水平滚动对齐中心

我试图在这里使用回收视图制作一个类似旋转木马的视图，我希望在滚动屏幕的时候，一个项目一个项目地出现在屏幕中间。我试过用 recyclerView.setScrollingTouchSlop(RecyclerView.TOUCH_SLOP_PAGING);

但是视图仍然可以平滑地滚动，我也尝试使用滚动侦听器来实现我自己的逻辑，如下所示:

recyclerView.setOnScrollListener(new OnScrollListener() {
@Override
public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
super.onScrollStateChanged(recyclerView, newState);
Log.v("Offset ", recyclerView.getWidth() + "");
if (newState == 0) {
try {
recyclerView.smoothScrollToPosition(layoutManager.findLastVisibleItemPosition());
recyclerView.scrollBy(20,0);
if (layoutManager.findLastVisibleItemPosition() >= recyclerView.getAdapter().getItemCount() - 1) {
Beam refresh = new Beam();
refresh.execute(createUrl());
}
} catch (Exception e) {
e.printStackTrace();
}
}

从右到左的滑动现在正常工作，但是反过来就不行了，我错过了什么？

117139 次浏览
小开

You need to use findFirstVisibleItemPosition for going in the opposite direction. And for detecting which direction the swipe was in, youll need to get either the fling velocity or the change in x. I approached this problem from a slightly different angle than you have.

Create a new class that extends the RecyclerView class and then override RecyclerView's fling method like so:

@Override
public boolean fling(int velocityX, int velocityY) {
LinearLayoutManager linearLayoutManager = (LinearLayoutManager) getLayoutManager();


//these four variables identify the views you see on screen.
int lastVisibleView = linearLayoutManager.findLastVisibleItemPosition();
int firstVisibleView = linearLayoutManager.findFirstVisibleItemPosition();
View firstView = linearLayoutManager.findViewByPosition(firstVisibleView);
View lastView = linearLayoutManager.findViewByPosition(lastVisibleView);


//these variables get the distance you need to scroll in order to center your views.
//my views have variable sizes, so I need to calculate side margins separately.
//note the subtle difference in how right and left margins are calculated, as well as
//the resulting scroll distances.
int leftMargin = (screenWidth - lastView.getWidth()) / 2;
int rightMargin = (screenWidth - firstView.getWidth()) / 2 + firstView.getWidth();
int leftEdge = lastView.getLeft();
int rightEdge = firstView.getRight();
int scrollDistanceLeft = leftEdge - leftMargin;
int scrollDistanceRight = rightMargin - rightEdge;


//if(user swipes to the left)
if(velocityX > 0) smoothScrollBy(scrollDistanceLeft, 0);
else smoothScrollBy(-scrollDistanceRight, 0);


return true;
}
小开

My solution:

/**
* Horizontal linear layout manager whose smoothScrollToPosition() centers
* on the target item
*/
class ItemLayoutManager extends LinearLayoutManager {


private int centeredItemOffset;


public ItemLayoutManager(Context context) {
super(context, LinearLayoutManager.HORIZONTAL, false);
}


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


public void setCenteredItemOffset(int centeredItemOffset) {
this.centeredItemOffset = centeredItemOffset;
}


/**
* ********** Inner Classes **********
*/


private class Scroller extends LinearSmoothScroller {


public Scroller(Context context) {
super(context);
}


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


@Override
public int calculateDxToMakeVisible(View view, int snapPreference) {
return super.calculateDxToMakeVisible(view, SNAP_TO_START) + centeredItemOffset;
}
}
}

I pass this layout manager to the RecycledView and set the offset required to center items. All my items have the same width so constant offset is ok

小开

Google I/O 2019 Update

ViewPager2 is here!

Google just announced at the talk 'What's New in Android' (aka 'The Android keynote') that they are working on a new ViewPager based on RecyclerView!

From the slides:

Like ViewPager, but better

  • Easy migration from ViewPager
  • Based on RecyclerView
  • Right-to-Left mode support
  • Allows vertical paging
  • Improved dataset change notifications

You can check the latest version here and the release notes here. There is also an official sample. Update Dec. 2021: sample has moved to this other repo.

Personal opinion: I think this is a really needed addition. I've recently had a lot of trouble with the PagerSnapHelper oscillating left right indefinitely - see the ticket I've opened.

