纠正：Android RecyclerView滚动到指定位置并置顶（滚动方法、移动置顶、定位滑动到指定位置item）

最近博主发现让RecyclerView滑动到某一位置并置顶的博客一大堆，抄的是完全一模一样。此外，虽然这些博客“解决”了这些问题，但这种解决方案过于浅显、粗暴，甚至都违背了开发思想。遂在此纠正这种错误。

RecyclerView提供了几种移动的方法

scrollToPosition

scrollTo

scrollBy

smoothScrollBy

smoothScrollToPosition

由于多数博客鱼龙混杂，本博客如果让你非常满意或解决了大家的根本性问题，希望多多支持在下方点赞和回复一下，举手之劳方便大家。

虽然里面有移动到指定位置的方法scrollToPosition（直接闪现至某一位位置）、smoothScrollToPosition（惯性滑动至某一位置）但是貌似都不尽人意，因为他们只保证能够展示出来，并不能保证在第一位。而此时如果你打开源码就会发现，原来全都是调用的LayoutManager移动方法，首先打开我们耳熟能详的LinearLayoutManager惊喜就在眼前

scrollToPosition

在scrollToPosition旁边有木有一个很像的方法

    @Override
    public void scrollToPosition(int position) {
        mPendingScrollPosition = position;
        mPendingScrollPositionOffset = INVALID_OFFSET;
        if (mPendingSavedState != null) {
            mPendingSavedState.invalidateAnchor();
        }
        requestLayout();
    }
    public void scrollToPositionWithOffset(int position, int offset) {
        mPendingScrollPosition = position;
        mPendingScrollPositionOffset = offset;
        if (mPendingSavedState != null) {
            mPendingSavedState.invalidateAnchor();
        }
        requestLayout();
    }

当看到offset时也许就会明白：没错，这个就是item移动后相对父控件的偏移值，传入0就会有你想要的

smoothScrollToPosition

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

而smoothScrollToPosition原来仅仅是new了一个LinearSmoothScroller然后调用startSmoothScroll

我们只需要自定义一个LinearSmoothScroller，之前写的有点仓促，仔细看LinearSmoothScroller的源码发现，其实谷歌已经埋下了伏笔，既然纠正就纠正到底吧

public class TopSmoothScroller extends LinearSmoothScroller {
    TopSmoothScroller(Context context) {
        super(context);
    }
    @Override
    protected int getHorizontalSnapPreference() {
        return SNAP_TO_START;//具体见源码注释
    }
    @Override
    protected int getVerticalSnapPreference() {
        return SNAP_TO_START;//具体见源码注释
    }
}

 然后调用LinearLayoutManager的startSmoothScroll即可

final TopSmoothScroller mScroller = new TopSmoothScroller(getActivity());
mScroller.setTargetPosition(integer);
mManager.startSmoothScroll(mScroller);

是否恍然大悟：其实我们并不需要什么bd，也不需要修改LinearLayoutManager，仅仅需要几行代码即可解决。

多看看源码，多思考思考，你也可以。

真理往往掌握在少数人手中，你是不是其中一员呢？

对于一些不会或质疑的人，此处追加demo：

public class RvHuaDongActivity extends BaseActivity {
    @BindView(R.id.rv_rvhuadong)
    RecyclerView mRv;
    private LinearLayoutManager mManager;

    //此处等于setContentView
    @Override
    protected int getLayouRes() {
        return R.layout.activity_rv_hua_dong;
    }

    //此处等于onCreate
    @Override
    protected void initData() {
        List list = new ArrayList<>();
        for (int i = 0; i < 100; i++) {
            list.add("position" + i);
        }
        mManager = new LinearLayoutManager(this);
        mRv.setLayoutManager(mManager);
        mRv.setAdapter(new MyAdapter(list));
    }

    @Override
    protected void setListener() {
    }

    @OnClick({R.id.tv_rvhuadong_GuanXing_1, R.id.tv_rvhuadong_GuanXing_2, R.id.tv_rvhuadong_GuanXing_3,
            R.id.tv_rvhuadong_ShanXian_1, R.id.tv_rvhuadong_ShanXian_2, R.id.tv_rvhuadong_ShanXian_3})
    public void onViewClicked(View view) {
        switch (view.getId()) {
            case R.id.tv_rvhuadong_GuanXing_1:
                int position1 = (int) (Math.random() * 100);
                Toast.makeText(this, "滑到：" + position1, Toast.LENGTH_SHORT).show();
                LinearSmoothScroller s1 = new TopSmoothScroller(getActivity());
                s1.setTargetPosition(position1);
                mManager.startSmoothScroll(s1);
                break;
            case R.id.tv_rvhuadong_GuanXing_2:
                LinearSmoothScroller s2 = new TopSmoothScroller(getActivity());
                s2.setTargetPosition(20);
                mManager.startSmoothScroll(s2);
                break;
            case R.id.tv_rvhuadong_GuanXing_3:
                LinearSmoothScroller s3 = new TopSmoothScroller(getActivity());
                s3.setTargetPosition(99);
                mManager.startSmoothScroll(s3);
                break;
            case R.id.tv_rvhuadong_ShanXian_1:
                int position2 = (int) (Math.random() * 100);
                Toast.makeText(this, "闪到：" + position2, Toast.LENGTH_SHORT).show();
                mManager.scrollToPositionWithOffset(position2, 0);
                break;
            case R.id.tv_rvhuadong_ShanXian_2:
                mManager.scrollToPositionWithOffset(20, 0);
                break;
            case R.id.tv_rvhuadong_ShanXian_3:
                mManager.scrollToPositionWithOffset(99, 0);
                break;
        }
    }

