自定义一个简易的RecyclerView的LinearLayuoutManager

package com.example.myapplication1.view;

import android.support.v7.widget.RecyclerView;
import android.view.View;
import android.view.ViewGroup;



public class MyLinearLayoutManager extends RecyclerView.LayoutManager {
    @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
        return new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
    }

    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        int offsetY = 0;
        for (int i = 0; i < getItemCount(); i++) {
            View viewForPosition = recycler.getViewForPosition(i);
            addView(viewForPosition);

            measureChildWithMargins(viewForPosition,0,0);
            int measuredHeight = viewForPosition.getMeasuredHeight();
            int measuredWidth = viewForPosition.getMeasuredWidth();

            layoutDecorated(viewForPosition,0,offsetY,measuredWidth,offsetY+measuredHeight);
            offsetY += measuredHeight;
        }
    }


    @Override
    public boolean canScrollVertically() {
        return true;
    }

    /**
     *
     * @param dy  往下滑动 dy < 0   ,往上滑动， dy > 0
     * @param recycler
     * @param state
     * @return
     */
    @Override
    public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {

        View firstView = getChildAt(0);
        int top = firstView.getTop();

        View lastView = getChildAt(getChildCount() - 1);
        int bottom = lastView.getBottom();
        //RecyclerView的实际可容纳空间的高度
        int height = getHeight() - getPaddingBottom() - getPaddingTop();

        //将偏移量转换成符合逻辑的，即：向下为正，向上为负
        int newDy = -dy;

        //顶部：
        //第一个view的顶部与RecyclerView的顶部刚好重合，不能再往下滑动了，newDy >0 代表向下滑动
        if (top == 0 && newDy > 0){
            return 0;
        }

        //top <0 说明recyclerView中的内容已经向上滑动一段距离
        //如果（top + newDy，即新的top）>0 就让它等于0，让它等于0的条件，就是 newDy = -top
        if (top < 0 && top + newDy > 0){
            newDy = -top;
        }

        //底部：== 0，最后一个View的底边 与RecyclerView的底边已重合，不能再往上滑动了
        //底部, < 0 的情况是内容过少 未充满RecyclerView，就是 最后一个itemView 的bottom 天生比 RecyclerView的 height 小
        if(bottom <= height && newDy <0){
            return 0;
        }

        //已经超出了RecyclerView的底边，但是要保证不能小于滑动到底边里面（即上部）
        //bottom + newDy < height 时，bottom + newDy = height，所以 newDy = height - bottom = -(bottom - height)
        if (bottom > height && bottom + newDy < height){
            newDy = - (bottom - height);
        }

        offsetChildrenVertical(newDy);
        return dy;
    }
}