android自定义View （一）MeasureSpec

浏览数：43 / 时间：2015年06月11日

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);
    }