实现点击RecyclerView的item时，将上一个item的底图滑块平移到目标item

实现RecyclerView item点击后的平移动画，可以通过使用RecyclerView的ItemDecoration来实现。

具体步骤如下：

1. 在RecyclerView的Adapter中添加接口，用来监听item的点击事件，例如：

public interface OnItemClickListener {
    void onItemClick(View view, int position);
}

public class MyAdapter extends RecyclerView.Adapter<MyViewHolder> {

    private OnItemClickListener mOnItemClickListener;

    public void setOnItemClickListener(OnItemClickListener listener) {
        mOnItemClickListener = listener;
    }

    // ...
}

2. 在RecyclerView的ItemDecoration中实现平移动画，例如：

public class MyItemDecoration extends RecyclerView.ItemDecoration {

    private Drawable mDivider;
    private int mDividerHeight;

    private int mLastSelectedPosition = -1;
    private int mCurrentSelectedPosition = -1;

    public MyItemDecoration(Context context) {
        mDivider = ContextCompat.getDrawable(context, R.drawable.divider);
        mDividerHeight = mDivider.getIntrinsicHeight();
    }

    public void setSelectedPosition(int position) {
        mLastSelectedPosition = mCurrentSelectedPosition;
        mCurrentSelectedPosition = position;
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);

        if (mLastSelectedPosition == -1 || mCurrentSelectedPosition == -1) {
            return;
        }

        View lastSelectedView = parent.getChildAt(mLastSelectedPosition);
        View currentSelectedView = parent.getChildAt(mCurrentSelectedPosition);

        if (lastSelectedView == null || currentSelectedView == null) {
            return;
        }

        int lastSelectedTop = lastSelectedView.getTop();
        int currentSelectedTop = currentSelectedView.getTop();

        int delta = currentSelectedTop - lastSelectedTop;

        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();

        int lastSelectedBottom = lastSelectedView.getBottom() - mDividerHeight;
        int currentSelectedBottom = currentSelectedView.getBottom() - mDividerHeight;

        mDivider.setBounds(left, lastSelectedBottom, right, lastSelectedBottom + mDividerHeight);
        mDivider.draw(c);

        mDivider.setBounds(left, currentSelectedBottom - delta, right, currentSelectedBottom - delta + mDividerHeight);
        mDivider.draw(c);
    }
}

3. 在RecyclerView的Activity或Fragment中设置ItemDecoration和OnItemClickListener，例如：

public class MyActivity extends AppCompatActivity implements MyAdapter.OnItemClickListener {

    private RecyclerView mRecyclerView;
    private MyAdapter mAdapter;
    private MyItemDecoration mItemDecoration;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my);

        mRecyclerView = findViewById(R.id.recyclerview);
        mAdapter = new MyAdapter();
        mItemDecoration = new MyItemDecoration(this);

        mRecyclerView.setAdapter(mAdapter);
        mRecyclerView.addItemDecoration(mItemDecoration);
        mAdapter.setOnItemClickListener(this);
    }

    @Override
    public void onItemClick(View view, int position) {
        mItemDecoration.setSelectedPosition(position);
        mRecyclerView.invalidateItemDecorations();
    }
}

这样，当RecyclerView的item被点击时，就会触发OnItemClickListener的回调方法，在里面设置当前选中的item的位置，并调用RecyclerView的invalidateItemDecorations()方法来刷新ItemDecoration，从而实现平移动画。