RecyclerView 水平滚动对齐在中心

Question

我正在尝试使用 RecyclerView 在此处制作类似轮播的视图，我希望该项目在滚动时捕捉到屏幕中间，一次一个项目。我试过使用recyclerView.setScrollingTouchSlop(RecyclerView.TOUCH_SLOP_PAGING);

但视图仍在平滑滚动，我也尝试使用滚动侦听器实现自己的逻辑，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();
                        }
                    }

现在从右向左滑动可以正常工作，但反过来不行，我在这里缺少什么？

Answer 1

有了LinearSnapHelper，现在这很容易。

您需要做的就是：

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

更新

自 25.1.0 起可用，PagerSnapHelper 可以帮助实现ViewPager 类似的效果。像使用 LinearSnapHelper 一样使用它。

旧的解决方法：

如果您希望它的行为类似于 ViewPager，请尝试以下操作：

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);

上面的实现只是根据速度的方向返回当前项目旁边的位置（居中），而不考虑大小。

前一个是包含在支持库版本 24.2.0 中的第一方解决方案。 这意味着您必须将其添加到您的应用模块的build.gradle 或更新它。

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

Answer 2

Google I/O 2019 更新

ViewPager2 来了！

Google just announced 在“What's New in Android”（又名“Android 主题演讲”）演讲中表示，他们正在开发基于 RecyclerView 的新 ViewPager！

来自幻灯片：

类似于 ViewPager，但更好

• 从 ViewPager 轻松迁移
• 基于 RecyclerView
• 支持从右到左的模式
• 允许垂直分页
• 改进的数据集更改通知

您可以查看最新版本here 和发行说明here。还有一个official sample。 2021 年 12 月更新：示例已移至 this other repo。

个人意见：我认为这是一个非常需要的补充。我最近在PagerSnapHelper oscillating left right indefinitely 遇到了很多麻烦 - 请参阅the ticket 我已经打开了。

---

新答案（2016 年）

您现在可以只使用SnapHelper。

如果您想要类似于ViewPager 的中心对齐捕捉行为，请使用PagerSnapHelper：

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

还有一个LinearSnapHelper。我试过了，如果你用能量一扔，那么它会用 1 次扔滚动 2 个项目。我个人不喜欢它，但请自行决定 - 尝试只需几秒钟。

---

原始答案（2016 年）

经过许多小时的尝试，在 SO 中找到了 3 种不同的解决方案，我终于构建了一个非常接近 ViewPager 中的行为的解决方案。

该解决方案基于@eDizzle solution，我相信我已经对其进行了足够的改进，可以说它几乎可以像ViewPager 一样工作。

重要提示：我的RecyclerView 项目宽度与屏幕完全相同。我没有尝试过其他尺寸。我也将它与水平 LinearLayoutManager 一起使用。如果你想要垂直滚动，我认为你需要调整代码。

这里有代码：

public class SnappyRecyclerView extends RecyclerView {

    // Use it with a horizontal LinearLayoutManager
    // Based on https://*.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);
            }
        }
    }

}

享受吧！

Answer 3

如果目标是让RecyclerView 模仿ViewPager 的行为，那么方法很简单

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);

通过使用PagerSnapHelper，您可以获得类似ViewPager的行为

Answer 4

您需要使用 findFirstVisibleItemPosition 来反方向前进。为了检测滑动的方向，您需要获得抛掷速度或 x 的变化。我从与您略有不同的角度解决了这个问题。

创建一个扩展 RecyclerView 类的新类，然后像这样重写 RecyclerView 的 fling 方法：

@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;
}

Answer 5

只需将padding 和margin 添加到recyclerView 和recyclerView item：

recyclerView 项目：

<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" />

并设置PagerSnapHelper：

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

dp 到 px：

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

结果：

Answer 6

我的解决方案：

/**
 * 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;
        }
    }
}

我将此布局管理器传递给 RecycledView 并设置居中项目所需的偏移量。我所有的项目都有相同的宽度，所以恒定的偏移量是可以的

Answer 7

PagerSnapHelper 不适用于 spanCount > 1 的GridLayoutManager，所以我在这种情况下的解决方案是：

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
    }
}