安卓PagedList更新

Question

我的问题是如何在PagedList中更新项目？

在我的例子中，有ListActivity和DetailsActivity。列表活动正在使用分页组件从网络获取帖子(仅限)，并使用分页适配器在回收器视图中显示它。当用户按下一些帖子，我需要获得帖子的详细信息，并在DetailsActivity上显示它。我正在向服务器发出另一个请求，它返回给我post详细信息。调用之后，服务器增加该帖子的viewsCount值，当用户返回到帖子列表时，我需要更新列表项中的计数器。

问题是，如何在PagedList中更新单个项目(post)，因为我不需要从头开始重新加载所有列表来更新单个项目。

Answer 1

PagedList是不可变的

我建议您不要使用分页库，因为它假定您的数据是不可变的，这在实际用例中几乎是不可能的。

您可以在official docs中查看它。上面写着：

如果您有更细粒度的更新信号，例如网络API发出更新列表中单个项目的信号，建议您将数据从网络加载到内存中。然后，通过包装内存快照的DataSource将数据呈现给PagedList。每次内存中的副本发生更改时，都会使以前的DataSource无效，并且可以创建包装快照的新状态的新快照。

即，

您必须从服务器获取数据，然后使用房间库将其存储到本地DB中，然后当您需要更新任何项目时，将项目更新到本地DB中，反过来， room 会将这些更改反映到UI中(使用LiveData)。

但是，您必须使用本地DB进行分页。

在空间的情况下，你会有类似这样的东西。

@Dao
interface FeedDao {
@Query("SELECT * FROM feed ")
fun selectPaged(): DataSource.Factory<Int, Feed>
}

但是，你还需要处理更多的事情，比如：

如果从远程服务器上删除了一个实体，该怎么办？我们怎么才能注意到这一点？

我们必须将它从本地DB 中删除。

你可以阅读this的文章来解决这个问题。它解释了分页库中的所有问题，以及如果您真的想要使用分页库时可能的解决方法。

我的建议是使用this库并实现您自己的分页。

Answer 2

PagedListAdapter使用的DiffUtil.ItemCallback只会在它认为是“新”项的情况下更新现有项。在我看来，您确实需要到服务器获取新的视图计数，但您也不希望重绘未受影响的项目。这可以通过向DiffUtil.ItemCallback添加视图计数属性来确定新项是什么来防止。具有新视图计数的项目将被重新绘制，而其他项目则不会。

我假设检索第一个列表的原始请求返回的对象中包含viewsCount字段。

Answer 3

我已经在recyclerview中实现了分页，你可以在git hub https://github.com/kunal-mahajan/GitRepositoryList上检查这个例子，你不会使用下面的代码一次又一次地重新加载数据。

步骤1:创建类DynamicLoadingAdapter

    import android.content.Context;
import android.graphics.Typeface;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.RecyclerView.ViewHolder;
import android.util.TypedValue;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.ProgressBar;
import android.widget.TextView;

import java.util.ArrayList;
import java.util.List;


public abstract class DynamicLoadingAdapter extends RecyclerView.Adapter<ViewHolder> {

    private static final int ITEM = 3;
    private static final int LOADING = 4;
    private static final int NO_DATA = 5;

    protected List records;
    protected Context context;
    private LoadingObj loadingObj = new LoadingObj();
    private NoDataAvailableObj noDataAvailableObj = new NoDataAvailableObj();

    public DynamicLoadingAdapter(Context context) {
        this.context = context;
        records = new ArrayList<>();
        records.add(loadingObj);
    }

    private int getIntToDP(int px) {
        return Math.round(TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, px, context.getResources().getDisplayMetrics()));
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        ViewHolder viewHolder = null;
        LayoutInflater inflater = LayoutInflater.from(parent.getContext());
        switch (viewType) {
            case ITEM:
                viewHolder = getViewHolder(parent, inflater);
                break;
            case LOADING:
                viewHolder = new DynamicLoadingAdapter.LoadingVH(getProgressBar());
                break;
            case NO_DATA:
                viewHolder = new ViewHolder(getNoDataTextView()) {
                };
        }
        return viewHolder;
    }

    protected abstract String getEmptyText();

