任务：
1）从SQLITE（1000个元素）中读取20个元素的数据
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 ArrayList 中删除不可见项目
4) 向上滚动 - 加载上一个项目并从 ArrayList 中删除不可见的项目
5）滚动条对应db中的记录数
当List size > Max_elements_in_memory我尝试从 ArrayList 中删除项目时，如果它对用户不可见。但是我的 RecyclerView 在 120 位置结束。如何解决它的任务？谢谢
EndlessRecyclerViewScrollListener
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);}
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);
}
}