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）

淩亂°似流年 2021-09-16 10:28 1061阅读 0赞

### 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<String> 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<BaseViewHolder> {
            private final List<String> mList;
    
            public MyAdapter(List<String> 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<String>()));
                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<BaseViewHolder> {
            private final List<String> mList;
    
            public ItemAdapter(List<String> 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

<?xml version="1.0" encoding="utf-8"?>
    <LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#fff"
        android:orientation="vertical">
    
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            >
    
            <TextView
                android:id="@+id/tv_rvhuadong_GuanXing_1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:padding="10dp"
                android:text="惯性随机滑动"
                android:textColor="#494949"
                android:textSize="15sp"
                />
    
            <TextView
                android:id="@+id/tv_rvhuadong_GuanXing_2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:padding="10dp"
                android:text="惯性滑动到20"
                android:textColor="#494949"
                android:textSize="15sp"
                />
    
            <TextView
                android:id="@+id/tv_rvhuadong_GuanXing_3"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:padding="10dp"
                android:text="惯性滑动到99"
                android:textColor="#494949"
                android:textSize="15sp"
                />
        </LinearLayout>
    
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            >
    
            <TextView
                android:id="@+id/tv_rvhuadong_ShanXian_1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:padding="10dp"
                android:text="随机闪现"
                android:textColor="#494949"
                android:textSize="15sp"
                />
    
            <TextView
                android:id="@+id/tv_rvhuadong_ShanXian_2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:padding="10dp"
                android:text="闪现到20"
                android:textColor="#494949"
                android:textSize="15sp"
                />
    
            <TextView
                android:id="@+id/tv_rvhuadong_ShanXian_3"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:padding="10dp"
                android:text="闪现到99"
                android:textColor="#494949"
                android:textSize="15sp"
                />
        </LinearLayout>
    
        <android.support.v7.widget.RecyclerView
            android:id="@+id/rv_rvhuadong"
            android:layout_width="match_parent"
            android:layout_height="match_parent"/>
    </LinearLayout>

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][https_blog.csdn.net_weimingjue_article_details_82805361]

[https_blog.csdn.net_weimingjue_article_details_82805361]: https://blog.csdn.net/weimingjue/article/details/82805361