void setLayoutParams(WindowManager.LayoutParams attrs, boolean newView) {
        synchronized (this) {
            ......
            mWindowAttributesChanged = true;
            scheduleTraversals();
        }
    }

然後再看看，scheduleTraversals的內容，出現了一個關鍵的類-Choreographer

   void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            if (!mUnbufferedInputDispatch) {
                scheduleConsumeBatchedInput();
            }
            notifyRendererOfFramePending();
            pokeDrawLockIfNeeded();
        }
    }
 

這個類的出現是由歷史的，這個關鍵的類是做什麼用的，可以轉到這裡仔細看看，http://blog.csdn.net/farmer_cc/article/details/18619429，裡面講得非常詳細，而且動畫和輸入事件等都會通過這種方式交給Choreographer來處理。繼續往下分析，當時機成熟之後，Choreographer會在doCallbacks呼叫這裡的TraversalRunnable的run方法

    final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            doTraversal();
        }
    }
    final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
 

   void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }

            performTraversals();

            if (mProfile) {
                Debug.stopMethodTracing();
                mProfile = false;
            }
        }
    }
 

關鍵程式碼來了，performTraversals完成measure、layout、draw的操作，這三個操作又分別呼叫onMessure、onLayout、onDraw。下面performTraversals程式碼，程式碼非常長，只顯示performMessure、performLayout、performDraw程式碼，和上面過程對應。

   private void performTraversals() {
        ......

        int desiredWindowWidth;
        int desiredWindowHeight;

        final int viewVisibility = getHostVisibility();
        boolean viewVisibilityChanged = mViewVisibility != viewVisibility
                || mNewSurfaceNeeded;

        WindowManager.LayoutParams params = null;
        if (mWindowAttributesChanged) {
            mWindowAttributesChanged = false;
            surfaceChanged = true;
            params = lp;
        }
        CompatibilityInfo compatibilityInfo = mDisplayAdjustments.getCompatibilityInfo();
        if (compatibilityInfo.supportsScreen() == mLastInCompatMode) {
            params = lp;
            mFullRedrawNeeded = true;
            mLayoutRequested = true;
            if (mLastInCompatMode) {
                params.privateFlags &= ~WindowManager.LayoutParams.PRIVATE_FLAG_COMPATIBLE_WINDOW;
                mLastInCompatMode = false;
            } else {
                params.privateFlags |= WindowManager.LayoutParams.PRIVATE_FLAG_COMPATIBLE_WINDOW;
                mLastInCompatMode = true;
            }
        }

        mWindowAttributesChangesFlag = 0;

        Rect frame = mWinFrame;
        if (mFirst) {
            mFullRedrawNeeded = true;
            mLayoutRequested = true;

            if (lp.type == WindowManager.LayoutParams.TYPE_STATUS_BAR_PANEL
                    || lp.type == WindowManager.LayoutParams.TYPE_INPUT_METHOD) {
                // NOTE -- system code, won't try to do compat mode.
                Point size = new Point();
                mDisplay.getRealSize(size);
                desiredWindowWidth = size.x;
                desiredWindowHeight = size.y;
            } else {
                DisplayMetrics packageMetrics =
                    mView.getContext().getResources().getDisplayMetrics();
                desiredWindowWidth = packageMetrics.widthPixels;
                desiredWindowHeight = packageMetrics.heightPixels;
            }

            // We used to use the following condition to choose 32 bits drawing caches:
            // PixelFormat.hasAlpha(lp.format) || lp.format == PixelFormat.RGBX_8888
            // However, windows are now always 32 bits by default, so choose 32 bits
            mAttachInfo.mUse32BitDrawingCache = true;
            mAttachInfo.mHasWindowFocus = false;
            mAttachInfo.mWindowVisibility = viewVisibility;
            mAttachInfo.mRecomputeGlobalAttributes = false;
            viewVisibilityChanged = false;
            mLastConfiguration.setTo(host.getResources().getConfiguration());
            mLastSystemUiVisibility = mAttachInfo.mSystemUiVisibility;
            // Set the layout direction if it has not been set before (inherit is the default)
            if (mViewLayoutDirectionInitial == View.LAYOUT_DIRECTION_INHERIT) {
                host.setLayoutDirection(mLastConfiguration.getLayoutDirection());
            }
            host.dispatchAttachedToWindow(mAttachInfo, 0);
            mAttachInfo.mTreeObserver.dispatchOnWindowAttachedChange(true);
            dispatchApplyInsets(host);
            //Log.i(TAG, "Screen on initialized: " + attachInfo.mKeepScreenOn);

        } else {
            desiredWindowWidth = frame.width();
            desiredWindowHeight = frame.height();
            if (desiredWindowWidth != mWidth || desiredWindowHeight != mHeight) {
                if (DEBUG_ORIENTATION) Log.v(TAG,
                        "View " + host + " resized to: " + frame);
                mFullRedrawNeeded = true;
                mLayoutRequested = true;
                windowSizeMayChange = true;
            }
        }

