修改已有的自定义view尺寸

1.重写父类onMeasure()方法，调用super.onMeasure()触发继承的view的测量。

2.调用getMeasureWidth()和getMeasureHeight()取得到被继承的view的宽高

3.调用setMeasureDimension()保存你最后修改的结果

全新定义view的尺寸

1.不用再写重写onMeasure()了，父类帮不了你了

2.父view对子view的限制。

//在父view执行
@Override
rotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    child.measure();
    //给子view设置了约束
    child.measure(childWidthMeasureSpec,childHeightMeasureSpec);
}

//在子view执行
@Override            
rotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//上面个参数就是上面两个数值的约束
    int measureWidt = XXXX;
    int measureHeight = XXXX;   
    //这里执行resolveSize()将你计算出的新尺寸和就尺寸进行修正
    measureWidth = resolveSize(measureWidth, widthMeasureSpec);
    measureHeight =resolveSize(measureHeight, heightMeasureSpec); 
    //再次保存该尺寸
    setMeasureDimention(measureWidth,measureHeight)
    
}

3.resolveSize()方法的源码解析

public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
        final int specMode = MeasureSpec.getMode(measureSpec);
        final int specSize = MeasureSpec.getSize(measureSpec);
        final int result;
        switch (specMode) {
            case MeasureSpec.AT_MOST:
            //如果是上限这种mode
            则比较一下有没有超过上限
                if (specSize < size) {
                    //如果没越界则用用户定义的尺寸
                    result = specSize | MEASURED_STATE_TOO_SMALL;
                } else {
                    //如果越界则用原尺寸
                    result = size;
                }
                break;
            case MeasureSpec.EXACTLY:
                //精确限制尺寸这种mode 则直接忽略用户定义的尺寸
                result = specSize;
                break;
            case MeasureSpec.UNSPECIFIED:
            //未定义，不执行任何的操作
            default:
                result = size;
        }
        return result | (childMeasuredState & MEASURED_STATE_MASK);
    }