RecyclerView存在大量Item时,当滚到底部时快速滑到顶部

在使用RecyclerView展示图片或者其他信息时，往往需要展示很多的Item,当滚到底部时又想回到顶部，如果一点一点的向上划去比较麻烦，而且用户体验不好。因此添加一个快速回到顶部的按钮是很有必要的，并且刚开始的时候这个按钮是隐藏的，当滑动超过超过一屏的时候才会出现，在滑动的过程中也是不会出现的。很多类似的项目都会使用到，但是如何在快速回到顶部的过程中不出现卡顿，体现的很是流畅，这点很重要。下面是我根据网上的方法自己修改的一个可以快速回到顶部的类，在此做一下备注，以便日后查看：

1. FastScrollManger.java

import android.content.Context;
import android.graphics.PointF;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.LinearSmoothScroller;
import android.support.v7.widget.RecyclerView;
import android.util.DisplayMetrics;

/**
 * Author: aaa
 * Date: 2017/3/16 11:16.
 * 快速回到RecyclerView 的顶部,不会出现卡顿
 */

public class FastScrollManger extends LinearLayoutManager {
    public FastScrollLinearLayoutManager(Context context) {
        super(context);
    }

    public FastScrollLinearLayoutManager(Context context, int orientation, boolean reverseLayout) {
        super(context, orientation, reverseLayout);
    }

    @Override
    public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {
        LinearSmoothScroller linearSmoothScroller = new LinearSmoothScroller(recyclerView.getContext()) {

            @Override
            public PointF computeScrollVectorForPosition(int targetPosition) {
                return FastScrollLinearLayoutManager.this.computeScrollVectorForPosition(targetPosition);
            }

            //控制速度。
            @Override
            protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
                return super.calculateSpeedPerPixel(displayMetrics);
            }
            @Override
            protected int calculateTimeForScrolling(int dx) {
                if (dx > 3000) {
                    dx = 3000;
                }

                int time = super.calculateTimeForScrolling(dx);
                return time;
            }
        };

        linearSmoothScroller.setTargetPosition(position);
        startSmoothScroll(linearSmoothScroller);
    }
}

• 2.使用

mRecyclerView = (RecyclerView) findViewById(R.id.activity_recyclerview);
        //LinearLayoutManager layout = new FastScrollManager(CustomActivity.this, LinearLayoutManager.VERTICAL, false);
        //mRecyclerView.setLayoutManager(layout);//竖直放置 （我用不上，看各自对RecyclerView的设置谨慎使用）
        ...
        mRecyclerView.setAdapter(mMyAdapter);
        mRecyclerView.addOnScrollListener(new MyRecyclerViewScrollListener());

3.控制显示与隐藏

//滑动监听
    private class MyRecyclerViewScrollListener extends RecyclerView.OnScrollListener {
        @Override
        public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
            super.onScrollStateChanged(recyclerView, newState);
            LinearLayoutManager manager = (LinearLayoutManager) recyclerView.getLayoutManager();
            int firstVisibleItemPosition = manager.findFirstVisibleItemPosition();
            // 当不滚动时
            if (newState == RecyclerView.SCROLL_STATE_IDLE) {
                // 判断是否滚动超过一屏
                if (firstVisibleItemPosition == 0) {
                    mImageViewRebackTop.setVisibility(View.INVISIBLE);
                } else {
                    mImageViewRebackTop.setVisibility(View.VISIBLE);
                }

            } else if (newState == RecyclerView.SCROLL_STATE_DRAGGING) {//拖动中
                mImageViewRebackTop.setVisibility(View.INVISIBLE);
            }
        }
    }

• • 4.点击回到顶部按钮的时候，回到顶部

  mRecyclerView.smoothScrollToPosition(0);

  
  
• 
  
• 5.布局附上

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <data>

    </data>

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical" >

        <android.support.v7.widget.RecyclerView
            android:id="@+id/classify_recyclerView"
            android:background="@color/classify_color_bg"
            android:layout_width="match_parent"
            android:layout_height="match_parent">

        </android.support.v7.widget.RecyclerView>

        <ImageView
            android:id="@+id/classify_activity_img"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:src="@drawable/ic_return_top"
            android:layout_alignParentBottom="true"
            android:layout_alignParentRight="true"
            android:padding="@dimen/dp8"
            android:scaleType="centerCrop"/>

    </RelativeLayout>

</layout>