest = true;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;validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);if (validLayoutRequesters != null) {final ArrayList<View> finalRequesters = validLayoutRequesters;getRunQueue().post(new Runnable() {@Overridepublic 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;}

在内部还是调用了View 的layout方法：

View.layout

    public void layout(int l, int t, int r, int b) {if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;}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;mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;}

具体的方法在onLayout中，同样来到FrameLayout的onLayout：

FrameLayout.onLayout

    @Overrideprotected void onLayout(boolean changed, int left, int top, int right, int bottom) {layoutChildren(left, top, right, bottom, false /* no force left gravity */);}

调用layoutChildren：

    void layoutChildren(int left, int top, int right, int bottom,boolean forceLeftGravity) {final int count = getChildCount();final int parentLeft = getPaddingLeftWithForeground();final int parentRight = right - left - getPaddingRightWithForeground();final int parentTop = getPaddingTopWithForeground();final int parentBottom = bottom - top - getPaddingBottomWithForeground();for (int i = 0; i < count; i++) {final View child = getChildAt(i);if (child.getVisibility() != GONE) {final LayoutParams lp = (LayoutParams) child.getLayoutParams();final int width = child.getMeasuredWidth();final int height = child.getMeasuredHeight();int childLeft;int childTop;int gravity = lp.gravity;if (gravity == -1) {gravity = DEFAULT_CHILD_GRAVITY;}final int layoutDirection = getLayoutDirection();final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {case Gravity.CENTER_HORIZONTAL:childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +lp.leftMargin - lp.rightMargin;break;case Gravity.RIGHT:if (!forceLeftGravity) {childLeft = parentRight - width - lp.rightMargin;break;}case Gravity.LEFT:default:childLeft = parentLeft + lp.leftMargin;}switch (verticalGravity) {case Gravity.TOP:childTop = parentTop + lp.topMargin;break;case Gravity.CENTER_VERTICAL:childTop = parentTop + (parentBottom - parentTop - height) / 2 +lp.topMargin - lp.bottomMargin;break;case Gravity.BOTTOM:childTop = parentBottom - height - lp.bottomMargin;break;default:childTop = parentTop + lp.topMargin;}child.layout(childLeft, childTop, childLeft + width, childTop + height);}}}

原理跟刚才的测量尺寸差不多，也是先计算自己的位置，然后再循环遍历子控件，调用子控件的layout方法，最后得出计算位置完毕。

最后来到第三步，performDraw，绘制布局：

ViewRootImpl.performDraw

    private void performDraw() {......draw(fullRedrawNeeded);......}

主要执行了draw方法：

ViewRootImpl.draw

    private void draw(boolean fullRedrawNeeded) {......if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {return;}......}

再来到drawSoftware方法：

ViewRootImpl.drawSoftware

    private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,boolean scalingRequired, Rect dirty) {......mView.draw(canvas);......}

