RecyclerView自动滑动

本章目录

• Part One：改变滑动速度
• Part Two：自动滑动

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RecyclerView的自动滑动本质上和我们先前讲的Android定时任务区别不大，都是设定一个时间段，然后执行下一步命令，只不过这次把定时执行的方法换成了smoothScrollToPosition。
顾名思义，smoothScrollToPosition翻译过来就是平滑地滚动到指定位置，也就是说RecyclerView通过调用smoothScrollToPosition可以滑动到指定的item。
但是smoothScrollToPosition有一个缺点，那就是尽管号称是平滑地滚动，实际来看，默认的滑动速度还是太快了。
打开RecyclerView的smoothScrollToPosition，可以看到注释有这么一句话：

smoothScrollToPosition注释.png
红框部分翻译过来就是LayoutManager是真正负责滚动操作的，如果想要自定义一个滚动逻辑，请重写LayoutManager的smoothScrollToPosition方法。
那么，具体该怎么做呢？我们一步一步来。

Part One：改变滑动速度

1. 自定义LayoutManager
   由于我们使用的是横向的LinearLayoutManager，所以我们要创建一个类继承它，并重写三个参数的构造方法。

package com.terana.mycustomview.customview;

import android.content.Context;
import android.support.v7.widget.LinearLayoutManager;

public class CustomLinearLayoutManager extends LinearLayoutManager{
    private float moveSpeedPerInch= 25f;//默认移动速度

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

2. 重写smoothScrollToPosition
   按着官方的指示，我们重写了smoothScrollToPosition方法，点到super看看父类的实现，发现除了需要自定义一个LinearSmoothScroller外，其它代码可不变。
   重写了LinearSmoothScroller后，我们需要重写一个calculateSpeedPerPixel方法，点开父类的实现看看发现，代码很简单

    /**
     * Calculates the scroll speed.
     *
     * @param displayMetrics DisplayMetrics to be used for real dimension calculations
     * @return The time (in ms) it should take for each pixel. For instance, if returned value is
     * 2 ms, it means scrolling 1000 pixels with LinearInterpolation should take 2 seconds.
     */
    protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
        return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
    }

MILLISECONDS_PER_INCH 是一个固定值25f，然后除以屏幕密度，获得移动1000像素所需要的时间。
所以，我们想让时间变慢，只需把MILLISECONDS_PER_INCH 换成我们预先定义的moveSpeedPerInch变量，然后扩大倍数即可。
具体代码为：

package com.terana.mycustomview.customview;

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

public class CustomLinearLayoutManager extends LinearLayoutManager{
    private float moveSpeedPerInch = 25f;//默认移动速度

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

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

    /**
     * 调用此方法，让smoothScrollToPosition的速度比原来放慢n倍
     */
    public void setSpeedSlow(int n){
        moveSpeedPerInch = 25f * n;
    }

    private class CustomLinearSmoothScroller extends LinearSmoothScroller{

        private CustomLinearSmoothScroller(Context context) {
            super(context);
        }

        @Override
        protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
            return moveSpeedPerInch / displayMetrics.densityDpi;
        }
    }
}

3. GalleryActivity调用
   好了，方法实现了，在GalleryActivity里把先前调用的LinearLayoutManger换成我们自定义的，然后调用setSpeedSlow方法即可减慢速度了。

    private void initRecyclerView() {
        RecyclerView recyclerView = findViewById(R.id.recyclerView_gallery);
        final List<PhotoBean> datas = getFakeDatas();
        final MyRecyclerViewAdapter adapter = new MyRecyclerViewAdapter(datas, this);
        final CustomLinearLayoutManager linearLayoutManager = new CustomLinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false);
        linearLayoutManager.setSpeedSlow(5);
        recyclerView.setLayoutManager(linearLayoutManager);
        new LinearSnapHelper().attachToRecyclerView(recyclerView);
        recyclerView.setAdapter(adapter);
    }

Part Two：自动滑动

滑动速度搞定了，我们就可以着手来做RecyclerView的自动滑动了，逻辑上并不复杂，我们之前都已经说过其实。

1. 复制定时任务代码
   把MainActivity的switchContent方法复制过来，然后改名为autoMoveRecyclerView

    private void initRecyclerView() {
        RecyclerView recyclerView = findViewById(R.id.recyclerView_gallery);
        final List<PhotoBean> datas = getFakeDatas();
        final MyRecyclerViewAdapter adapter = new MyRecyclerViewAdapter(datas, this);
        final CustomLinearLayoutManager linearLayoutManager = new CustomLinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false);
        linearLayoutManager.setSpeedSlow(5);
        recyclerView.setLayoutManager(linearLayoutManager);
        new LinearSnapHelper().attachToRecyclerView(recyclerView);
        recyclerView.setAdapter(adapter);
        autoMoveRecyclerView(recyclerView, datas);
    }

    private int index = 0;
    private void autoMoveRecyclerView(final RecyclerView recyclerView, final List<PhotoBean> datas) {
        final Timer timer = new Timer();
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        if (index == datas.size()){
                            index = 0;
                            timer.cancel();
                        }
                        recyclerView.smoothScrollToPosition(index);
                        index++;
                    }
                });
            }
        }, 50, 4000);
    }

2. 手动滑动
   自动滑动做好了，万一用户想手动滑动，这样我们的index就乱了，所以要做下处理，给RecyclerView添加一个滑动监听，获取静止状态，也就是不滑动时的位置，改为index索引。

    private void initRecyclerView() {
        RecyclerView recyclerView = findViewById(R.id.recyclerView_gallery);
        final List<PhotoBean> datas = getFakeDatas();
        final MyRecyclerViewAdapter adapter = new MyRecyclerViewAdapter(datas, this);
        final CustomLinearLayoutManager linearLayoutManager = new CustomLinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false);
        linearLayoutManager.setSpeedSlow(5);
        recyclerView.setLayoutManager(linearLayoutManager);
        new LinearSnapHelper().attachToRecyclerView(recyclerView);
        recyclerView.setAdapter(adapter);
        autoMoveRecyclerView(recyclerView, datas);
        recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
            @Override
            public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                if (newState == RecyclerView.SCROLL_STATE_IDLE){
                    CustomLinearLayoutManager manager = (CustomLinearLayoutManager) recyclerView.getLayoutManager();
                    index = manager.findLastVisibleItemPosition();
                }
            }
        });
    }

3. 最后就是滑动到末尾进行跳转了

    private void autoMoveRecyclerView(final RecyclerView recyclerView, final List<PhotoBean> datas) {
        final Timer timer = new Timer();
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        if (index == datas.size()){
                            timer.cancel();
                            Intent intent = new Intent(GalleryActivity.this, HeartActivity.class);
                            startActivity(intent);
                        }
                        recyclerView.smoothScrollToPosition(index);
                        index++;
                    }
                });
            }
        }, 50, 4000);
    }

最终效果为：

自动滚动.gif