android RecyclerView实现列表定位

昨天使用ListView实现了列表定位的功能，然后又研究了下使用RecyclerView的实现方式，下面我们就探讨下实现思路。

ListView实现方式看这里
android仿朋友圈列表精准定位

先上图:

recyclerview_location.gif

得益于RecyclerView的分离式结构，这里使用了三种布局分别测试：
LinearLayoutManager、GridLayoutManager、StaggeredGridLayoutManager

先说下列表数据，这里采用ArrayList添加字符串对象

List list = new ArrayList<>();
        for (int i = 0; i < 50; i++) {
            if (i % 2 == 0) {
                list.add("我是传说中的Java" + i + "\n" + "\n"
                        + "因为我身体比较长" + "\n" + "\n"
                        + "再加长一点" + "\n" + "\n"
                        + "O(∩_∩)O哈哈哈~");
            } else if (i % 3 == 0) {
                list.add("KotLin" + i);
            } else if (i % 5 == 0) {
                list.add("ObjectC" + i);
            } else if (i % 7 == 0) {
                list.add("PHP" + i);
            } else {
                list.add("Linux" + i);
            }
        }

适配器继承RecyclerView.Adapter使用ViewHolder设置具体的值，这里只贴出必须要覆写的三个方法

@Override
    public RecyclerHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemView = LayoutInflater.from(mContext).inflate(R.layout.recycler_item, parent, false);
        return new RecyclerHolder(itemView);
    }

    @Override
    public void onBindViewHolder(RecyclerHolder holder, int position) {
        holder.tvInfo.setText(mItems.get(position));
    }

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

static class RecyclerHolder extends RecyclerView.ViewHolder {

        private TextView tvInfo;

        public RecyclerHolder(View itemView) {
            super(itemView);
            tvInfo = (TextView) itemView.findViewById(R.id.textView);
        }
    }

布局采用CardView，美观与实用同在

第一种线性布局结构LinearLayoutManager:

LinearLayoutManager manager = new LinearLayoutManager(this,
                        LinearLayoutManager.VERTICAL, false);
                mRecyclerView.setLayoutManager(manager);

第二种宫格布局结构GridLayoutManager:

GridLayoutManager manager = new GridLayoutManager(this, 3);
                mRecyclerView.setLayoutManager(manager);

第三章流式布局结构StaggeredGridLayoutManager:

StaggeredGridLayoutManager manager =
                        new StaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL);
                mRecyclerView.setLayoutManager(manager);

这三种布局我使用了菜单来切换，title采用的是ToolBar

使用ToolBar加载这个菜单布局并设置菜单点击事件监听

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        toolbar.inflateMenu(R.menu.actionbar_menu);

toolbar.setOnMenuItemClickListener(new Toolbar.OnMenuItemClickListener() {
            @Override
            public boolean onMenuItemClick(MenuItem item) {
                int type = 0;
                switch (item.getItemId()) {
                    case R.id.linearlayout:
                        type = 0;
                        break;
                    case R.id.gridlayout:
                        type = 1;
                        break;
                    case R.id.staggeredlayout:
                        type = 2;
                        break;
                }
                changeLayoutManager(type);
                adapter.notifyDataSetChanged();
                location(type);
                PreferenceTool.putInt("manage", type);
                PreferenceTool.commit();
                return true;
            }
        });

好了，接下来就是重中之重了，其实实现方式和listview差不多，都是在滚动事件中保存position和top，当然了，这里多保存了一个切换布局，recyclerview的添加滚动监听和listview不太一样，不是setOnScrollListener而是addOnScrollListener，匿名内部类使用的RecyclerView.OnScrollListener而ListView使用的是AbsListView.OnScrollListener
这里只覆写onScrollStateChanged方法就能达到我们想要的效果啦

@Override
public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
}

注意第二个参数，它代表的是列表滚动的状态，共有三种状态

Paste_Image.png

分别表示当前列表不在滚动、列表正在被手指拖拽、列表在以动画的方式滚动到一个最终的位置；所以我们直接判断当前列表是否停止滚动，所有保存状态的代码都在判断条件内
if (newState == RecyclerView.SCROLL_STATE_IDLE)

由于采用了切换布局的方式，所以要首先判断manager具体属于哪一个的实例
if (mRecyclerView.getLayoutManager() instanceof LinearLayoutManager)

然后强制转换layoutManager

LinearLayoutManager linearLayoutManager =
(LinearLayoutManager) mRecyclerView.getLayoutManager();

找到当前屏幕内第一个可见的item的position
linearLayoutManager.findFirstVisibleItemPosition()

接着根据这个position再找到对应的itemView

View itemView = linearLayoutManager.findViewByPosition(firstVisiblePosition)

如果这个View不为空就保存它的position和top值，这里的top值表示的是这个view距离顶部的偏移量

if (itemView != null) {
   int top = itemView.getTop();
   PreferenceTool.putInt("position", firstVisiblePosition);
   PreferenceTool.putInt("top", top);
   PreferenceTool.commit();
}

GridLayoutManager的使用方式和LinearLayoutManager一样，这里就不赘述了，具体说下StaggeredGridLayoutManager

由于我们设置的是两列，所以要找到的当前屏幕内可见的item有两个

int[] firstVisiblePositionArray = staggeredGridLayoutManager
         .findFirstVisibleItemPositions(new int[2]);

我们只需找到整形数组的第一个view就可以了

View itemView = staggeredGridLayoutManager.findViewByPosition(firstVisiblePositionArray[0]);
                        if (itemView != null) {
                            int top = itemView.getTop();
                            PreferenceTool.putInt("position", firstVisiblePositionArray[0]);
                            PreferenceTool.putInt("top", top);
                            PreferenceTool.commit();
                        }

