UI绘制的详细步骤

1. 测量performMeasure

->view.measure
->view.onMeasure
->view.setMeasuredDimension
->setMeasuredDimensionRaw

2. 布局performLayout

->view.layout
->view.onLayout

3. 绘制performDraw

->ViewRootImpl.draw(fullRedrawNeeded)
->ViewRootImpl.drawSoftware
->view.draw(Canvas)

1. 测量performMeasure：

先看ViewRootImpl.class源码中的performMeasure方法：

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

调用了View.class的measure方法

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

//读取缓存
        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);

            ...

            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
//调用onMeasure
                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;
            }

            ...

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;

//设置缓存
        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }



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;
        }
        setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}


private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;

        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}

代码逻辑还算是比较清晰的，调用步骤一如开头总结出来的，为了更好的理解View绘制的过程，我们再来理解一个类 MeasureSpec.class

了解MeasureSpec

View的布局参数 包含 模式 + 尺寸 的信息
而MeasureSpec 作为一个32位的int值，则记录了 模式 + 尺寸 这些信息
00 000000000000000000000000000000
【前2位代表模式SpecMode，后30位代表尺寸SpecSize】

private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
3左移30位，转为二进制：
11 000000000000000000000000000000
~MODE_MASK 则为：
00 111111111111111111111111111111

• public static final int UNSPECIFIED = 0 << MODE_SHIFT;
  0左移30位，转为二进制：
  00 000000000000000000000000000000
  父容器不对View做任何限制，一般系统内部使用

• public static final int EXACTLY = 1 << MODE_SHIFT;
  1左移30位，转为二进制：
  01 000000000000000000000000000000
  父容器检测出View的大小，View的大小就是SpecSize LayoutParams match_parent 以及 固定大小

• public static final int AT_MOST = 2 << MODE_SHIFT;
  2左移30位，转为二进制：
  10 000000000000000000000000000000
  父容器制定一个可用大小，View的大小不能超过这个值，LayoutParams wrap_content

如何生成一个MeasureSpec呢？Android已经给你提供方法了，传入一个mode，一个size，makeMeasureSpec方法就会返回一个MeasureSpec给你。而通过getMode和getSize就能从传入的MeasureSpec中取出mode和size。

public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,  
        @MeasureSpecMode int mode) {
    if (sUseBrokenMakeMeasureSpec) {
        return size + mode;
    } else {
        return (size & ~MODE_MASK) | (mode & MODE_MASK);
    }
}

public static int getMode(int measureSpec) {
    return (measureSpec & MODE_MASK);
}


public static int getSize(int measureSpec) {
    return (measureSpec & ~MODE_MASK);
}

View的测量-确定DecorView的MeasureSpec

DecorView的MeasureSpec由窗口大小和自身LayoutParams共同决定，遵循如下规则：

1. LayoutParams.MATCH_PARENT ：精确模式，窗口大小
2. LayoutParams.WRAP_CONTENT ：最大模式，最大为窗口大小
3. 固定大小：精确模式，大小为LayoutParams的大小

回想刚才的分析流程，performMeasure方法里执行了mView.measure方法，measure方法中又执行了onMeasure方法，而对于我们要探究的DecorView来说，我们就要看一下继承自FrameLayout的DecorView.class是否重写了onMeasure方法，而不是只看View.class中的实现了，不同View子类对于onMeasure、onLayout、onDraw的重写，是我们的学习重点。

DecorView的onMeasure方法

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
       ...

       super.onMeasure(widthMeasureSpec, heightMeasureSpec);

       ...
}

可以看到里面还调用了super.onMeasure，那我们再来看看DecorView父类FrameLayout的onMeasure实现：

    @Override
    protected 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++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                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);
                    }
                }
            }
        }

        // Account for padding too
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // Check against our foreground's minimum height and width
        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) {
            for (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);
                }

                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }

其中，在for循环中调用了ViewGroup.class的 measureChildWithMargins方法

protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

在ViewGroup.class的 measureChildWithMargins方法之中，以及在FrameLayout.class的setMeasuredDimension方法之后，也都调用了ViewGroup.class的getChildMeasureSpec方法：

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

for循环之后，调用了View.class的setMeasuredDimension方法，以及setMeasuredDimensionRaw方法：

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;
}

• onMeasure方法传入的参数widthMeasureSpec，heightMeasureSpec就是当前容器的测量规格。
• for循环调用measureChildWithMargins，而这个方法就是测量FrameLayout中子控件的宽高，其中的getChildMeasureSpec方法就是获取子控件的测量规格。

getChildMeasureSpec(int spec, int padding, int childDimension)这个方法有三个参数：
spec 表示父容器的测量规格，
padding 就不说了，
childDimension 子控件的布局参数（MarginLayoutParams）对应的尺寸。
方法中根据父容器的不同mode模式，给子控件的resultSize resultMode赋值。
最后通过MeasureSpec.makeMeasureSpec(resultSize, resultMode)方法将其打包。

• for循环不是把子View都测量了个遍么，然后调用了一个方法setMeasuredDimension，这个方法是设定父控件自身的宽高的，因为父控件的宽高可能是和子控件的宽高也有关系的。