因为mView是FrameLayout，所以来到FrameLayout的draw方法：

FrameLayout.draw

    public void draw(Canvas canvas) {final int privateFlags = mPrivateFlags;final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;/** Draw traversal performs several drawing steps which must be executed* in the appropriate order:**      1. Draw the background*      2. If necessary, save the canvas' layers to prepare for fading*      3. Draw view's content*      4. Draw children*      5. If necessary, draw the fading edges and restore layers*      6. Draw decorations (scrollbars for instance)*/// Step 1, draw the background, if neededint saveCount;if (!dirtyOpaque) {drawBackground(canvas);}// skip step 2 & 5 if possible (common case)final int viewFlags = mViewFlags;boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;if (!verticalEdges && !horizontalEdges) {// Step 3, draw the contentif (!dirtyOpaque) onDraw(canvas);// Step 4, draw the childrendispatchDraw(canvas);// Overlay is part of the content and draws beneath Foregroundif (mOverlay != null && !mOverlay.isEmpty()) {mOverlay.getOverlayView().dispatchDraw(canvas);}// Step 6, draw decorations (foreground, scrollbars)onDrawForeground(canvas);// we're done...return;}/** Here we do the full fledged routine...* (this is an uncommon case where speed matters less,* this is why we repeat some of the tests that have been* done above)*/boolean drawTop = false;boolean drawBottom = false;boolean drawLeft = false;boolean drawRight = false;float topFadeStrength = 0.0f;float bottomFadeStrength = 0.0f;float leftFadeStrength = 0.0f;float rightFadeStrength = 0.0f;// Step 2, save the canvas' layersint paddingLeft = mPaddingLeft;final boolean offsetRequired = isPaddingOffsetRequired();if (offsetRequired) {paddingLeft += getLeftPaddingOffset();}int left = mScrollX + paddingLeft;int right = left + mRight - mLeft - mPaddingRight - paddingLeft;int top = mScrollY + getFadeTop(offsetRequired);int bottom = top + getFadeHeight(offsetRequired);if (offsetRequired) {right += getRightPaddingOffset();bottom += getBottomPaddingOffset();}final ScrollabilityCache scrollabilityCache = mScrollCache;final float fadeHeight = scrollabilityCache.fadingEdgeLength;int length = (int) fadeHeight;// clip the fade length if top and bottom fades overlap// overlapping fades produce odd-looking artifactsif (verticalEdges && (top + length > bottom - length)) {length = (bottom - top) / 2;}// also clip horizontal fades if necessaryif (horizontalEdges && (left + length > right - length)) {length = (right - left) / 2;}if (verticalEdges) {topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));drawTop = topFadeStrength * fadeHeight > 1.0f;bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));drawBottom = bottomFadeStrength * fadeHeight > 1.0f;}if (horizontalEdges) {leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));drawLeft = leftFadeStrength * fadeHeight > 1.0f;rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));drawRight = rightFadeStrength * fadeHeight > 1.0f;}saveCount = canvas.getSaveCount();int solidColor = getSolidColor();if (solidColor == 0) {final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;if (drawTop) {canvas.saveLayer(left, top, right, top + length, null, flags);}if (drawBottom) {canvas.saveLayer(left, bottom - length, right, bottom, null, flags);}if (drawLeft) {canvas.saveLayer(left, top, left + length, bottom, null, flags);}if (drawRight) {canvas.saveLayer(right - length, top, right, bottom, null, flags);}} else {scrollabilityCache.setFadeColor(solidColor);}// Step 3, draw the contentif (!dirtyOpaque) onDraw(canvas);// Step 4, draw the childrendispatchDraw(canvas);// Step 5, draw the fade effect and restore layersfinal Paint p = scrollabilityCache.paint;final Matrix matrix = scrollabilityCache.matrix;final Shader fade = scrollabilityCache.shader;if (drawTop) {matrix.setScale(1, fadeHeight * topFadeStrength);matrix.postTranslate(left, top);fade.setLocalMatrix(matrix);p.setShader(fade);canvas.drawRect(left, top, right, top + length, p);}if (drawBottom) {matrix.setScale(1, fadeHeight * bottomFadeStrength);matrix.postRotate(180);matrix.postTranslate(left, bottom);fade.setLocalMatrix(matrix);p.setShader(fade);canvas.drawRect(left, bottom - length, right, bottom, p);}if (drawLeft) {matrix.setScale(1, fadeHeight * leftFadeStrength);matrix.postRotate(-90);matrix.postTranslate(left, top);fade.setLocalMatrix(matrix);p.setShader(fade);canvas.drawRect(left, top, left + length, bottom, p);}if (drawRight) {matrix.setScale(1, fadeHeight * rightFadeStrength);matrix.postRotate(90);matrix.postTranslate(right, top);fade.setLocalMatrix(matrix);p.setShader(fade);canvas.drawRect(right - length, top, right, bottom, p);}canvas.restoreToCount(saveCount);// Overlay is part of the content and draws beneath Foregroundif (mOverlay != null && !mOverlay.isEmpty()) {mOverlay.getOverlayView().dispatchDraw(canvas);}// Step 6, draw decorations (foreground, scrollbars)onDrawForeground(canvas);}

官方的注释写得好详细，来翻译一下就行了，绘制主要分为6个步骤：
1. 绘制背景
2. 保存画布图层
3. 绘制视图
4. 绘制视图的子控件
5. 绘制渐变边缘，恢复刚才保存的画布图层
6. 绘制前景或者进度条等装饰

总结

经过XML布局文件的加载和WindowManager对Window的操作，经过测量大小，计算位置，绘制三个步骤最后展现给了用户。