New answer (2016)

You can now just use a SnapHelper.

If you want a center-aligned snapping behavior similar to ViewPager then use PagerSnapHelper:

SnapHelper snapHelper = new PagerSnapHelper();
snapHelper.attachToRecyclerView(recyclerView);

There is also a LinearSnapHelper. I've tried it and if you fling with energy then it scrolls 2 items with 1 fling. Personally I didn't like it, but just decide by yourself - trying it only takes seconds.

Original answer (2016)

After many hours of trying 3 different solutions found here in SO I've finally built a solution that mimics very closely the behavior found in a ViewPager.

The solution is based on the @eDizzle solution, which I believe I've improved enough to say that it works almost like a ViewPager.

Important: my RecyclerView items width is exactly the same as the screen. I haven't tried with other sizes. Also I use it with an horizontal LinearLayoutManager. I think that you will need to adapt the code if you want vertical scroll.

Here you have the code:

public class SnappyRecyclerView extends RecyclerView {


// Use it with a horizontal LinearLayoutManager
// Based on https://stackoverflow.com/a/29171652/4034572


public SnappyRecyclerView(Context context) {
super(context);
}


public SnappyRecyclerView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
}


public SnappyRecyclerView(Context context, @Nullable AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}


@Override
public boolean fling(int velocityX, int velocityY) {


LinearLayoutManager linearLayoutManager = (LinearLayoutManager) getLayoutManager();


int screenWidth = Resources.getSystem().getDisplayMetrics().widthPixels;


// views on the screen
int lastVisibleItemPosition = linearLayoutManager.findLastVisibleItemPosition();
View lastView = linearLayoutManager.findViewByPosition(lastVisibleItemPosition);
int firstVisibleItemPosition = linearLayoutManager.findFirstVisibleItemPosition();
View firstView = linearLayoutManager.findViewByPosition(firstVisibleItemPosition);


// distance we need to scroll
int leftMargin = (screenWidth - lastView.getWidth()) / 2;
int rightMargin = (screenWidth - firstView.getWidth()) / 2 + firstView.getWidth();
int leftEdge = lastView.getLeft();
int rightEdge = firstView.getRight();
int scrollDistanceLeft = leftEdge - leftMargin;
int scrollDistanceRight = rightMargin - rightEdge;


if (Math.abs(velocityX) < 1000) {
// The fling is slow -> stay at the current page if we are less than half through,
// or go to the next page if more than half through


if (leftEdge > screenWidth / 2) {
// go to next page
smoothScrollBy(-scrollDistanceRight, 0);
} else if (rightEdge < screenWidth / 2) {
// go to next page
smoothScrollBy(scrollDistanceLeft, 0);
} else {
// stay at current page
if (velocityX > 0) {
smoothScrollBy(-scrollDistanceRight, 0);
} else {
smoothScrollBy(scrollDistanceLeft, 0);
}
}
return true;


} else {
// The fling is fast -> go to next page


if (velocityX > 0) {
smoothScrollBy(scrollDistanceLeft, 0);
} else {
smoothScrollBy(-scrollDistanceRight, 0);
}
return true;


}


}


@Override
public void onScrollStateChanged(int state) {
super.onScrollStateChanged(state);


// If you tap on the phone while the RecyclerView is scrolling it will stop in the middle.
// This code fixes this. This code is not strictly necessary but it improves the behaviour.


if (state == SCROLL_STATE_IDLE) {
LinearLayoutManager linearLayoutManager = (LinearLayoutManager) getLayoutManager();


int screenWidth = Resources.getSystem().getDisplayMetrics().widthPixels;


// views on the screen
int lastVisibleItemPosition = linearLayoutManager.findLastVisibleItemPosition();
View lastView = linearLayoutManager.findViewByPosition(lastVisibleItemPosition);
int firstVisibleItemPosition = linearLayoutManager.findFirstVisibleItemPosition();
View firstView = linearLayoutManager.findViewByPosition(firstVisibleItemPosition);


// distance we need to scroll
int leftMargin = (screenWidth - lastView.getWidth()) / 2;
int rightMargin = (screenWidth - firstView.getWidth()) / 2 + firstView.getWidth();
int leftEdge = lastView.getLeft();
int rightEdge = firstView.getRight();
int scrollDistanceLeft = leftEdge - leftMargin;
int scrollDistanceRight = rightMargin - rightEdge;


if (leftEdge > screenWidth / 2) {
smoothScrollBy(-scrollDistanceRight, 0);
} else if (rightEdge < screenWidth / 2) {
smoothScrollBy(scrollDistanceLeft, 0);
}
}
}


}

Enjoy!

