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;    }}
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