<resources>
    <declare-styleable name="CustomView">
        <attr name="background" format="color"/> //定義屬性：名稱 + 格式類型---顏色、尺寸、整形、字符串類型
        <attr name="size" format="dimension"/>
        <attr name="AttrFirst" format="integer"/>
        <attr name="AttrSecond" format="string"/>
    </declare-styleable>
</resources>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" 
    xmlns:custom="http://schemas.android.com/apk/res-auto" // 自定義的命名空間
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context="com.example.custom.MainActivity">
    <com.example.custom.CustomView
        android:id="@+id/custom_view"
        android:layout_width="100dp"
        android:layout_height="100dp"
        custom:background="@color/colorPrimary"
        custom:size="20dp"
        custom:AttrFirst="1"
        custom:AttrSecond="CustomView"/>
</LinearLayout>

public CustomView(Context context, AttributeSet attrs) {
    super(context, attrs);
    int n = attrs.getAttributeCount(); //獲得自定義view的屬性數(shù)量
    for (int i =0; i < n; i++) {
       Log.d("CustomView", "attrs No " + i + ", name: " + attrs.getAttributeName(i) + ", value: " + attrs.getAttributeValue(i));
    }
    int custon_background = attrs.getAttributeIntValue("http://schemas.android.com/apk/res-auto", "background", Color.BLUE);
    int custom_size = attrs.getAttributeIntValue("http://schemas.android.com/apk/res-auto", "size", 10);
    int AttrFirst = attrs.getAttributeIntValue("http://schemas.android.com/apk/res-auto", "AttrFirst", 100));
    String AttrSecond = attrs.getAttributeValue("http://schemas.android.com/apk/res-auto", "AttrSecond"));
}

public class CustomView extends View {
    private Paint mPaint;
    private int mHeight;
    private int mWidth;
    private int custom_size;
    private float scale = 1f;
    private final int SIZE = 30;
    public CustomView(Context context) {
        this(context, null);
    }
    public CustomView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }
    public CustomView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.CustomView, defStyleAttr, R.style.AppTheme); //通過obtainStyledAttributes()方法獲取屬性值
        int custon_background = a.getColor(R.styleable.CustomView_background, Color.BLUE); 
        custom_size = a.getDimensionPixelSize(R.styleable.CustomView_size, 10);
        int AttrFirst = arr.getInt(R.styleable.CustomView_AttrFirst, 100);
        String AttrSecond = arr.getString(R.styleable.CustomView_AttrSecond);
        a.recycle();
        mPaint = new Paint();
        mPaint.setColor(custom_background);
        mPaint.setAntiAlias(true);
        mPaint.setStyle(Paint.Style.FILL);
    }
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { //計算出實際的高(mMeasuredHeight)和寬(mMeasureWidth)傳入setMeasuredDimension()方法完成單個View的測量
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        int measuredHeight, measuredWidth;
        if (widthMode == MeasureSpec.EXACTLY) {
            measuredWidth = widthSize;
        } else {
            measuredWidth = SIZE;
        }
        if (heightMode == MeasureSpec.EXACTLY) {
            measuredHeight = heightSize;
        } else {
            measuredHeight = SIZE;
        }
        setMeasuredDimension(measuredWidth, measuredHeight);
    }
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) { //onLayout決定具體View的大小和位置, 對當(dāng)前視圖和其所有子View設(shè)置它們在父視圖中具體位置
        super.onLayout(changed, left, top, right, bottom);
        mHeight = getHeight(); //getWidth,和getLeft等這些函數(shù)都是View相對于其父View的位置。而getMeasuredWidth,getMeasuredHeight是測量后該View的實際值
        mWidth = getWidth();
    }
    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawCircle(mWidth/2, mHeight/2, custom_size * scale, mPaint);
    }
    private ValueAnimator mAnimator;
    public void startAnimation() {
        mAnimator = ValueAnimator.ofFloat(1, 2);
        mAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                scale = (float) animation.getAnimatedValue();
                postInvalidate(); 
            }
        });
        mAnimator.setRepeatCount(-1); //重復(fù)次數(shù)，-1表示無限循環(huán)   
        mAnimator.setRepeatMode(ValueAnimator.REVERSE); //重復(fù)模式：RESTART---重新開始，REVERSE---恢復(fù)初始狀態(tài)再開始
        mAnimator.start();
    }
    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
    }
    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        mAnimator.end();  //關(guān)閉動畫
    }
    @Override
    protected Parcelable onSaveInstanceState() {
        return super.onSaveInstanceState();
    }
    @Override
    protected void onRestoreInstanceState(Parcelable state) {
        super.onRestoreInstanceState(state);
    }
}  

public class View implements Drawable.Callback, KeyEvent.Callback,
        AccessibilityEventSource {
    ...
