RecyclerView 平滑滚动到中心位置。安卓

Question

我正在为我的RecyclerView 使用水平布局管理器。 我需要用下一种方式制作RecyclerView：当单击某个项目时-将smoothScrool 放到该位置并将该项目放在RecyclerView 的中心（如果可能，例如，20 中的10 个项目）。

所以，我对smoothScrollToPosition()没有问题，但是如何将项目放在RecyclerView的中心？？

谢谢！

Answer 1

是的，这是可能的。

通过实现RecyclerView.SmoothScroller的方法onTargetFound(View, State, Action)。

/**
 * Called when the target position is laid out. This is the last callback SmoothScroller
 * will receive and it should update the provided {@link Action} to define the scroll
 * details towards the target view.
 * @param targetView    The view element which render the target position.
 * @param state         Transient state of RecyclerView
 * @param action        Action instance that you should update to define final scroll action
 *                      towards the targetView
 */
abstract protected void onTargetFound(View targetView, State state, Action action);

特别是在LinearLayoutManager 和LinearSmoothScroller：

public class CenterLayoutManager extends LinearLayoutManager {

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

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

    public CenterLayoutManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    @Override
    public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {
        RecyclerView.SmoothScroller smoothScroller = new CenterSmoothScroller(recyclerView.getContext());
        smoothScroller.setTargetPosition(position);
        startSmoothScroll(smoothScroller);
    }

    private static class CenterSmoothScroller extends LinearSmoothScroller {

        CenterSmoothScroller(Context context) {
            super(context);
        }

        @Override
        public int calculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd, int snapPreference) {
            return (boxStart + (boxEnd - boxStart) / 2) - (viewStart + (viewEnd - viewStart) / 2);
        }
    }
}

Answer 2

对答案的改进 - 无需覆盖 LinearLayoutManager

来自上一个答案：

public class CenterSmoothScroller extends LinearSmoothScroller {

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

    @Override
    public int calculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd, int snapPreference) {
        return (boxStart + (boxEnd - boxStart) / 2) - (viewStart + (viewEnd - viewStart) / 2);
    }
}

这里如何使用它：

RecyclerView.LayoutManager lm = new GridLayoutManager(...): // or whatever layout manager you need

...

RecyclerView.SmoothScroller smoothScroller = new CenterSmoothScroller(recyclerView.getContext());

smoothScroller.setTargetPosition(position);

lm.startSmoothScroll(smoothScroller);

Answer 3

以防万一有人在接受的答案中需要类的 Kotlin 等效项。

class CenterLayoutManager : LinearLayoutManager {
    constructor(context: Context) : super(context)
    constructor(context: Context, orientation: Int, reverseLayout: Boolean) : super(context, orientation, reverseLayout)
    constructor(context: Context, attrs: AttributeSet, defStyleAttr: Int, defStyleRes: Int) : super(context, attrs, defStyleAttr, defStyleRes)

    override fun smoothScrollToPosition(recyclerView: RecyclerView, state: RecyclerView.State, position: Int) {
        val centerSmoothScroller = CenterSmoothScroller(recyclerView.context)
        centerSmoothScroller.targetPosition = position
        startSmoothScroll(centerSmoothScroller)
    }

    private class CenterSmoothScroller(context: Context) : LinearSmoothScroller(context) {
        override fun calculateDtToFit(viewStart: Int, viewEnd: Int, boxStart: Int, boxEnd: Int, snapPreference: Int): Int = (boxStart + (boxEnd - boxStart) / 2) - (viewStart + (viewEnd - viewStart) / 2)
    }
}

Answer 4

根据@Boda 的回答，如果您想控制平滑滚动速度（以获得更好的动画效果），您可以使用以下方法：

class CenterLayoutManager : LinearLayoutManager {
    constructor(context: Context) : super(context)
    constructor(context: Context, orientation: Int, reverseLayout: Boolean) : super(
        context,
        orientation,
        reverseLayout
    )

    constructor(context: Context, attrs: AttributeSet, defStyleAttr: Int, defStyleRes: Int) : super(
        context,
        attrs,
        defStyleAttr,
        defStyleRes
    )

    override fun smoothScrollToPosition(
        recyclerView: RecyclerView,
        state: RecyclerView.State,
        position: Int
    ) {
        val centerSmoothScroller = CenterSmoothScroller(recyclerView.context)
        centerSmoothScroller.targetPosition = position
        startSmoothScroll(centerSmoothScroller)
    }

    private class CenterSmoothScroller(context: Context) : LinearSmoothScroller(context) {
        override fun calculateDtToFit(
            viewStart: Int,
            viewEnd: Int,
            boxStart: Int,
            boxEnd: Int,
            snapPreference: Int
        ): Int = (boxStart + (boxEnd - boxStart) / 2) - (viewStart + (viewEnd - viewStart) / 2)

        override fun calculateSpeedPerPixel(displayMetrics: DisplayMetrics): Float {
            return MILLISECONDS_PER_INCH / displayMetrics.densityDpi
        }
    }

    companion object {
        // This number controls the speed of smooth scroll
        private const val MILLISECONDS_PER_INCH = 150f
    }
}

用法：

recyclerView.layoutManager = CenterLayoutManager(requireContext(), LinearLayoutManager.HORIZONTAL, false)
recyclerView.smoothScrollToPosition(selectedPosition)

Answer 5

从现在（2019 年 2 月）开始，我可以轻松地在 ListView 中使用此代码

(ListView)word_list_view.smoothScrollToPositionFromTop(your_item_index, center_position.y);

RecyclerView 未验证，我猜应该是一样的。