Java 通过控制内存中的最大元素（ArrayList）实现无止境的RecyclerView

任务：

1） 通过20个元素从SQLITE（1000个元素）读取数据

2） 设置ArrayList的最大大小（例如50）

3） 向下滚动-加载更多项目并从中删除不可见项目 ArrayList

4） 向上滚动-加载上一个项目并从中删除不可见的项目 ArrayList

5） 滚动条对应于数据库中的记录数

当

列表大小>内存中的最大元素数
时，如果ArrayList对用户不可见，我会尝试将其删除。但我的RecyclerView在120位置结束。如何解决它的任务？谢谢

endlessRecycleViewScrollListener

public abstract class EndlessRecyclerViewScrollListener extends RecyclerView.OnScrollListener {

    // The minimum amount of items to have below your current scroll position
    // before loading more.
    private int visibleThreshold = 5;

    // The current offset index of data you have loaded
    private int currentPage = 0;

    // The total number of items in the dataset after the last load
    private int previousTotalItemCount = 0;

    // True if we are still waiting for the last set of data to load.
    private boolean loading = true;

    // Sets the starting page index
    private int startingPageIndex = 0;

    RecyclerView.LayoutManager mLayoutManager;

    public EndlessRecyclerViewScrollListener(LinearLayoutManager layoutManager) {

        this.mLayoutManager = layoutManager;
    }

    public EndlessRecyclerViewScrollListener(GridLayoutManager layoutManager) {

        this.mLayoutManager = layoutManager;
        visibleThreshold = visibleThreshold * layoutManager.getSpanCount();
    }

    public EndlessRecyclerViewScrollListener(StaggeredGridLayoutManager layoutManager) {

        this.mLayoutManager = layoutManager;
        visibleThreshold = visibleThreshold * layoutManager.getSpanCount();

    }

    public int getLastVisibleItem(int[] lastVisibleItemPositions) {

        int maxSize = 0;

        for (int i = 0; i < lastVisibleItemPositions.length; i++) {

            if (i == 0) {
                maxSize = lastVisibleItemPositions[i];
            }

            else if (lastVisibleItemPositions[i] > maxSize) {
                maxSize = lastVisibleItemPositions[i];
            }
        }
        return maxSize;
    }

    // This happens many times a second during a scroll, so be wary of the code you place here.
    // We are given a few useful parameters to help us work out if we need to load some more data,
    // but first we check if we are waiting for the previous load to finish.

    @Override
    public void onScrolled(RecyclerView view, int dx, int dy) {

        int lastVisibleItemPosition = 0;
        int totalItemCount = mLayoutManager.getItemCount();

        if (mLayoutManager instanceof StaggeredGridLayoutManager) {

            int[] lastVisibleItemPositions = ((StaggeredGridLayoutManager) mLayoutManager).findLastVisibleItemPositions(null);

            // get maximum element within the list
            lastVisibleItemPosition = getLastVisibleItem(lastVisibleItemPositions);

        } else if (mLayoutManager instanceof GridLayoutManager) {

            lastVisibleItemPosition = ((GridLayoutManager) mLayoutManager).findLastVisibleItemPosition();

        } else if (mLayoutManager instanceof LinearLayoutManager) {

            lastVisibleItemPosition = ((LinearLayoutManager) mLayoutManager).findLastVisibleItemPosition();

        }

        // If the total item count is zero and the previous isn't, assume the
        // list is invalidated and should be reset back to initial state

        if (totalItemCount < previousTotalItemCount) {

            this.currentPage = this.startingPageIndex;
            this.previousTotalItemCount = totalItemCount;

            if (totalItemCount == 0) {
                this.loading = true;
            }
        }

        // If it’s still loading, we check to see if the dataset count has
        // changed, if so we conclude it has finished loading and update the current page
        // number and total item count.

        if (loading && (totalItemCount > previousTotalItemCount)) {
            loading = false;
            previousTotalItemCount = totalItemCount;
        }