阶段总结：

View的MeasureSpec由父容器的MeasureSpec和自身的LayoutParams决定

• 子View长宽为固定，则无视父类模式，肯定为EXACTLY模式，且长宽都为childSize
• 子View长宽为match_parent，则跟随父类的模式，且长宽都为parentSize，UNSPECIFIED模式为0
• 子View长宽为wrap_content，则模式都为AT_MOST，且长宽都为暂定为parentSize
  【注：parentSize都是父控件的剩余大小，减去padding之类的长度】
  

ViewGroup
->measure
->onMeasure【通过measureChildWithMargins方法测量子控件的宽高，调用子控件的measure】
->setMeasureDimension
->setMeasureDimensionRaw【保存自己的宽高】

View
->measure
->onMeasure
->setMeasureDimension() 【getDefaultSize() 】
->setMeasureDimensionRaw

注意View.class的setMeasureDimension

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

public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
//子控件为wrap_content时
        case MeasureSpec.AT_MOST: 
//子控件为match_parent时
        case MeasureSpec.EXACTLY: 
//都赋值为父控件的specSize
            result = specSize;
            break;
        }
        return result;
}

实战技巧

自定义View时，如果不重写onMeasure，则match_parent和wrap_content的效果一样。 此时需要视情况决定是否重写onMeasure方法，另需注意，当自定义类型为容器ViewGroup时，重写onMeasure方法要测量子View宽高后再设置自身宽高，非ViewGroup的View则不需测量子View。

2. 布局performLayout

先看ViewRootImpl.class源码中的performLayout方法：

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
            int desiredWindowHeight) {
        mLayoutRequested = false;
        mScrollMayChange = true;
        mInLayout = true;
//这个mView就是顶层View
        final View host = mView;
        if (host == null) {
            return;
        }
      
        try {
            host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

            ...
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        mInLayout = false;
    }

看见 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());，追踪一下View.class的layout方法：

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);
            ...
        }
      ...
}

可以看到有一处调用setFrame方法，就是对mLeft，mTop，mBottom，mRight进行赋值，确定了这四个值，View的位置可以说就固定下来了。然后调用了onLayout(changed, l, t, r, b)，但是点进去发现View.class的onLayout方法是空方法，也就是说是留给子类重写的。如果我们自定义的是容器类型的，则需在onLayout里摆放子View的位置，如果我们自定义的是View，则无需重写。
距离FrameLayout：

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

//遍历子控件对子控件进行摆放，并调用子View的layout方法，形成递归操作
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);
            }
        }
    }

阶段总结

• ViewGroup
  ->layout【确定自己的位置，4个点的位置】
  ->onLayout【负责子View的布局，自定义时需要重写】

• View
  ->layout【确定自己的位置，4个点的位置】

3. 绘制performDraw

先看ViewRootImpl.class源码

private void performDraw() {
        ...
        try {
            boolean canUseAsync = draw(fullRedrawNeeded);
            if (usingAsyncReport && !canUseAsync) {
                mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
                usingAsyncReport = false;
            }
        } finally {
            mIsDrawing = false;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        ...
    }




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


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

这时候，代码追踪draw方法来到了View.class，通过观察注释即可得知draw的绘制步骤
* 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)

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);

            drawAutofilledHighlight(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);

            // Step 7, draw the default focus highlight
            drawDefaultFocusHighlight(canvas);

            if (debugDraw()) {
                debugDrawFocus(canvas);
            }

            // we're done...
            return;
        }

        ...
    }

因为涉及到子View的绘制，我们看一下ViewGroup.class中对dispatchDraw的实现

    @Override
    protected void dispatchDraw(Canvas canvas) {
        ...
//遍历子控件，通过drawChild绘制子控件
        for (int i = 0; i < childrenCount; i++) {
            while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
                final View transientChild = mTransientViews.get(transientIndex);
                if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                        transientChild.getAnimation() != null) {
                    more |= drawChild(canvas, transientChild, drawingTime);
                }
                transientIndex++;
                if (transientIndex >= transientCount) {
                    transientIndex = -1;
                }
            }

            final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
            final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
            if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                more |= drawChild(canvas, child, drawingTime);
            }
        }
        ... 
}


protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        return child.draw(canvas, this, drawingTime);
}

阶段总结

• ViewGroup
  ->绘制背景drawBackground(Canvas)
  -> 绘制自己onDraw(Canvas)【自定义时需重写】
  -> 绘制子View dispatchDraw(Canvas)
  -> 绘制前景，滚动条等装饰 onDrawForeground(Canvas)

• View
  ->绘制背景drawBackground(Canvas)
  -> 绘制自己onDraw(Canvas)【自定义时需重写】
  -> 绘制前景，滚动条等装饰 onDrawForeground(Canvas)

总结

自定义View时

• ViewGroup
  -> onMeasure
  -> onLayout
  -> onDraw【可选，例如容器中装载的都是系统控件，则不用重写】

• View
  -> onMeasure
  -> onDraw【可选】