A MeasureSpec encapsulates the layout requirements passed from parent to child. Each MeasureSpec represents a requirement
for either the width or the height. A MeasureSpec is comprised of a size and a mode. There are three possible modes:
- UNSPECIFIED
- The parent has not imposed any constraint on the child. It can be whatever size it wants.
- EXACTLY
- The parent has determined an exact size for the child. The child is going to be given those bounds regardless of how big it wants to be.
- AT_MOST
- The child can be as large as it wants up to the specified size.
1.使用 View.resolveSize(int size,int measureSpec)
public static int resolveSize(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; case MeasureSpec.AT_MOST: result = Math.min(size, specSize); break; case MeasureSpec.EXACTLY: result = specSize; break; } return result; }
2.源码
public static class MeasureSpec { 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(int size, int mode) { return size + mode; } public static int getMode(int measureSpec) { return (measureSpec & MODE_MASK); } public static int getSize(int measureSpec) { return (measureSpec & ~MODE_MASK); } public static String toString(int measureSpec) { int mode = getMode(measureSpec); int size = getSize(measureSpec); StringBuilder sb = new StringBuilder("MeasureSpec: "); if (mode == UNSPECIFIED) sb.append("UNSPECIFIED "); else if (mode == EXACTLY) sb.append("EXACTLY "); else if (mode == AT_MOST) sb.append("AT_MOST "); else sb.append(mode).append(" "); sb.append(size); return sb.toString(); } }
3.示例ListView.measureItem(View child)
private void measureItem(View child) { ViewGroup.LayoutParams p = child.getLayoutParams(); if (p == null) { p = new ViewGroup.LayoutParams( ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT); } int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec, mListPadding.left + mListPadding.right, p.width); int lpHeight = p.height; int childHeightSpec; if (lpHeight > 0) { childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY); } else { childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED); } child.measure(childWidthSpec, childHeightSpec); }