    public void requestLayout() {
        if (mMeasureCache != null) mMeasureCache.clear();
        if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
            // Only trigger request-during-layout logic if this is the view requesting it,
            // not the views in its parent hierarchy
            ViewRootImpl viewRoot = getViewRootImpl();
            if (viewRoot != null && viewRoot.isInLayout()) {
                if (!viewRoot.requestLayoutDuringLayout(this)) {
                    return;
                }
            }
            mAttachInfo.mViewRequestingLayout = this;
        }
        mPrivateFlags |= PFLAG_FORCE_LAYOUT;
        mPrivateFlags |= PFLAG_INVALIDATED;
        if (mParent != null && !mParent.isLayoutRequested()) {
            mParent.requestLayout(); //ViewParent接口的requestLayout()方法
        }
        if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
            mAttachInfo.mViewRequestingLayout = null;
        }
    }
    public final void measure(int widthMeasureSpec, int heightMeasureSpec) { //為整個View樹計算實際的大小, 需要遞歸的去計算每一個子視圖的大小
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
            int oHeight = insets.top  + insets.bottom;
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }
        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) 
           mMeasureCache = new LongSparseLongArray(2);
        final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
        final boolean isExactly   = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY && MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
        final boolean matchingSize = isExactly &&
                getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec) &&
                getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
        if (forceLayout || !matchingSize && (widthMeasureSpec != mOldWidthMeasureSpec || heightMeasureSpec != mOldHeightMeasureSpec)) {
            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
            resolveRtlPropertiesIfNeeded();
            int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                onMeasure(widthMeasureSpec, heightMeasureSpec);
                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            } else {
                long value = mMeasureCache.valueAt(cacheIndex);
                // Casting a long to int drops the high 32 bits, no mask needed
                setMeasuredDimensionRaw((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }
            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }
            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }
        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;
        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 | (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }
    //對于每個View的實際寬高都是由父視圖和本身視圖決定的
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { //計算出實際的高(mMeasuredHeight)和寬(mMeasureWidth)傳入setMeasuredDimension()方法完成單個View的測量
        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;
        }
        setMeasuredDimensionRaw(measuredWidth, measuredHeight);
    }
    private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;
        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
    }
    public void layout(int l, int t, int r, int b) { //為整個View樹計算實際的位置, 需要遞歸的去計算每一個子視圖的位置 
        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) {
            //調(diào)用onLayout回調(diào)方法，具體實現(xiàn)由重寫了onLayout方法的ViewGroup的子類去實現(xiàn)
            onLayout(changed, l, t, r, b); //l，t，r，b四個值是子View相對于父View的值
            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;
    }
    //onLayout被定義為抽象方法，繼承ViewGroup時必須要重寫該方法
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    }
    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 needed
        int 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 content
            if (!dirtyOpaque) onDraw(canvas);
            // Step 4, draw the children
            dispatchDraw(canvas);
            // Step 6, draw decorations (scrollbars)
            onDrawScrollBars(canvas);
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }
            // we're done...
            return;
        }
        ...
        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);
        // Step 4, draw the children
        dispatchDraw(canvas);
        // Step 5, draw the fade effect and restore layers
        final 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) {
            ...
        }
        if (drawLeft) {
            ...
        }
        if (drawRight) {
            ...
        }
        canvas.restoreToCount(saveCount);
        // Step 6, draw decorations (scrollbars)
        onDrawScrollBars(canvas);
        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas); // 核心 
        }
    }
    protected void dispatchDraw(Canvas canvas) {
    }
    public void postInvalidate() {
        postInvalidateDelayed(0);
    }
    public void postInvalidateDelayed(long delayMilliseconds) {
        // We try only with the AttachInfo because there's no point in invalidating
        // if we are not attached to our window
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            attachInfo.mViewRootImpl.dispatchInvalidateDelayed(this, delayMilliseconds);
        }
    }
    ...
