自定义View过程.png

用户主动调用 request，只会出发 measure 和 layout 过程，而不会执行 draw 过程

Paste_Image.png

onMeasure.png

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

    int viewWidth = circleRadius + this.getPaddingLeft() + this.getPaddingRight();
    int viewHeight = circleRadius + this.getPaddingTop() + this.getPaddingBottom();

    int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    int widthSize = MeasureSpec.getSize(widthMeasureSpec);
    int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    int heightSize = MeasureSpec.getSize(heightMeasureSpec);

    int width;
    int height;

    //Measure Width
    if (widthMode == MeasureSpec.EXACTLY) {
        //Must be this size
        width = widthSize;
    } else if (widthMode == MeasureSpec.AT_MOST) {
        //Can't be bigger than...
        width = Math.min(viewWidth, widthSize);
    } else {
        //Be whatever you want
        width = viewWidth;
    }

    //Measure Height
    if (heightMode == MeasureSpec.EXACTLY || widthMode == MeasureSpec.EXACTLY) {
        //Must be this size
        height = heightSize;
    } else if (heightMode == MeasureSpec.AT_MOST) {
        //Can't be bigger than...
        height = Math.min(viewHeight, heightSize);
    } else {
        //Be whatever you want
        height = viewHeight;
    }

    setMeasuredDimension(width, height);
}

measure的核心方法

• measure(int widthMeasureSpec,int heightMeasureSpec):是view中final方法，不能重写，但是最终都会曲调用onMeasure方法
• onMeasure(int widthMeasureSpec,int heightMeasureSpec):发方法中两个参数是父控件对子view的测量要求，在该方法中需要结合测量模式和测量大小以及边界距离值确定view的最终大小
• setMeasureDimension(int width,int height):在onMeasure中得到最终的大小之后，一定要调用这个方法进行最终的确定，否则会报错。

mCustomView.setPadding(0, top, 0, 0);等方法最终也会去调用requestLayout，调整padding的值，其中参数负数则代表反方向。