小开

With LinearSnapHelper, this is now very easy.

All you need to do is this:

SnapHelper helper = new LinearSnapHelper();
helper.attachToRecyclerView(recyclerView);

Update

Available since 25.1.0, PagerSnapHelper can help achieve ViewPager like effect. Use it as you would use the LinearSnapHelper.

Old workaround:

If you wish for it to behave akin to the ViewPager, try this instead:

LinearSnapHelper snapHelper = new LinearSnapHelper() {
@Override
public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY) {
View centerView = findSnapView(layoutManager);
if (centerView == null)
return RecyclerView.NO_POSITION;


int position = layoutManager.getPosition(centerView);
int targetPosition = -1;
if (layoutManager.canScrollHorizontally()) {
if (velocityX < 0) {
targetPosition = position - 1;
} else {
targetPosition = position + 1;
}
}


if (layoutManager.canScrollVertically()) {
if (velocityY < 0) {
targetPosition = position - 1;
} else {
targetPosition = position + 1;
}
}


final int firstItem = 0;
final int lastItem = layoutManager.getItemCount() - 1;
targetPosition = Math.min(lastItem, Math.max(targetPosition, firstItem));
return targetPosition;
}
};
snapHelper.attachToRecyclerView(recyclerView);

The implementation above just returns the position next to the current item (centered) based on the direction of the velocity, regardless of the magnitude.

The former one is a first party solution included in the Support Library version 24.2.0. Meaning you have to add this to your app module's build.gradle or update it.

compile "com.android.support:recyclerview-v7:24.2.0"
小开

If the goal is to make the RecyclerView mimic the behavior of ViewPager there is quite easy approach 

RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.recycler_view);


LinearLayoutManager layoutManager = new LinearLayoutManager(context, LinearLayoutManager.HORIZONTAL, false);
SnapHelper snapHelper = new PagerSnapHelper();
recyclerView.setLayoutManager(layoutManager);
snapHelper.attachToRecyclerView(mRecyclerView);

By using PagerSnapHelper you can get the behavior like ViewPager

小开

Just add padding and margin to recyclerView and recyclerView item:

recyclerView item:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/parentLayout"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_marginLeft="8dp" <!-- here -->
android:layout_marginRight="8dp" <!-- here  -->
android:layout_width="match_parent"
android:layout_height="200dp">


<!-- child views -->


</RelativeLayout>

recyclerView:

<androidx.recyclerview.widget.RecyclerView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:paddingLeft="8dp" <!-- here -->
android:paddingRight="8dp" <!-- here -->
android:clipToPadding="false" <!-- important!-->
android:scrollbars="none" />

and set PagerSnapHelper :

int displayWidth = Resources.getSystem().getDisplayMetrics().widthPixels;
parentLayout.getLayoutParams().width = displayWidth - Utils.dpToPx(16) * 4;
SnapHelper snapHelper = new PagerSnapHelper();
snapHelper.attachToRecyclerView(recyclerView);

dp to px:

public static int dpToPx(int dp) {
return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, Resources.getSystem().getDisplayMetrics());
}

result:

enter image description here

小开

PagerSnapHelper doesn't work with GridLayoutManager with spanCount > 1, so my solution under this circumstance is:

class GridPagerSnapHelper : PagerSnapHelper() {
override fun findTargetSnapPosition(layoutManager: RecyclerView.LayoutManager?, velocityX: Int, velocityY: Int): Int {
val forwardDirection = if (layoutManager?.canScrollHorizontally() == true) {
velocityX > 0
} else {
velocityY > 0
}
val centerPosition = super.findTargetSnapPosition(layoutManager, velocityX, velocityY)
return centerPosition +
if (forwardDirection) (layoutManager as GridLayoutManager).spanCount - 1 else 0
}
}

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