Android应用上面的View显示出来都必须经过测量，布局，和绘制这三个过程。我们知道PhoneWindow中的DecorView是界面最顶层的View，那么，最先绘制的View肯定是DecorView。在ViewRootImpl中的performTraversals方法中中，依次对DecorView进行测量，布局，和绘制：
private void performTraversals() {…performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);…performLayout(lp, desiredWindowWidth, desiredWindowHeight);…performDraw();… }
最先开始对DecorView进行测量，然后对其子View进行测量，一直到最上层的View，直到所有的View都测量完毕。在ViewRootImpl中对DecorView进行测量时，调用的是performMeasure方法：
(int childWidthMeasureSpec, int childHeightMeasureSpec) {Trace.traceBegin(Trace.TRACE_TAG_VIEW, “measure”);try {mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);} finally {Trace.traceEnd(Trace.TRACE_TAG_VIEW);}}
在performMeasure方法里面，调用了View的measure方法:
(int widthMeasureSpec, int heightMeasureSpec) {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 byteslong 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 flagmPrivateFlags &= ~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 backonMeasure(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 neededsetMeasuredDimensionRaw((int) (value >> 32), (int) value);mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;}((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}
这个方法是final修饰，子类不能重写。在它的33行，调用了onMeasure(widthMeasureSpec, heightMeasureSpec)，找到onMeasure方法：
(int widthMeasureSpec, int heightMeasureSpec) {setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));}
这个方法子类是可以重写的，DecorView重写了方法，并在里面调用 super.onMeasure(widthMeasureSpec, heightMeasureSpec)方法，DecorView继承自FrameLayout，那么会调用FrameLayout的onMeasure方法：
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 toomaxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();// Check against our minimum height and widthmaxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());// Check against our foreground’s minimum height and widthfinal 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();int childWidthMeasureSpec;int childHeightMeasureSpec;if (lp.width == LayoutParams.MATCH_PARENT) {childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -getPaddingLeftWithForeground() – getPaddingRightWithForeground() -lp.leftMargin – lp.rightMargin,MeasureSpec.EXACTLY);} else {childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,getPaddingLeftWithForeground() + getPaddingRightWithForeground() +lp.leftMargin + lp.rightMargin,lp.width);}if (lp.height == LayoutParams.MATCH_PARENT) {childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -getPaddingTopWithForeground() – getPaddingBottomWithForeground() -lp.topMargin – lp.bottomMargin,MeasureSpec.EXACTLY);} else {childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,getPaddingTopWithForeground() + getPaddingBottomWithForeground() +lp.topMargin + lp.bottomMargin,lp.height);}child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}}}发光并非太阳的专利，你也可以发光
