Android中View繪製流程分析

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建立Window

在Activity的attach方法中通過調用PolicyManager.makeNewWindo建立Window,將一個View add到WindowManager時，WindowManagerImpl建立一個ViewRoot來管理該視窗的根View。並通過ViewRoot.setView方法把該View傳給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為整個Window介面的最頂層View。

Activity中的Window對象幫我們建立了一個PhoneWindow內部類DecorView（父類為FrameLayout）視窗頂層視圖，然後通過LayoutInflater將xml內容布局解析成View樹形結構添加到DecorView頂層視圖中id為content的FrameLayout父容器上面。Activity的content內容布局最終會添加到DecorView視窗頂層視圖上面。

    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並關聯View

WindowManagerImpl儲存DecorView到mViews，建立對應的ViewRoot；

ViewRoot用於管理視窗的根View，並和global window manger進行互動。ViewRoot中有一個nested class： W，W是一個Binder子類，用於接收global window manager的各種訊息， 如按鍵訊息， 觸摸訊息等。 ViewRoot有一個W類型的成員mWindow，ViewRoot在Constructor中建立一個W的instance並賦值給mWindow。 ViewRoot是Handler的子類， W會通過Looper把訊息傳遞給ViewRoot。 ViewRoot在setView方法中把mWindow傳給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管理系統與GUI呈現系統之間的橋樑，需要注意它並不是一個View類型，。
它的主要作用如下：
1、向DecorView分發收到的使用者發起的event事件，如按鍵，觸屏，軌跡球等事件；
2、與WindowManagerService互動，完成整個Activity的GUI的繪製。

View繪製基本流程

這裡先給出Android系統View的繪製流程：依次執行View類裡面的如下三個方法：
measure(int ,int) :測量View的大小
layout(int ,int ,int ,int) ：設定子View的位置
draw(Canvas) ：繪製View內容到Canvas畫布上

整個View樹的繪圖流程是在ViewRoot.java類的performTraversals()函數展開的，該函數做的執行過程可簡單概況為根據之前設定的狀態，判斷是否需要重新計算視圖大小(measure)、是否重新需要安置視圖的位置(layout)、以及是否需要重繪 (draw)

 mesarue()測量過程

主要作用：為整個View樹計算實際的大小，即設定實際的高(mMeasuredHeight)和寬(mMeasureWidth)，每個View的控制項的實際寬高都是由父視圖和本身視圖決定的。
具體的調用如下：

ViewRootImpl 的performTraversals方法中，調用measureHierarchy，然後調用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根對象地屬性mView(其類型一般為ViewGroup類型)調用measure()方法去計算View樹的大小，回調
View/ViewGroup對象的onMeasure()方法，該方法實現的功能如下：    
1、設定本View視圖的最終大小，該功能的實現通過調用setMeasuredDimension()方法去設定實際的高(mMeasuredHeight)和寬(mMeasureWidth) 

2、如果該View對象是個ViewGroup類型，需要重寫onMeasure()方法，對其子視圖進行遍曆的measure()過程。
對每個子視圖的measure()過程，是通過調用父類ViewGroup.java類裡的measureChildWithMargins()方法去實現，該方法內部只是簡單地調用了View對象的measure()方法。
整個measure調用流程就是個樹形的遞迴過程
measure()方法兩個參數都是父View傳遞過來的，也就是代表了父view的規格。他由兩部分組成，高2位表示MODE，定義在MeasureSpec類（View的內部類）中，有三種類型，MeasureSpec.EXACTLY表示確定大小， MeasureSpec.AT_MOST表示最大大小， MeasureSpec.UNSPECIFIED不確定。低30位表示size，也就是父View的大小。對於系統Window類的DecorVIew對象Mode一般都為MeasureSpec.EXACTLY ，而size分別對應螢幕寬高。對於子View來說大小是由父View和子View共同決定的。

    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版面配置階段主要作用 ：為將整個根據子視圖的大小以及布局參數將View樹放到合適的位置上。
具體的調用如下：
ViewRootImpl 的performTraversals方法中，調用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()開始View樹的布局，繼而回調給View/ViewGroup類中的layout()方法。具體流程如下
1 、layout方法會設定該View視圖位於父視圖的座標軸，即mLeft，mTop，mLeft，mBottom(調用setFrame()函數去實現)，接下來回調onLayout()方法(如果該View是ViewGroup對象，需要實現該方法，對每個子視圖進行布局)。
2、如果該View是個ViewGroup類型，需要遍曆每個子視圖chiildView，調用該子視圖的layout()方法去設定它的座標值。

    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()繪圖過程ViewRootImpl 的performTraversals方法中，調用了mView的draw方法 mView.draw()開始繪製，draw()方法實現的功能如下：
1 、繪製該View的背景
2 、為顯示漸層框做一些準備操作      
3、調用onDraw()方法繪製視圖本身   (每個View都需要重載該方法，ViewGroup不需要實現該方法)
4、調用dispatchDraw ()方法繪製子視圖(如果該View類型不為ViewGroup，即不包含子視圖，不需要重載該方法)
值得說明的是，ViewGroup類已經為我們重寫了dispatchDraw ()的功能實現，應用程式一般不需要重寫該方法，但可以重載父類函數實現具體的功能。
dispatchDraw()方法內部會遍曆每個子視圖，調用drawChild()去重新回調每個子視圖的draw()方法。
5、繪製捲軸
重新整理視圖Android中實現view的更新有兩個方法，一個是invalidate，另一個是postInvalidate，其中前者是在UI線程自身中使用，而後者在非UI線程中使用。 
requestLayout()方法 ：會導致調用measure()過程 和 layout()過程 。
 說明：只是對View樹重新布局layout過程包括measure()和layout()過程，不會調用draw()過程，但不會重新繪製
任何視圖包括該調用者本身。

一般引起invalidate()操作的函數如下：
1、直接調用invalidate()方法，請求重新draw()，但只會繪製調用者本身。
2、setSelection()方法 ：請求重新draw()，但只會繪製調用者本身。
3、setVisibility()方法 ： 當View可視狀態在INVISIBLE轉換VISIBLE時，會間接調用invalidate()方法，繼而繪製該View。
4 、setEnabled()方法 ： 請求重新draw()，但不會重新繪製任何視圖包括該調用者本身。
 

內容參考源碼，借鑒了網上的一些分析。

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Android中View繪製流程分析