    @NonNull
    protected abstract ViewHolder getViewHolder(ViewGroup parent, LayoutInflater inflater);

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        if (getItemViewType(position) == ITEM)
            setValuesOnBind(holder, position);
    }

    protected abstract void setValuesOnBind(ViewHolder holder, int position);

    @Override
    public int getItemViewType(int position) {
        Object o = records.get(position);
        if (o instanceof LoadingObj)
            return LOADING;

        if (o instanceof NoDataAvailableObj)
            return NO_DATA;

        return ITEM;
    }

    void setLoadingFinished() {
        records.remove(records.size() - 1);
        if (records.size() == 0) records.add(new NoDataAvailableObj());
        notifyDataSetChanged();
    }

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

    int getItemLoadedCount() {
        int l = records.size();
        if (records.get(records.size() - 1).equals(loadingObj))
            l--;

        if (l > 0 && records.get(0).equals(noDataAvailableObj))
            l--;

        return l;
    }

    void add(List list) {
        for (int i = 0; i < list.size(); i++) {
            records.add(records.size() - 1, list.get(i));
        }

        notifyDataSetChanged();
    }

    private ProgressBar getProgressBar() {
        ProgressBar progressBar = new ProgressBar(context);
        int h = getIntToDP(30);
        int w = ViewGroup.LayoutParams.MATCH_PARENT;
        LinearLayout.LayoutParams progressParams = new LinearLayout.LayoutParams(w, h);
        int margin = getIntToDP(5);
        progressParams.setMargins(margin, margin, margin, margin);
        progressParams.gravity = Gravity.CENTER;
        progressBar.setLayoutParams(progressParams);
        return progressBar;
    }

    public TextView getNoDataTextView() {
        LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT);
        TextView textView = new TextView(context);
        textView.setGravity(Gravity.CENTER);
        textView.setText(getEmptyText());
        textView.setTypeface(null, Typeface.ITALIC);
        params.setMargins(0, getIntToDP(7), 0, getIntToDP(7));
        textView.setLayoutParams(params);
        return textView;
    }

    private static class LoadingObj {
    }

    private class LoadingVH extends ViewHolder {
        public LoadingVH(View itemView) {
            super(itemView);
        }
    }

    private class NoDataAvailableObj {
    }
}

步骤2:创建类: DynamicLoadingListHelper

import android.content.Context;
import android.support.v7.widget.DefaultItemAnimator;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.ProgressBar;

import java.util.List;

/**
 * Created by Kunal.Mahajan on 7/23/2018.
 */

public abstract class DynamicLoadingListHelper {

    private final DynamicLoadingAdapter adapter;
    private final Context context;
    private LinearLayout containerLayout;
    private RecyclerView rv;
    private int totalRecords = -1;
    private int pageCount = 0;

    public DynamicLoadingListHelper(Context context, LinearLayout containerLayout, DynamicLoadingAdapter adapter) {
        this.adapter = adapter;
        this.context = context;
        this.containerLayout = containerLayout;
        init();
    }

    public void init() {
        final LinearLayoutManager lm = new LinearLayoutManager(context, LinearLayoutManager.VERTICAL, false);
        rv = new RecyclerView(context);
        rv.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
        containerLayout.addView(rv);
        rv.setLayoutManager(lm);
        rv.setItemAnimator(new DefaultItemAnimator());
        rv.setAdapter(adapter);

        rv.addOnScrollListener(new RecyclerView.OnScrollListener() {
            int lastLoadCountReq = Integer.MIN_VALUE;

            @Override
            public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                super.onScrollStateChanged(recyclerView, newState);
                if (lm.findLastVisibleItemPosition() == adapter.getItemCount() - 1 && newState == RecyclerView.SCROLL_STATE_IDLE && adapter.getItemLoadedCount() < totalRecords && adapter.getItemLoadedCount() > lastLoadCountReq) {
                    lastLoadCountReq = adapter.getItemLoadedCount();
                    loadNextPage();
                }
            }
        });

        rv.addOnChildAttachStateChangeListener(new RecyclerView.OnChildAttachStateChangeListener() {
            LinearLayoutManager layoutManager = (LinearLayoutManager) rv.getLayoutManager();