        ......

            final HardwareRenderer hardwareRenderer = mAttachInfo.mHardwareRenderer;
            if (hardwareRenderer != null && hardwareRenderer.isEnabled()) {
                if (hwInitialized
                        || mWidth != hardwareRenderer.getWidth()
                        || mHeight != hardwareRenderer.getHeight()) {
                    hardwareRenderer.setup(mWidth, mHeight, mAttachInfo,
                            mWindowAttributes.surfaceInsets);
                    if (!hwInitialized) {
                        hardwareRenderer.invalidate(mSurface);
                        mFullRedrawNeeded = true;
                    }
                }
            }

            if (!mStopped || mReportNextDraw) {
                boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
                        (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
					......
                     // Ask host how big it wants to be
                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

                    // Implementation of weights from WindowManager.LayoutParams
                    // We just grow the dimensions as needed and re-measure if
                    // needs be
                    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);
                        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                    }

                    layoutRequested = true;
                }
            }
        } else {
            ......
        }

        final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);
        boolean triggerGlobalLayoutListener = didLayout
                || mAttachInfo.mRecomputeGlobalAttributes;
        if (didLayout) {
            performLayout(lp, desiredWindowWidth, desiredWindowHeight);

            ......
        }

        ......
        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();
                }

                performDraw();
            }
        } else {
            ......
        }

        mIsInTraversal = false;
    }

上述三個過程中，measure操作最複雜，三個操作的相同點都是從父view遍歷到子view，然後呼叫操作。首先簡要的說一個meassure操作，ViewRootImpl呼叫performMessure操作，裡面呼叫的是DecorView的measure，由於DecorView其實是整合FrameLayout,最終轉化為呼叫View的measure操作。measure的流程，可以參考：http://blog.csdn.net/a553181867/article/details/51494058

   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);
        }
    }

補充說明一下measure： View的測量規格是由自身的規格和父View的規格共同決定，共三類，AT_MOST、EXACTLY、UNSPECIFIED，如果當前View的大小在佈局中是確定的，比如300dip，那麼它就是EXACTLY模式，不管父View的測量規格是咋樣的，如果當前View在佈局中是match_parent，那麼根據父類的測量規格，如果父類是EXACTLY，那麼當前view也就變成了EXACTLLY,如果父類是AT_MOST,那麼當前View也是AT_MOST,大小是父類大小，如果當前View是wrap_content，那麼不管父類是EXACTLY還是AT_MOST，子類都是AT_MOST，大小是父類大小，至於特殊的測量規格UNSPECIFIED，目前還不清楚怎麼出現的，那麼當前View在此模式下，如果是match_parent還是wrap_content都是UNSPECIFIED，預設大小是0，由於DecorView基本是EXACTLY模式因此，只要子view不是EXACTLY模式的，那麼都不會超過這個值。 至於常用佈局FrameLayout、LinearLayout、RelativeLayout效率問題，可以在網上搜索一下，同樣的層次下，基本上FrameLayout > LinearLayout > RelativeLayout，RelativeLayout 會進行2次measure操作，水平和垂直，而當LinearLayout的子View有weight屬性時，也會進行2次measure操作，不過第一次時measure沒有weight屬性的子view，第二次才是有weight屬性的子view，因此效率還是比RelativeLayout效率高。FrameLayout的基本上不會有佈局依賴的問題，一次遍歷即可。當然最影響效率還是佈局層次，所以最好是減少佈局層次。(參考：http://blog.csdn.net/guyuealian/article/details/52162774、http://www.tuicool.com/articles/uQ3MBnj) layout操作ViewGroup中沒有實現，LinearLayout、RelativeLayout、FrameLayout有具體的實現，可以自行參考。 draw操作基本上就是把View通過呼叫Canvas介面進行繪製，具體的介面繪製看下一節，這裡補充一下其他的。 從draw的繪製流程可以知道，第一步是如果背景不是透明的，就繪製背景，第二步是繪製內容，由於ViewGroup是容器，本身沒有內容，如果背景透明，onDraw方法會被跳過，直接執行，dispatchDraw方法，因此，當重寫ViewGroup的時候，最好在dispatchDraw方法中呼叫繪製.

   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);

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(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
        ......

