自定义View学习笔记-2

理解MeasureSpec

MeasureSpec代表一个32位int值，高2位代表SpecMode，低30位代表SpecSize，SpecMode是指测量模式，

而SpecSize是指在某种测量模式下的规格大小；MeasureSpec里边通过将SpecMode和SpecSize打包成一个int值来避免过多的对象内存分配；

SpecMode和SpecSize也是一个int值，一组SpecMode和SpecSize可以打包为一个MeasureSpec，而一个MeasureSpec可以通过解包的形式

来得出其原始的SpecMode和SpecSize，需要注意的是这里提到的MeasureSpec是指MeasureSpec所代表的int值，而并非MeasureSpec本身

SpecMode有三个类，每一类都表示其各含义：

UNSPECIFIED

父容器不对View有任何限制，要多大给多大，这种情况一般用于系统内部，表示一个种测量的状态

EXACTLY

父容器已经检查出View所需要的精确大小，这个时候View的最终大小就是SpecSize所指定的值。它对应于LayoutParams中的match_parent和具体的

数值这两种模式

AT_MOST

父容器指定了一个可用大小即SpecSize,View的大小不能大于这个值，具体是什么值要看不同View的具体实现。它对应于LayoutParams中的wrap_content

继承ViewGroup派生特殊的Layout

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    int measuredWidth = 0;
    int measureHeight = 0;
    final int childCount = getChildCount();
    measureChildren(widthMeasureSpec, heightMeasureSpec);

    int widthSpaceSize = MeasureSpec.getSize(widthMeasureSpec);
    int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
    int heightSapceSize = MeasureSpec.getSize(heightMeasureSpec);
    int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
    //判断是否有子元素  没有设宽高为0 接着再判断是否采用了wrap_content做相应处理
    if (childCount == 0) {
        setMeasuredDimension(0, 0);
    } else if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
        final View childView = getChildAt(0);
        measuredWidth = childView.getMeasuredWidth() * childCount;
        measureHeight = childView.getMeasuredHeight();
        setMeasuredDimension(measuredWidth, measureHeight);
    } else if (heightSpecMode == MeasureSpec.AT_MOST) {
        final View childView = getChildAt(0);
        measureHeight = childView.getMeasuredHeight();
        setMeasuredDimension(widthSpaceSize, childView.getMeasuredHeight());
    } else if (widthSpecMode == MeasureSpec.AT_MOST) {
        final View childView = getChildAt(0);
        measuredWidth = childView.getMeasuredWidth() * childCount;
        setMeasuredDimension(measuredWidth, heightSapceSize);
    }
}

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    //完成子元素定位
    int childLeft = 0;
    final int childCount = getChildCount();
    mChildrenSize = childCount;

    for (int i = 0; i < childCount; i++) {
        final View childView = getChildAt(i);
        if (childView.getVisibility() != View.GONE) {
            final int childWidth = childView.getMeasuredWidth();
            mChildrenWidth = childWidth;
            childView.layout(childLeft, 0, childLeft + childWidth, childView.getMeasuredHeight());
            childLeft += childWidth;
        }
    }
}