}  

public final class ViewRootImpl implements ViewParent,
        View.AttachInfo.Callbacks, HardwareRenderer.HardwareDrawCallbacks {
    ...
    final class ViewRootHandler extends Handler {
        ...
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
            case MSG_INVALIDATE:
                ((View) msg.obj).invalidate();
                break;
            case MSG_INVALIDATE_RECT:
                final View.AttachInfo.InvalidateInfo info = (View.AttachInfo.InvalidateInfo) msg.obj;
                info.target.invalidate(info.left, info.top, info.right, info.bottom);
                info.recycle();
                break;
            case MSG_PROCESS_INPUT_EVENTS:
                mProcessInputEventsScheduled = false;
                doProcessInputEvents();
                break;
            case MSG_DISPATCH_APP_VISIBILITY:
                handleAppVisibility(msg.arg1 != 0);
                break;
            case MSG_DISPATCH_GET_NEW_SURFACE:
                handleGetNewSurface();
                break;
            case MSG_RESIZED: {
                // Recycled in the fall through...
                SomeArgs args = (SomeArgs) msg.obj;
                if (mWinFrame.equals(args.arg1)
                        && mPendingOverscanInsets.equals(args.arg5)
                        && mPendingContentInsets.equals(args.arg2)
                        && mPendingStableInsets.equals(args.arg6)
                        && mPendingVisibleInsets.equals(args.arg3)
                        && args.arg4 == null) {
                    break;
                }
                } // fall through...
            case MSG_RESIZED_REPORT:
                if (mAdded) {
                    ...
                    requestLayout();
                }
                break;
            case MSG_WINDOW_MOVED:
                if (mAdded) {
                    ...
                    requestLayout();
                }
                break;
            case MSG_WINDOW_FOCUS_CHANGED: {
                if (mAdded) {
                    boolean hasWindowFocus = msg.arg1 != 0;
                    mAttachInfo.mHasWindowFocus = hasWindowFocus;
                    profileRendering(hasWindowFocus);
                    if (hasWindowFocus) {
                        boolean inTouchMode = msg.arg2 != 0;
                        ensureTouchModeLocally(inTouchMode);
                        if (mAttachInfo.mHardwareRenderer != null && mSurface.isValid()){
                            mFullRedrawNeeded = true;
                            try {
                                final WindowManager.LayoutParams lp = mWindowAttributes;
                                final Rect surfaceInsets = lp != null ? lp.surfaceInsets : null;
                                mAttachInfo.mHardwareRenderer.initializeIfNeeded(
                                        mWidth, mHeight, mSurface, surfaceInsets);
                            } catch (OutOfResourcesException e) {
                                Log.e(TAG, "OutOfResourcesException locking surface", e);
                                try {
                                    if (!mWindowSession.outOfMemory(mWindow)) {
                                        Slog.w(TAG, "No processes killed for memory; killing self");
                                        Process.killProcess(Process.myPid());
                                    }
                                } catch (RemoteException ex) {
                                }
                                // Retry in a bit.
                                sendMessageDelayed(obtainMessage(msg.what, msg.arg1, msg.arg2), 500);
                                return;
                            }
                        }
                    }
                    mLastWasImTarget = WindowManager.LayoutParams
                            .mayUseInputMethod(mWindowAttributes.flags);
                    InputMethodManager imm = InputMethodManager.peekInstance();
                    if (mView != null) {
                        if (hasWindowFocus && imm != null && mLastWasImTarget &&
                                !isInLocalFocusMode()) {
                            imm.startGettingWindowFocus(mView);
                        }
                        mAttachInfo.mKeyDispatchState.reset();
                        mView.dispatchWindowFocusChanged(hasWindowFocus);
                        mAttachInfo.mTreeObserver.dispatchOnWindowFocusChange(hasWindowFocus);
                    }
                    // Note: must be done after the focus change callbacks,
                    // so all of the view state is set up correctly.
                    if (hasWindowFocus) {
                        if (imm != null && mLastWasImTarget && !isInLocalFocusMode()) {
                            imm.onWindowFocus(mView, mView.findFocus(),
                                    mWindowAttributes.softInputMode,
                                    !mHasHadWindowFocus, mWindowAttributes.flags);
                        }
                        // Clear the forward bit.  We can just do this directly, since
                        // the window manager doesn't care about it.