        // Step 6, draw decorations (foreground, scrollbars)
        onDrawForeground(canvas);
    }

三、介面的具體繪製過程 在ViewRootImpl中呼叫了performDraw之後，接著呼叫draw方法，裡面有硬繪製和軟繪製,

     private void performDraw() {
        ......
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
        try {
            draw(fullRedrawNeeded);
        } finally {
            mIsDrawing = false;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }

            ......
    }

開啟硬體加速，則採用硬繪製(Android 6.0 mHardwareRenderer是ThreadedRenderer)，否則，採用軟繪製

     private void draw(boolean fullRedrawNeeded) {
                .....
                mAttachInfo.mHardwareRenderer.draw(mView, mAttachInfo, this);
            } else {
                ......

                if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                    return;
                }
            }
        }.....
    }

首先看看ThreadedRenderer的draw方法，然後是是updateRootDisplayList

  void draw(View view, AttachInfo attachInfo, HardwareDrawCallbacks callbacks) {
        attachInfo.mIgnoreDirtyState = true;

        final Choreographer choreographer = attachInfo.mViewRootImpl.mChoreographer;
        choreographer.mFrameInfo.markDrawStart();

        updateRootDisplayList(view, callbacks);

        ......
    }

   private void updateRootDisplayList(View view, HardwareDrawCallbacks callbacks) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Record View#draw()");
        updateViewTreeDisplayList(view);

        if (mRootNodeNeedsUpdate || !mRootNode.isValid()) {
            DisplayListCanvas canvas = mRootNode.start(mSurfaceWidth, mSurfaceHeight);
            try {
                final int saveCount = canvas.save();
                canvas.translate(mInsetLeft, mInsetTop);
                callbacks.onHardwarePreDraw(canvas);

                canvas.insertReorderBarrier();
                canvas.drawRenderNode(view.updateDisplayListIfDirty());
                canvas.insertInorderBarrier();

                callbacks.onHardwarePostDraw(canvas);
                canvas.restoreToCount(saveCount);
                mRootNodeNeedsUpdate = false;
            } finally {
                mRootNode.end(canvas);
            }
        }
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

     private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
            boolean scalingRequired, Rect dirty) {

        // Draw with software renderer.
        final Canvas canvas;
        try {
            final int left = dirty.left;
            final int top = dirty.top;
            final int right = dirty.right;
            final int bottom = dirty.bottom;

            canvas = mSurface.lockCanvas(dirty);

            // The dirty rectangle can be modified by Surface.lockCanvas()
            //noinspection ConstantConditions
            if (left != dirty.left || top != dirty.top || right != dirty.right
                    || bottom != dirty.bottom) {
                attachInfo.mIgnoreDirtyState = true;
            }

            // TODO: Do this in native
            canvas.setDensity(mDensity);
        } catch (Surface.OutOfResourcesException e) {
            handleOutOfResourcesException(e);
            return false;
        } catch (IllegalArgumentException e) {
            Log.e(TAG, "Could not lock surface", e);
            // Don't assume this is due to out of memory, it could be
            // something else, and if it is something else then we could
            // kill stuff (or ourself) for no reason.
            mLayoutRequested = true;    // ask wm for a new surface next time.
            return false;
        }

        try {
            if (DEBUG_ORIENTATION || DEBUG_DRAW) {
                Log.v(TAG, "Surface " + surface + " drawing to bitmap w="
                        + canvas.getWidth() + ", h=" + canvas.getHeight());
                //canvas.drawARGB(255, 255, 0, 0);
            }

            // If this bitmap's format includes an alpha channel, we
            // need to clear it before drawing so that the child will
            // properly re-composite its drawing on a transparent
            // background. This automatically respects the clip/dirty region
            // or
            // If we are applying an offset, we need to clear the area
            // where the offset doesn't appear to avoid having garbage
            // left in the blank areas.
            if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
                canvas.drawColor(0, PorterDuff.Mode.CLEAR);
            }

            dirty.setEmpty();
            mIsAnimating = false;
            mView.mPrivateFlags |= View.PFLAG_DRAWN;

            if (DEBUG_DRAW) {
                Context cxt = mView.getContext();
                Log.i(TAG, "Drawing: package:" + cxt.getPackageName() +
                        ", metrics=" + cxt.getResources().getDisplayMetrics() +
                        ", compatibilityInfo=" + cxt.getResources().getCompatibilityInfo());
            }
            try {
                canvas.translate(-xoff, -yoff);
                if (mTranslator != null) {
                    mTranslator.translateCanvas(canvas);
                }
                canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
                attachInfo.mSetIgnoreDirtyState = false;

                mView.draw(canvas);

                drawAccessibilityFocusedDrawableIfNeeded(canvas);
            } finally {
                if (!attachInfo.mSetIgnoreDirtyState) {
                    // Only clear the flag if it was not set during the mView.draw() call
                    attachInfo.mIgnoreDirtyState = false;
                }
            }
        } finally {
            try {
                surface.unlockCanvasAndPost(canvas);
            } catch (IllegalArgumentException e) {
                Log.e(TAG, "Could not unlock surface", e);
                mLayoutRequested = true;    // ask wm for a new surface next time.
                //noinspection ReturnInsideFinallyBlock
                return false;
            }

            if (LOCAL_LOGV) {
                Log.v(TAG, "Surface " + surface + " unlockCanvasAndPost");
            }
        }
        return true;
    }

Surface中呼叫unlockCanvasAndPost開始繪製啦 繪製的資料基本上都在Surface上，可以看原始碼知道，在Surface上分配了一個快取，然後有一個指標指向這個地方，每個View都會有一個Surface。 下一節是事件的分發流程，比起前面的要簡單一些。