扯了这么多，真正能够实现定位的只有下面这行代码
scrollToPositionWithOffset(int position, int offset)
郁不郁闷！！！

Paste_Image.png

源码中说的很明白了，position指的是item的索引，而offset指的是itemview的上边界和recyclerview的上边界之间的距离，这个距离可正可负；至此大功告成，下面贴下activity的源码仅供参考

public class RecyclerViewPositedAct extends AppCompatActivity {

    RecyclerView mRecyclerView;
    RecyclerAdapter adapter;

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

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        toolbar.inflateMenu(R.menu.actionbar_menu);
        toolbar.setOnMenuItemClickListener(new Toolbar.OnMenuItemClickListener() {
            @Override
            public boolean onMenuItemClick(MenuItem item) {
                int type = 0;
                switch (item.getItemId()) {
                    case R.id.linearlayout:
                        type = 0;
                        break;
                    case R.id.gridlayout:
                        type = 1;
                        break;
                    case R.id.staggeredlayout:
                        type = 2;
                        break;
                }
                changeLayoutManager(type);
                adapter.notifyDataSetChanged();
                location(type);
                PreferenceTool.putInt("manage", type);
                PreferenceTool.commit();
                return true;
            }
        });

        List list = new ArrayList<>();
        for (int i = 0; i < 50; i++) {
            if (i % 2 == 0) {
                list.add("我是传说中的Java" + i + "\n" + "\n"
                        + "因为我身体比较长" + "\n" + "\n"
                        + "再加长一点" + "\n" + "\n"
                        + "O(∩_∩)O哈哈哈~");
            } else if (i % 3 == 0) {
                list.add("KotLin" + i);
            } else if (i % 5 == 0) {
                list.add("ObjectC" + i);
            } else if (i % 7 == 0) {
                list.add("PHP" + i);
            } else {
                list.add("Linux" + i);
            }
        }

        mRecyclerView = (RecyclerView) findViewById(R.id.recycler_view);
        mRecyclerView.setHasFixedSize(true);

        int type = PreferenceTool.getInt("manage", 0);

        changeLayoutManager(type);
        adapter = new RecyclerAdapter(this, list);
        mRecyclerView.setAdapter(adapter);
        location(type);

        mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
            @Override
            public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                super.onScrollStateChanged(recyclerView, newState);
                if (newState == RecyclerView.SCROLL_STATE_IDLE) {
                    int firstVisiblePosition;
                    if (mRecyclerView.getLayoutManager() instanceof LinearLayoutManager) {
                        LinearLayoutManager linearLayoutManager =
                                (LinearLayoutManager) mRecyclerView.getLayoutManager();
                        firstVisiblePosition = linearLayoutManager.findFirstVisibleItemPosition();
                        View itemView = linearLayoutManager.findViewByPosition(firstVisiblePosition);
                        if (itemView != null) {
                            int top = itemView.getTop();
                            PreferenceTool.putInt("position", firstVisiblePosition);
                            PreferenceTool.putInt("top", top);
                            PreferenceTool.commit();
                        }
                    } else if (mRecyclerView.getLayoutManager() instanceof GridLayoutManager) {
                        GridLayoutManager gridLayoutManager = (GridLayoutManager)
                                mRecyclerView.getLayoutManager();
                        firstVisiblePosition = gridLayoutManager.findFirstVisibleItemPosition();
                        View itemView = gridLayoutManager.findViewByPosition(firstVisiblePosition);
                        if (itemView != null) {
                            int top = itemView.getTop();
                            PreferenceTool.putInt("position", firstVisiblePosition);
                            PreferenceTool.putInt("top", top);
                            PreferenceTool.commit();
                        }
                    } else if (mRecyclerView.getLayoutManager() instanceof StaggeredGridLayoutManager) {
                        StaggeredGridLayoutManager staggeredGridLayoutManager = (StaggeredGridLayoutManager)
                                mRecyclerView.getLayoutManager();
                        int[] firstVisiblePositionArray = staggeredGridLayoutManager
                                .findFirstVisibleItemPositions(new int[2]);
                        View itemView = staggeredGridLayoutManager.findViewByPosition(firstVisiblePositionArray[0]);
                        if (itemView != null) {
                            int top = itemView.getTop();
                            PreferenceTool.putInt("position", firstVisiblePositionArray[0]);
                            PreferenceTool.putInt("top", top);
                            PreferenceTool.commit();
                        }
                    }
                }
            }
        });
    }

    private void changeLayoutManager(int type) {
        switch (type) {
            case 0: {
                LinearLayoutManager manager = new LinearLayoutManager(this,
                        LinearLayoutManager.VERTICAL, false);
                mRecyclerView.setLayoutManager(manager);
            }
            break;
            case 1: {
                GridLayoutManager manager = new GridLayoutManager(this, 3);
                mRecyclerView.setLayoutManager(manager);
            }
            break;
            case 2: {
                StaggeredGridLayoutManager manager =
                        new StaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL);
                mRecyclerView.setLayoutManager(manager);
            }

            break;
        }
    }

    /**
     * 定位列表
     *
     * @param type
     */
    private void location(int type) {
        int top = PreferenceTool.getInt("top", 0);
        int position = PreferenceTool.getInt("position", 0);
        if (type == 0) {
            ((LinearLayoutManager) mRecyclerView.getLayoutManager())
                    .scrollToPositionWithOffset(position, top);
        } else if (type == 1) {
            ((GridLayoutManager) mRecyclerView.getLayoutManager())
                    .scrollToPositionWithOffset(position, top);
        } else if (type == 2) {
            ((StaggeredGridLayoutManager) mRecyclerView.getLayoutManager())
                    .scrollToPositionWithOffset(position, top);
        }
    }
}

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