            @Override
            public void onChildViewAttachedToWindow(View view) {

                boolean isScrolled = layoutManager.findViewByPosition(0) != layoutManager.findViewByPosition(layoutManager.findFirstCompletelyVisibleItemPosition());
                if (isScrolled)
                    return;

                if (!(view instanceof ProgressBar))
                    return;

                if (isRecyclerScrollable()) {
                    loadNextPage();
                }
            }

            public void onChildViewDetachedFromWindow(View view) {

            }
        });

        loadNextPage();
    }

    private void loadNextPage() {
        pageCount++;
        loadData(pageCount);
    }

    private boolean isRecyclerScrollable() {
        LinearLayoutManager layoutManager = (LinearLayoutManager) rv.getLayoutManager();
        RecyclerView.Adapter adapter = rv.getAdapter();

        if (layoutManager == null || adapter == null) return false;

        return layoutManager.findLastCompletelyVisibleItemPosition() < totalRecords;
    }

    protected abstract void loadData(int offset);

    public void dataLoaded(List list, int totalPage) {

        if (totalPage <= 0) {
            adapter.setLoadingFinished();
            return;
        }

        this.totalRecords = totalPage;
        adapter.add(list);
        if (adapter.getItemLoadedCount() >= totalPage)
            adapter.setLoadingFinished();
    }
}

步骤3:创建您自己的PaginationAdaptor，例如：

import android.content.Context;
import android.content.Intent;
import android.net.Uri;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;

import com.gitfeaturelisting.R;
import com.gitfeaturelisting.component.DynamicLoadingAdapter;
import com.gitfeaturelisting.pojo.Item;
import com.squareup.picasso.Picasso;

/**
 * Created by Kunal.Mahajan on 7/23/2018.
 */

public class RepoPaginationAdaptor extends DynamicLoadingAdapter {

    public RepoPaginationAdaptor(Context applicationContext) {
        super(applicationContext);
    }

    @Override
    protected String getEmptyText() {
        return "No repo available";
    }


    @NonNull
    @Override
    protected RecyclerView.ViewHolder getViewHolder(ViewGroup parent, LayoutInflater inflater) {
        View v = inflater.inflate(R.layout.layout_repo_info_short, parent, false);
        final RecyclerView.ViewHolder viewHolder = new RepoInfoVH(v);
        return viewHolder;
    }

    @Override
    protected void setValuesOnBind(RecyclerView.ViewHolder holder, int position) {
        Item r = (Item) records.get(position);
        RepoInfoVH rvh = (RepoInfoVH) holder;
        rvh.tvName.setText(r.getDescription());
        rvh.tvTitle.setText(r.getName());
        rvh.tvRate.setText(context.getString(R.string.updated_on) + " " + (r.getUpdatedAt()));
        Picasso.get().load(Uri.parse(r.getOwner().getAvatarUrl())).into(rvh.ivPic);
        rvh.llMain.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(context, ActivityRepoDetail.class);
                Item r = (Item) records.get(rvh.getLayoutPosition());
                i.putExtra(ActivityRepoDetail.KEY_REPO_DETAIL, r);
                context.startActivity(i);
            }
        });
    }


    private class
    RepoInfoVH extends RecyclerView.ViewHolder {
        ImageView ivPic;
        TextView tvName;
        TextView tvTitle;
        TextView tvRate;
        LinearLayout llMain;

        public RepoInfoVH(View jobView) {
            super(jobView);
            ivPic = jobView.findViewById(R.id.layout_repo_info_short_iv_repo_pic);
            tvName = (jobView.findViewById(R.id.layout_repo_info_short_tv_name));
            tvTitle = (jobView.findViewById(R.id.layout_repo_info_short_tv_title));
            tvRate = (jobView.findViewById(R.id.layout_repo_info_short_tv_bottom));
            llMain = (jobView.findViewById(R.id.layout_repo_info_short_ll));
        }
    }
}