private static final int MODE_SHIFT = 30; private static final int MODE_MASK = 0x3 << MODE_SHIFT; public static final int UNSPECIFIED = 0 << MODE_SHIFT; public static final int EXACTLY = 1 << MODE_SHIFT; public static final int AT_MOST = 2 << MODE_SHIFT; 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); } }
View#measure
计算完MeasureSpec，DecorView就该执行measure方法了。
public final void measure(int widthMeasureSpec, int heightMeasureSpec) { ··· final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT; ··· final boolean needsLayout = specChanged && (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize); if (forceLayout || needsLayout) { ··· 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 { ··· } ··· } ··· } ··· }
public static int getChildMeasureSpec(int spec, int padding, int childDimension) { //计算父View的size和mode int specMode = MeasureSpec.getMode(spec); int specSize = MeasureSpec.getSize(spec); //计算刨除padding的size 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); }
上面代码，其实通过一张表格，就能很清楚的解释转换规律
竖列代表LayoutParams的宽或高属性\横行父View传递的Mode
EXACTLY
AT_MOST
UNSPECIFIED
精确值
EXACTLY + childSize
EXACTLY + childSize
EXACTLY + childSize
match_parent
EXACTLY + parentSize
AT_MOST + parentSize
UNSPECIFIED + parentSize
warp_content
AT_MOST + parentSize
AT_MOST + parentSize
UNSPECIFIED + parentSize
View#getMeasuredState
public final int getMeasuredState() { return (mMeasuredWidth&MEASURED_STATE_MASK) | ((mMeasuredHeight>>MEASURED_HEIGHT_STATE_SHIFT) & (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT)); }
public static final int MEASURED_STATE_TOO_SMALL = 0x01000000; public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) { //计算父view传递的size和mode final int specMode = MeasureSpec.getMode(measureSpec); final int specSize = MeasureSpec.getSize(measureSpec); final int result; switch (specMode) { case MeasureSpec.AT_MOST: if (specSize < size) { //如果父View给的size小于自身测绘出的size， //则在第24位上加上measure_state标记。 result = specSize | MEASURED_STATE_TOO_SMALL; } else { result = size; } break; case MeasureSpec.EXACTLY: result = specSize; break; case MeasureSpec.UNSPECIFIED: default: result = size; } //如果child在第24为上有measure_state标记，则在result的第24位上也加上measure_state标记，然后返回result。 return result | (childMeasuredState & MEASURED_STATE_MASK); }
图为onMeasure时序图