use onLayout implements LinearLayout in vertical orientation

import android.content.Context;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewGroup;

public class MyLayout extends ViewGroup {
    public MyLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    //onMeasure被Android在onLayout之前调用。
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        //所有子View加起来总的高度和宽度。
        int measuredWidth = 0;
        int measuredHeight = 0;

        int count = getChildCount();
        for (int i = 0; i < count; i++) {
            View v = this.getChildAt(i);
            if (v.getVisibility() != View.GONE) {
                this.measureChild(v, widthMeasureSpec, heightMeasureSpec);

                measuredWidth = Math.max(measuredWidth, v.getMeasuredWidth());
                measuredHeight += v.getMeasuredHeight();
            }
        }

        //仔细检查！不要疏忽掉一些padding的值。
        measuredWidth += getPaddingLeft() + getPaddingRight();
        measuredHeight += getPaddingTop() + getPaddingBottom();

        this.setMeasuredDimension(measuredWidth, measuredHeight);
    }

    // Android系统在onMeasure之后调用onLayout。
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        // 在left，top，right，bottom这四个坐标参数“框”出来一个盛放子View的矩形空间，在此空间内一个一个摆放子View。

        int count = this.getChildCount();
        for (int i = 0; i < count; i++) {
            View v = getChildAt(i);
            if (v.getVisibility() != View.GONE) {
                int childHeight = v.getMeasuredHeight();

                //在细分的(left, top, right, top + childHeight)框出来的矩形空间内摆放这一子View。
                v.layout(left, top, right, top + childHeight);

                //把顶点的锚定位置往下移。
                top = top + childHeight;
            }
        }
    }
}