        // If it isn’t currently loading, we check to see if we have breached
        // the visibleThreshold and need to reload more data.
        // If we do need to reload some more data, we execute onLoadMore to fetch the data.
        // threshold should reflect how many total columns there are too

        if (!loading && (lastVisibleItemPosition + visibleThreshold) > totalItemCount) {

            currentPage++;
            onLoadMore(currentPage, totalItemCount, view);
            loading = true;
        }
    }

    // Call this method whenever performing new searches
    public void resetState() {

        this.currentPage = this.startingPageIndex;
        this.previousTotalItemCount = 0;
        this.loading = true;
    }

    // Defines the process for actually loading more data based on page
    public abstract void onLoadMore(int page, int totalItemsCount, RecyclerView view);}

公共抽象类endlessRecycleWebScrollListener扩展了RecycleView.OnScrollListener{
//低于当前滚动位置的最小项目数
//在加载更多之前。
私有int visibleThreshold=5；
//已加载数据的当前偏移索引
private int currentPage=0；
//上次加载后数据集中的项目总数
private int previousTotalItemCount=0；
//如果仍在等待加载最后一组数据，则为True。
私有布尔加载=真；
//设置起始页索引
私有int startingPageIndex=0；
RecyclerView.LayoutManager mllayoutmanager；
public endlessRecycleViewScrollListener（LinearLayoutManager布局管理器）{
this.mLayoutManager=layoutManager；
}
public endlessRecycleViewScrollListener（GridLayoutManager布局管理器）{
this.mLayoutManager=layoutManager；
VisibleThrreshold=VisibleThrreshold*layoutManager.getSpanCount（）；
}
public EndlessRecycleViewScrollListener（交错边缘布局管理器布局管理器）{
this.mLayoutManager=layoutManager；
VisibleThrreshold=VisibleThrreshold*layoutManager.getSpanCount（）；
}
public int getLastVisibleItem（int[]lastVisibleItemPositions）{
int maxSize=0；
对于（int i=0；imaxSize）{
maxSize=lastVisibleItemPositions[i]；
}
}
返回最大大小；
}
//在滚动过程中，这种情况在一秒钟内会发生很多次，因此请小心放置在此处的代码。
//我们得到了一些有用的参数，以帮助我们确定是否需要加载更多数据，
//但首先我们检查是否在等待前一次加载完成。
@凌驾
已克隆的公共无效（RecyclerView视图、int dx、int dy）{
int lastVisibleItemPosition=0；
int totalItemCount=mLayoutManager.getItemCount（）；
if（交错边缘布局管理器的mLayoutManager实例）{
int[]lastVisibleItemPositions=（（StaggedGridLayoutManager）mLayoutManager）。findLastVisibleItemPositions（null）；
//获取列表中的最大元素
lastVisibleItemPosition=getLastVisibleItem（lastVisibleItemPositions）；
}else if（网格布局管理器的mLayoutManager实例）{
lastVisibleItemPosition=（（GridLayoutManager）mLayoutManager）.findLastVisibleItemPosition（）；
}else if（线性布局管理器的mLayoutManager实例）{
lastVisibleItemPosition=（（LinearLayoutManager）mLayoutManager）。findLastVisibleItemPosition（）；
}
//如果项目总数为零，而前一个不是，则假定
//列表无效，应重置回初始状态
if（totalItemCountpreviousTotalItemCount））{
加载=假；
previousTotalItemCount=totalItemCount；
}
//如果当前未加载，我们将检查是否已违反
//VisibleSThreshold已关闭，需要重新加载更多数据。
//如果确实需要重新加载一些数据，则执行onLoadMore来获取数据。
//阈值应反映出有多少个总列
如果（！loading&&（lastVisibleItemPosition+visibleThreshold）>totalItemCount）{
currentPage++；
onLoadMore（当前页面、totalItemCount、视图）；
加载=真；
}
}
//执行新搜索时调用此方法
公共状态（）{
this.currentPage=this.startingPageIndex；
this.previousTotalItemCount=0；
这是。加载=真；
}
//定义根据页面实际加载更多数据的过程
公共抽象void onLoadMore（int-page，int-totalItemsCount，RecyclerView视图）；}

