本章目录
- Part One：改变滑动速度
- Part Two：自动滑动
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
- 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);
}
}
- 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;
}
}
}
- 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。
- 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);
}
- 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();
}
}
});
}
- 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
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