Android Paging数据刷新及原理解析_pagingdataadapter.refresh 正在刷新调用次方法有效吗

Paging

刷新数据以及解析

从刷新讲起

流程

Paging的确很好用，省了很多步骤，但跟以往简单的刷新不同的是，Paging比较麻烦，不能直接清空Adapter里面的数据了和直接请求第一页的数据。以前清除列表，再重新请求第一页数据就可以了。那么Paging如何刷新呢？大致上也和以前用法一样，无外乎就是清除数据并加载新数据

清除Adapter里面的数据

submitList()通过传入null值，如下源码：

public void submitList(final PagedList<T> pagedList) {
   
    ...

    // incrementing generation means any currently-running diffs are discarded when they finish
    final int runGeneration = ++mMaxScheduledGeneration;

    if (pagedList == null) {
   
        int removedCount = getItemCount();
        if (mPagedList != null) {
   
            mPagedList.removeWeakCallback(mPagedListCallback);
            mPagedList = null;
        } else if (mSnapshot != null) {
   
            mSnapshot = null;
        }
        // dispatch update callback after updating mPagedList/mSnapshot
        mUpdateCallback.onRemoved(0, removedCount);
        if (mListener != null) {
   
            mListener.onCurrentListChanged(null);
        }
        return;
    }

    ...
}
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也就是在传入值为null的情况下，最终会执行

mUpdateCallback.onRemoved(0, removedCount);
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ListUpdateCallback是一个接口，其实现类如下：

/**
 * ListUpdateCallback that dispatches update events to the given adapter.
 *
 * @see DiffUtil.DiffResult#dispatchUpdatesTo(RecyclerView.Adapter)
 */
public final class AdapterListUpdateCallback implements ListUpdateCallback {
   
    @NonNull
    private final RecyclerView.Adapter mAdapter;

    /**
     * Creates an AdapterListUpdateCallback that will dispatch update events to the given adapter.
     *
     * @param adapter The Adapter to send updates to.
     */
    public AdapterListUpdateCallback(@NonNull RecyclerView.Adapter adapter) {
   
        mAdapter = adapter;
    }

    /** {@inheritDoc} */
    @Override
    public void onInserted(int position, int count) {
   
        mAdapter.notifyItemRangeInserted(position, count);
    }

    /** {@inheritDoc} */
    @Override
    public void onRemoved(int position, int count) {
   
        mAdapter.notifyItemRangeRemoved(position, count);
    }

    /** {@inheritDoc} */
    @Override
    public void onMoved(int fromPosition, int toPosition) {
   
        mAdapter.notifyItemMoved(fromPosition, toPosition);
    }

    /** {@inheritDoc} */
    @Override
    public void onChanged(int position, int count, Object payload) {
   
        mAdapter.notifyItemRangeChanged(position, count, payload);
    }
}
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也就是onRemoved(0, removedCount)还是调用了RecyclerView.Adapter的notifyItemRangeRemoved()，因此，如果在submitList()传入null值，最终会让Adapter移除从位置0开始的getItemCount()个数据，即删除全部数据

请求第一页的数据

那么还有一个问题，就是加载第一页数据。Paging并没有根据页数请求数据的方法。即使接口是有提供这个方法，那么只能看DataSource的loadInitial()是在什么情况下被调用的了

在本项目中，DataSource继承自PageKeyedDataSource，查看在该类中loadInitial()被调用情况

@Override
final void dispatchLoadInitial(@Nullable Key key, int initialLoadSize, int pageSize,
                               boolean enablePlaceholders, @NonNull Executor mainThreadExecutor,
                               @NonNull PageResult.Receiver<Value> receiver) {
   
    LoadInitialCallbackImpl<Key, Value> callback =
        new LoadInitialCallbackImpl<>(this, enablePlaceholders, receiver);
    loadInitial(new LoadInitialParams<Key>(initialLoadSize, enablePlaceholders), callback);

    // If initialLoad's callback is not called within the body, we force any following calls
    // to post to the UI thread. This constructor may be run on a background thread, but
    // after constructor, mutation must happen on UI thread.
    callback.mCallbackHelper.setPostExecutor(mainThreadExecutor)
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