android架构组件之paging源码解析

RecyclerView使用paging就是多了对数据的拉取，使得RecyclerView的数据和显示更加的解耦，RecyclerView对paging的使用多了如下几步：

        instance = CustomAdapter.getInstance(this);
        factory = new CustomPageDataSourceFactory<>();
        build = new LivePagedListBuilder<Integer, String>(factory,
                new PagedList.Config.Builder().setPageSize(20).setInitialLoadSizeHint(20)
                        .setPrefetchDistance(3).build()).setInitialLoadKey(4).build();
        build.observe(this, it -> instance.submitList(it));

也就是将数据设置到adapter中，看到这，我们该想paging的源码该从哪里入手呢？想想就应该知道应该是从LivePagedListBuilder入手，好的，那就从LivePagedListBuilder的build（）方法中去看看：

LivePagedListBuilder的build()方法：

    public LiveData<PagedList<Value>> build() {
        return create(mInitialLoadKey, mConfig, mBoundaryCallback, mDataSourceFactory,
                ArchTaskExecutor.getMainThreadExecutor(), mFetchExecutor);
    }

就是简单调用了它的create()方法，那就看下它的create（）方法：

    private static <Key, Value> LiveData<PagedList<Value>> create(
            @Nullable final Key initialLoadKey,//ItemKeyedDataSource会用到
            @NonNull final PagedList.Config config,//加载数据时的一些配置信息，比如初始加载多少，每页加载多少
            @Nullable final PagedList.BoundaryCallback boundaryCallback,
            @NonNull final DataSource.Factory<Key, Value> dataSourceFactory,
            @NonNull final Executor notifyExecutor,
            @NonNull final Executor fetchExecutor//获取数据的线程池) {
        return new ComputableLiveData<PagedList<Value>>(fetchExecutor) {
            @Nullable
            private PagedList<Value> mList;
            @Nullable
            private DataSource<Key, Value> mDataSource;

            private final DataSource.InvalidatedCallback mCallback =
                    new DataSource.InvalidatedCallback() {
                        @Override
                        public void onInvalidated() {
                            invalidate();
                        }
                    };

            @Override
            protected PagedList<Value> compute() {
                @Nullable Key initializeKey = initialLoadKey;
                if (mList != null) {
                    //noinspection unchecked
                    initializeKey = (Key) mList.getLastKey();
                }

                do {
                    if (mDataSource != null) {
                        mDataSource.removeInvalidatedCallback(mCallback);
                    }

                    mDataSource = dataSourceFactory.create();
                    mDataSource.addInvalidatedCallback(mCallback);

                    mList = new PagedList.Builder<>(mDataSource, config)
                            .setNotifyExecutor(notifyExecutor)
                            .setFetchExecutor(fetchExecutor)
                            .setBoundaryCallback(boundaryCallback)
                            .setInitialKey(initializeKey)
                            .build();
                } while (mList.isDetached());
                return mList;
            }
        }.getLiveData();
    }

构建了一个ComputableLiveData对象，并把拉取数据的线程池传递进去，同时调用它的getLiveData（）方法，接下来就去看下ComputableLiveData的构造方法：

    public ComputableLiveData(@NonNull Executor executor) {
        mExecutor = executor;
        mLiveData = new LiveData<T>() {
            @Override
            protected void onActive() {
                mExecutor.execute(mRefreshRunnable);
            }
        };
    }

这里了创建了一个LiveData对象，并在它的onActive（）方法中在传进来的线程池执行了mRefreshRunnable任务，这里就来看下这个任务里面执行了什么：

final Runnable mRefreshRunnable = new Runnable() {
        @WorkerThread
        @Override
        public void run() {
            boolean computed;
            do {
                computed = false;
                // compute can happen only in 1 thread but no reason to lock others.
                if (mComputing.compareAndSet(false, true)) {
                    // as long as it is invalid, keep computin