private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {int childWidthMeasureSpec;int childHeightMeasureSpec;boolean windowSizeMayChange = false;if (DEBUG_ORIENTATION || DEBUG_LAYOUT) Log.v(TAG,"Measuring " + host + " in display " + desiredWindowWidth+ "x" + desiredWindowHeight + "...");boolean goodMeasure = false;if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) { // 在WRAP_CONTENT的情况下，先从一个prefered值开始measure// On large screens, we don't want to allow dialogs to just// stretch to fill the entire width of the screen to display// one line of text.  First try doing the layout at a smaller// size to see if it will fit.final DisplayMetrics packageMetrics = res.getDisplayMetrics();res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true);int baseSize = 0;if (mTmpValue.type == TypedValue.TYPE_DIMENSION) {baseSize = (int)mTmpValue.getDimension(packageMetrics);}if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": baseSize=" + baseSize);if (baseSize != 0 && desiredWindowWidth > baseSize) { // 如果baseSize真小的话，用baseSize先measure一遍试试childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("+ host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {goodMeasure = true; // measure的结果合适} else {// Didn't fit in that size... try expanding a bit.baseSize = (baseSize+desiredWindowWidth)/2; // 加大baseSize重新执行上述过程if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": next baseSize="+ baseSize);childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("+ host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {if (DEBUG_DIALOG) Log.v(TAG, "Good!");goodMeasure = true;}}}}if (!goodMeasure) { // 如果用baseSize measure的结果不合适，则老老实实用提供的参数重新measure一遍childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {windowSizeMayChange = true;}}if (DBG) {System.out.println("======================================");System.out.println("performTraversals -- after measure");host.debug();}return windowSizeMayChange;}/*** Figures out the measure spec for the root view in a window based on it's* layout params.** @param windowSize*            The available width or height of the window** @param rootDimension*            The layout params for one dimension (width or height) of the*            window.** @return The measure spec to use to measure the root view.*/private static int getRootMeasureSpec(int windowSize, int rootDimension) { // 通过具体的windowSize和提供的specint measureSpec;                                                       // 构造一个合适的Root MeasureSpecswitch (rootDimension) {  // 这里的windowSize就是设备的宽、高，rootDimension就是xml文件里指定的layoutparamcase ViewGroup.LayoutParams.MATCH_PARENT:// Window can't resize. Force root view to be windowSize. 设置root view就是window这么大measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);break;case ViewGroup.LayoutParams.WRAP_CONTENT:// Window can resize. Set max size for root view. 设置root view最多是window这么大measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);break;default:// Window wants to be an exact size. Force root view to be that size. 某一个具体的大小measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);break;}return measureSpec;}

    private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");try {            // 调用root view的measure方法，从此进入到view层次结构，顺便也把MeasureSpec带了进去。。。mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);} finally {Trace.traceEnd(Trace.TRACE_TAG_VIEW);}}

/*** <p>* This is called to find out how big a view should be. The parent* supplies constraint information in the width and height parameters.* </p>** <p>* The actual measurement work of a view is performed in* {@link #onMeasure(int, int)}, called by this method. Therefore, only* {@link #onMeasure(int, int)} can and must be overridden by subclasses.* </p>*** @param widthMeasureSpec Horizontal space requirements as imposed by the*        parent* @param heightMeasureSpec Vertical space requirements as imposed by the*        parent** @see #onMeasure(int, int)*/public final void measure(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);if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||widthMeasureSpec != mOldWidthMeasureSpec ||heightMeasureSpec != mOldHeightMeasureSpec) {// first clears the measured dimension flagmPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;resolveRtlPropertiesIfNeeded();int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :mMeasureCache.indexOfKey(key);if (cacheIndex < 0 || sIgnoreMeasureCache) {// measure ourselves, this should set the measured dimension flag backonMeasure(widthMeasureSpec, heightMeasureSpec); // 注意这个调用，这个方法是本文的重点mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;} else {long value = mMeasureCache.valueAt(cacheIndex);// Casting a long to int drops the high 32 bits, no mask neededsetMeasuredDimension((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 developerif ((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}