    private class MyAdapter extends RecyclerView.Adapter {
        private final List mList;

        public MyAdapter(List list) {
            mList = list;
        }

        @Override
        public BaseViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            LinearLayout ll = new LinearLayout(getActivity());
            ll.setLayoutParams(new RecyclerView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT));
            ll.setOrientation(LinearLayout.VERTICAL);

            AppCompatTextView tv = new AppCompatTextView(RvHuaDongActivity.this);
            tv.setTextSize(30);
            tv.setBackgroundColor(0xffeeeeee);
            ll.addView(tv);

            RecyclerView rv = new RecyclerView(getActivity());
            rv.setLayoutManager(new LinearLayoutManager(getActivity()));
            rv.setNestedScrollingEnabled(true);
            rv.setAdapter(new ItemAdapter(new ArrayList()));
            ll.addView(rv, ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT);
            return new BaseViewHolder(ll);
        }

        @Override
        public void onBindViewHolder(BaseViewHolder holder, int position) {
            ViewGroup vg = (ViewGroup) holder.itemView;
            TextView tv = (TextView) vg.getChildAt(0);
            tv.setText(mList.get(position));

            RecyclerView rv = (RecyclerView) vg.getChildAt(1);
            ItemAdapter adapter = (ItemAdapter) rv.getAdapter();
            adapter.mList.clear();
            for (int i = 0; i < 6; i++) {
                adapter.mList.add("item" + i);
            }
            adapter.notifyDataSetChanged();//在bind时确定好数据
        }

        @Override
        public int getItemCount() {
            return mList.size();
        }
    }

    private class ItemAdapter extends RecyclerView.Adapter {
        private final List mList;

        public ItemAdapter(List list) {
            mList = list;
        }


        @Override
        public BaseViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            AppCompatTextView tv = new AppCompatTextView(RvHuaDongActivity.this);
            tv.setTextSize(30);
            tv.setBackgroundColor(0xffeeeeee);
            tv.setLayoutParams(new RecyclerView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT));
            return new BaseViewHolder(tv);
        }

        @Override
        public void onBindViewHolder(BaseViewHolder holder, int position) {
            TextView tv = (TextView) holder.itemView;
            tv.setText(mList.get(position));
            if (position >= mList.size() - 2) {
                tv.getLayoutParams().height = 600;
            } else {
                tv.getLayoutParams().height = ViewGroup.LayoutParams.WRAP_CONTENT;
            }
            tv.setLayoutParams(tv.getLayoutParams());
        }

        @Override
        public int getItemCount() {
            return mList.size();
        }
    }

    public static class TopSmoothScroller extends LinearSmoothScroller {
        TopSmoothScroller(Context context) {
            super(context);
        }

        @Override
        protected int getHorizontalSnapPreference() {
            return SNAP_TO_START;//具体见源码注释
        }

        @Override
        protected int getVerticalSnapPreference() {
            return SNAP_TO_START;//具体见源码注释
        }
    }
}

activity_rv_hua_dong.xml

support包27.0.2，目前除了startSmoothScroll进行中然后立即调用scrollToPositionWithOffset（谁会这么做）位置会出现偏差 ，其他连续调用等操作均没有任何问题

注：

1.终于明白下面评论里的最后一个条目置顶的事了，对于类似rv嵌套rv置顶的问题（尤其是最后一个置顶）：首先你的内层rv高度（如果是竖着的）必须是wrap，为了效率和复用内层rv要在bind初始化好基本数据LayoutManager、Adapter、setNestedScrollingEnabled(true);，在外层rv bind时必须确定好你内层rv的数据（我上面的例子都有写）。

2.置顶失败的举例：①rv嵌套内层的数据需要再次请求网络动态获取的②adapter里图片高度是网络请求决定的③rv设置数据用的是handler或子线程（这3个问题的根本原因是，滑到最后一个条目了了，你还在异步加载数据，rv发现滑不动自然就把最后一个条目停在最底下了，然后你才把数据返回来早就晚了）④用了一些支持header、footer的Adapter的，条目可能需要+1才能滑到

具体的使用场景及效果可以参考微信的通讯录

转载请注明出处：王能的博客https://blog.csdn.net/weimingjue/article/details/82805361