                        mWindowAttributes.softInputMode &=
                                ~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
                        ((WindowManager.LayoutParams)mView.getLayoutParams())
                                .softInputMode &=
                                    ~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
                        mHasHadWindowFocus = true;
                    }
                    if (mView != null && mAccessibilityManager.isEnabled()) {
                        if (hasWindowFocus) {
                            mView.sendAccessibilityEvent(
                                    AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED);
                        }
                    }
                }
            } break;
            ...
            }
        }
    }
    final ViewRootHandler mHandler = new ViewRootHandler();
    public void dispatchInvalidateDelayed(View view, long delayMilliseconds) {
        Message msg = mHandler.obtainMessage(MSG_INVALIDATE, view);
        mHandler.sendMessageDelayed(msg, delayMilliseconds);
    }
    @Override
    public void requestLayout() {
        if (!mHandlingLayoutInLayoutRequest) {
            checkThread();
            mLayoutRequested = true; // 
            scheduleTraversals(); // mTraversalRunnable => doTraversal => performTraversals
        }
    }
    @Override
    public boolean isLayoutRequested() {
        return mLayoutRequested;
    }
    void invalidate() {
        mDirty.set(0, 0, mWidth, mHeight);
        if (!mWillDrawSoon) {
            scheduleTraversals(); // mTraversalRunnable => doTraversal => performTraversals
        }
    }
    final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            doTraversal();
        }
    }
    final TraversalRunnable mTraversalRunnable = new TraversalRunnable();  
    void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().postSyncBarrier();
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            if (!mUnbufferedInputDispatch) {
                scheduleConsumeBatchedInput();
            }
            notifyRendererOfFramePending();
        }
    }
    void unscheduleTraversals() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().removeSyncBarrier(mTraversalBarrier);
            mChoreographer.removeCallbacks(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        }
    }
    void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().removeSyncBarrier(mTraversalBarrier);
            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "performTraversals");
            try {
                performTraversals();
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_VIEW);
            }
            if (mProfile) {
                Debug.stopMethodTracing();
                mProfile = false;
            }
        }
    }
    private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try {
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); // 調(diào)用用View.java的measure()方法
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }
    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()); // 調(diào)用用View.java的layout()方法
            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;
    }
    //performDraw() => draw() => drawSoftware()
    private void performDraw() {
        if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
            return;
        }
        final boolean fullRedrawNeeded = mFullRedrawNeeded;
        mFullRedrawNeeded = false;
        mIsDrawing = true;
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
        try {
            draw(fullRedrawNeeded); 
            // draw()調(diào)用 drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)
            // private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scalingRequired, Rect dirty)
        } finally {
            mIsDrawing = false;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        // For whatever reason we didn't create a HardwareRenderer, end any
        // hardware animations that are now dangling
        if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
            final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
            for (int i = 0; i < count; i++) {
                mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
            }
            mAttachInfo.mPendingAnimatingRenderNodes.clear();
        }
        if (mReportNextDraw) {
            mReportNextDraw = false;
            if (mAttachInfo.mHardwareRenderer != null) {
                mAttachInfo.mHardwareRenderer.fence();
            }
            if (LOCAL_LOGV) {
                Log.v(TAG, "FINISHED DRAWING: " + mWindowAttributes.getTitle());
            }
            if (mSurfaceHolder != null && mSurface.isValid()) {
                mSurfaceHolderCallback.surfaceRedrawNeeded(mSurfaceHolder);
                SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
                if (callbacks != null) {
                    for (SurfaceHolder.Callback c : callbacks) {
                        if (c instanceof SurfaceHolder.Callback2) {
                            ((SurfaceHolder.Callback2)c).surfaceRedrawNeeded(
                                    mSurfaceHolder);
                        }
                    }
                }
            }
            try {
                mWindowSession.finishDrawing(mWindow);
            } catch (RemoteException e) {
            }
        }
    }
    ...
}