    /*** <p>* Measure the view and its content to determine the measured width and the* measured height. This method is invoked by {@link #measure(int, int)} and* should be overriden by subclasses to provide accurate and efficient* measurement of their contents.* </p>** <p>* <strong>CONTRACT:</strong> When overriding this method, you* <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the* measured width and height of this view. Failure to do so will trigger an* <code>IllegalStateException</code>, thrown by* {@link #measure(int, int)}. Calling the superclass'* {@link #onMeasure(int, int)} is a valid use.* </p>** <p>* The base class implementation of measure defaults to the background size,* unless a larger size is allowed by the MeasureSpec. Subclasses should* override {@link #onMeasure(int, int)} to provide better measurements of* their content.* </p>** <p>* If this method is overridden, it is the subclass's responsibility to make* sure the measured height and width are at least the view's minimum height* and width ({@link #getSuggestedMinimumHeight()} and* {@link #getSuggestedMinimumWidth()}).* </p>** @param widthMeasureSpec horizontal space requirements as imposed by the parent.*                         The requirements are encoded with*                         {@link android.view.View.MeasureSpec}.* @param heightMeasureSpec vertical space requirements as imposed by the parent.*                         The requirements are encoded with*                         {@link android.view.View.MeasureSpec}.** @see #getMeasuredWidth()* @see #getMeasuredHeight()* @see #setMeasuredDimension(int, int)* @see #getSuggestedMinimumHeight()* @see #getSuggestedMinimumWidth()* @see android.view.View.MeasureSpec#getMode(int)* @see android.view.View.MeasureSpec#getSize(int)*/protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));}/*** <p>This method must be called by {@link #onMeasure(int, int)} to store the* measured width and measured height. Failing to do so will trigger an* exception at measurement time.</p>** @param measuredWidth The measured width of this view.  May be a complex* bit mask as defined by {@link #MEASURED_SIZE_MASK} and* {@link #MEASURED_STATE_TOO_SMALL}.* @param measuredHeight The measured height of this view.  May be a complex* bit mask as defined by {@link #MEASURED_SIZE_MASK} and* {@link #MEASURED_STATE_TOO_SMALL}.*/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; // 赋值mMeasuredHeight = measuredHeight;mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET; // 设置标志位}/*** Returns the suggested minimum height that the view should use. This* returns the maximum of the view's minimum height* and the background's minimum height* ({@link android.graphics.drawable.Drawable#getMinimumHeight()}).* <p>* When being used in {@link #onMeasure(int, int)}, the caller should still* ensure the returned height is within the requirements of the parent.** @return The suggested minimum height of the view.*/protected int getSuggestedMinimumHeight() {// 找到view的最小大小，没background的时候返回mMinHeight，这个你可以在xml中指定return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());// 否则返回mMinHeight和background的最小值里的较大者}/*** Returns the suggested minimum width that the view should use. This* returns the maximum of the view's minimum width)* and the background's minimum width*  ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).* <p>* When being used in {@link #onMeasure(int, int)}, the caller should still* ensure the returned width is within the requirements of the parent.** @return The suggested minimum width of the view.*/protected int getSuggestedMinimumWidth() {return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());}/*** Utility to return a default size. Uses the supplied size if the* MeasureSpec imposed no constraints. Will get larger if allowed* by the MeasureSpec.** @param size Default size for this view* @param measureSpec Constraints imposed by the parent* @return The size this view should be.*/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: // parent imposed的spec没指定，则用自己的值default sizeresult = size;break;case MeasureSpec.AT_MOST:case MeasureSpec.EXACTLY: // 否则不论是精确指定或是至多，都用spec提供的值result = specSize;break;}return result;}

