普通View的MeasureSpec的创建规则

2021-09-30 本文已影响0人雨来

可以先读的文章：
https://cloud.tencent.com/developer/article/1459335

我先借用两张图：

1、

image.png

2、

image.png

上面两张图的由来：(前面几个判断我是加了注释了)

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {

        //spc 传的是（自己onMeasure方法接收到 widthMeausreSpec 和 heightMeasureSpec） 这个方法是自己测量子view的时候用的

        //padding  这个参数则自己的 getPaddingLeft()和getPaddingRight()  childDimension 也是子view的 layoutParmars.width 或 layoutParmars.height

        int specMode = MeasureSpec.getMode(spec); //自己的模式
        int specSize = MeasureSpec.getSize(spec); //自己的尺寸(父亲给你的尺寸)  上有父（参考上）下有子（拿到子） 最后决定自己的高度

        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) {

                    //如果 自己的mode是 exactly 并且 子View的尺寸是一个确定的值 也就是exactly 那么 子view的尺寸就是自己的 childDimension (也就像 一个线性布局 设置固定高度为 100个dp 里面再有一个TextView高度设置成
                    // 固定高度为 30dp 那么这个 TextView的高度就是 30个dp)
                    resultSize = childDimension;

                    //如果 自己的mode 是exactly 并且子view的 尺寸是一个具体的值并且大于0 那么 子view的 mode 就是  MeasureSpec.EXACTLY     resultMode = MeasureSpec.EXACTLY;
                    resultMode = MeasureSpec.EXACTLY;
                } else if (childDimension == LayoutParams.MATCH_PARENT) {
                    //子view的的 尺寸  如果 父mode 是exactly(确切的值 如 100dp ) 那么子view的 尺寸LayoutParams.MATCH_PARENT 那么 子view的尺寸就是 size 子view的尺寸是父view给自己的自己的参考尺寸
                    // Child wants to be our size. So be it. （孩子想成为我们的尺寸 那么给它 自己有多大就给子view多大）
                    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);
    }

通过上面的方法我们应该明天上面两张图怎么看了
这三种模式（mode）是通过父view给我们也就是自身的mode