MainActivity.class

public class MainActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks<Cursor>{

    private static int DEFAULT_MAX_ELEMENTS_IN_MEMORY = 50;
    private static int DEFAULT_MAX_ELEMENTS_IN_RECYCLER_VIEW = 20;
    private int from = 0;

    @BindView(R.id.recycler_view)
    RecyclerView recyclerView;

    private UsersAdapter usersAdapter;
    private List<User> usersList;

    // Store a member variable for the listener
    private EndlessRecyclerViewScrollListener scrollListener;

    private LinearLayoutManager mLayoutManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);

        res = getResources();

        usersList = new ArrayList<>();
        usersAdapter = new UsersAdapter(this, usersList);

        mLayoutManager = new LinearLayoutManager(this);

        int orientation = getResources().getConfiguration().orientation;
        if(orientation == Configuration.ORIENTATION_PORTRAIT){
            recyclerView.setLayoutManager(mLayoutManager);
        } else{
            recyclerView.setLayoutManager(mLayoutManager);
        }

        recyclerView.setHasFixedSize(true);
        recyclerView.setItemAnimator(null);
        recyclerView.setAdapter(usersAdapter);

        scrollListener = new EndlessRecyclerViewScrollListener(mLayoutManager) {
            @Override
            public void onLoadMore(int page, int totalItemsCount, RecyclerView view) {

                loadNextDataFromDb(page);
            }
        };
        // Adds the scroll listener to RecyclerView
        recyclerView.addOnScrollListener(scrollListener);

        getSupportLoaderManager().initLoader(R.id.users_loader, Bundle.EMPTY, this);

    }

    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        switch (id) {
            case R.id.users_loader:
                return new UsersLoader(this, from, DEFAULT_MAX_ELEMENTS_IN_RECYCLER_VIEW);

            default:
                return null;
        }
    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        int id = loader.getId();

        if (usersList.size() > 0)
        if (usersList.size() - DEFAULT_MAX_ELEMENTS_IN_RECYCLER_VIEW > DEFAULT_MAX_ELEMENTS_IN_MEMORY) {
            for (int i = 0; i < DEFAULT_MAX_ELEMENTS_IN_RECYCLER_VIEW; i++) {
                usersList.remove(i);
            }
            /*usersList.clear();
            usersAdapter.notifyDataSetChanged();
            scrollListener.resetState();*/
        }
            usersList.addAll((List) UsersTable.listFromCursor(data));
            usersAdapter.notifyDataSetChanged();

            getLoaderManager().destroyLoader(id);
    }

    @Override
    public void onLoaderReset(Loader<Cursor> loader) {

    }

    private void loadNextDataFromDb(int offset) {
        from = offset * DEFAULT_MAX_ELEMENTS_IN_RECYCLER_VIEW;
        getSupportLoaderManager().restartLoader(R.id.users_loader, Bundle.EMPTY, this);
    }

}

public类MainActivity扩展AppCompatActivity实现LoaderManager.LoaderCallbacks{
私有静态int默认值\u最大\u内存中的\u元素=50；
私有静态int DEFAULT\u MAX\u ELEMENTS\u IN\u RECYCLER\u VIEW=20；
私有int from=0；
@BindView（R.id.recycler\U视图）
回收视图回收视图；
私有用户适配器用户适配器；
私有列表用户列表；
//存储侦听器的成员变量
私有endlessRecycleViewScrollListener滚动Listener；
私人直线酒店经理；
@凌驾
创建时受保护的void（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）；
setContentView（R.layout.activity_main）；
黄油刀