    @Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {int count = getChildCount();// matchparent child标记final boolean measureMatchParentChildren =MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;mMatchParentChildren.clear(); // 清空列表int maxHeight = 0;int maxWidth = 0;int childState = 0;// 遍历children，初步找出maxHeight和maxWidth，顺便构造mMatchParentChildren列表for (int i = 0; i < count; i++) {final View child = getChildAt(i);            // 默认情况下，只measure非GONE的child，但你可以设置mMeasureAllChildren来打破这一限制if (mMeasureAllChildren || child.getVisibility() != GONE) {                // 调用parent，ViewGroup提供的方法。。。measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);                // 我们平时在xml文件中设置的android:layout_xxx其实就是这里的LayoutParamsfinal 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); // 添加matchparent child}}}}// 进一步调整maxWidth、maxHeight的值，考虑foreground padding、minimum height/width，还有foreground的最小值// 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());}// 设置FrameLayout自身的measuredWidth、measuredHeightsetMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),resolveSizeAndState(maxHeight, heightMeasureSpec,childState << MEASURED_HEIGHT_STATE_SHIFT));count = mMatchParentChildren.size();if (count > 1) { // 如果有matchparent child的话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) {                    // 如果是MATCH_PARENT的话，由于parent已经measure过了，所以就相当于child指定了确定值，                    // 所以用的MeasureSpec.EXACTLY。。。childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -getPaddingLeftWithForeground() - getPaddingRightWithForeground() -lp.leftMargin - lp.rightMargin,// 在算child大小的时候要去掉parent的padding，child自己指定的各种marginMeasureSpec.EXACTLY);} else {                    // 否则，根据parent的measureSpec，已经用掉的大小，child的layoutparam的信息，创建一个合适的MeasureSpecchildWidthMeasureSpec = 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);}// 用新的childWidthMeasureSpec、childHeightMeasureSpec再次measure childchild.measure(childWidthMeasureSpec, childHeightMeasureSpec);}}}

    /*** Ask one of the children of this view to measure itself, taking into* account both the MeasureSpec requirements for this view and its padding* and margins. The child must have MarginLayoutParams The heavy lifting is* done in getChildMeasureSpec.** @param child The child to measure* @param parentWidthMeasureSpec The width requirements for this view* @param widthUsed Extra space that has been used up by the parent*        horizontally (possibly by other children of the parent)* @param parentHeightMeasureSpec The height requirements for this view* @param heightUsed Extra space that has been used up by the parent*        vertically (possibly by other children of the parent)*/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 // 考虑上parent的padding和child的margin+ widthUsed, lp.width);final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin+ heightUsed, lp.height);// 这里如果child是个ViewGroup类型，则实际会递归下去。。。child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}/*** Does the hard part of measureChildren: figuring out the MeasureSpec to* pass to a particular child. This method figures out the right MeasureSpec* for one dimension (height or width) of one child view.** The goal is to combine information from our MeasureSpec with the* LayoutParams of the child to get the best possible results. For example,* if the this view knows its size (because its MeasureSpec has a mode of* EXACTLY), and the child has indicated in its LayoutParams that it wants* to be the same size as the parent, the parent should ask the child to* layout given an exact size.** @param spec The requirements for this view* @param padding The padding of this view for the current dimension and*        margins, if applicable* @param childDimension How big the child wants to be in the current*        dimension* @return a MeasureSpec integer for the child*/ public static int getChildMeasureSpec(int spec, int padding, int childDimension) {        // 这个方法是协商型的，最终结果既可能直接由spec（parent提供的），也可能由childDimension决定        // 所以我们知道了，一个View的大小不是简单的单方面决定的，而是通过一系列条件协商的结果，        // 有时会尊重parent的spec，有时会坚持自己的dimension要求int specMode = MeasureSpec.getMode(spec);int specSize = MeasureSpec.getSize(spec);int size = Math.max(0, specSize - padding); // 可用的大小int resultSize = 0;int resultMode = 0;switch (specMode) {// Parent has imposed an exact size on uscase MeasureSpec.EXACTLY: // parent说child你应该是个确定的大小if (childDimension >= 0) { // child正好设置了确定的大小resultSize = childDimension; // 让child是那个确定的大小resultMode = MeasureSpec.EXACTLY; // 设置mode为EXACTLY} else if (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size. So be it.resultSize = size; // 其他情况下都是parent spec中的大小，只是mode不同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; // 不能超过sizeresultMode = MeasureSpec.AT_MOST;}break;// Parent has imposed a maximum size on uscase MeasureSpec.AT_MOST: // parent说child你应该最大是某个值。。。if (childDimension >= 0) { // child指定确定值了，则听child的// Child wants a specific size... so be itresultSize = 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 becase MeasureSpec.UNSPECIFIED: // parent没对child的大小有啥要求if (childDimension >= 0) { // child指定了确定的值，听child的// Child wants a specific size... let him have itresultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} else if (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size... find out how big it should// beresultSize = 0;resultMode = MeasureSpec.UNSPECIFIED;} else if (childDimension == LayoutParams.WRAP_CONTENT) {// Child wants to determine its own size.... find out how// big it should beresultSize = 0;resultMode = MeasureSpec.UNSPECIFIED;}break;}return MeasureSpec.makeMeasureSpec(resultSize, resultMode);}