android/arch/paging/TiledPagedList.java

/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.arch.paging;

import android.support.annotation.AnyThread;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.annotation.WorkerThread;

import java.util.concurrent.Executor;

class TiledPagedList<T> extends PagedList<T>
        implements PagedStorage.Callback {
    private final PositionalDataSource<T> mDataSource;

    private PageResult.Receiver<T> mReceiver = new PageResult.Receiver<T>() {
        // Creation thread for initial synchronous load, otherwise main thread
        // Safe to access main thread only state - no other thread has reference during construction
        @AnyThread
        @Override
        public void onPageResult(@PageResult.ResultType int type,
                @NonNull PageResult<T> pageResult) {
            if (pageResult.isInvalid()) {
                detach();
                return;
            }

            if (isDetached()) {
                // No op, have detached
                return;
            }

            if (mStorage.getPageCount() == 0) {
                mStorage.initAndSplit(
                        pageResult.leadingNulls, pageResult.page, pageResult.trailingNulls,
                        pageResult.positionOffset, mConfig.pageSize, TiledPagedList.this);
            } else {
                mStorage.insertPage(pageResult.positionOffset, pageResult.page,
                        TiledPagedList.this);
            }

            if (mBoundaryCallback != null) {
                boolean deferEmpty = mStorage.size() == 0;
                boolean deferBegin = !deferEmpty
                        && pageResult.leadingNulls == 0
                        && pageResult.positionOffset == 0;
                int size = size();
                boolean deferEnd = !deferEmpty
                        && ((type == PageResult.INIT && pageResult.trailingNulls == 0)
                                || (type == PageResult.TILE
                                        && pageResult.positionOffset
                                                == (size - size % mConfig.pageSize)));
                deferBoundaryCallbacks(deferEmpty, deferBegin, deferEnd);
            }
        }
    };

    @WorkerThread
    TiledPagedList(@NonNull PositionalDataSource<T> dataSource,
            @NonNull Executor mainThreadExecutor,
            @NonNull Executor backgroundThreadExecutor,
            @Nullable BoundaryCallback<T> boundaryCallback,
            @NonNull Config config,
            int position) {
        super(new PagedStorage<T>(), mainThreadExecutor, backgroundThreadExecutor,
                boundaryCallback, config);
        mDataSource = dataSource;

        final int pageSize = mConfig.pageSize;
        mLastLoad = position;

        if (mDataSource.isInvalid()) {
            detach();
        } else {
            final int firstLoadSize =
                    (Math.max(Math.round(mConfig.initialLoadSizeHint / pageSize), 2)) * pageSize;

            final int idealStart = position - firstLoadSize / 2;
            final int roundedPageStart = Math.max(0, Math.round(idealStart / pageSize) * pageSize);

            mDataSource.loadInitial(true, roundedPageStart, firstLoadSize,
                    pageSize, mMainThreadExecutor, mReceiver);
        }
    }

    @Override
    boolean isContiguous() {
        return false;
    }

    @Nullable
    @Override
    public Object getLastKey() {
        return mLastLoad;
    }

    @Override
    protected void dispatchUpdatesSinceSnapshot(@NonNull PagedList<T> pagedListSnapshot,
            @NonNull Callback callback) {
        //noinspection UnnecessaryLocalVariable
        final PagedStorage<T> snapshot = pagedListSnapshot.mStorage;

        if (snapshot.isEmpty()
                || mStorage.size() != snapshot.size()) {
            throw new IllegalArgumentException("Invalid snapshot provided - doesn't appear"
                    + " to be a snapshot of this PagedList");
        }

        // loop through each page and signal the callback for any pages that are present now,
        // but not in the snapshot.
        final int pageSize = mConfig.pageSize;
        final int leadingNullPages = mStorage.getLeadingNullCount() / pageSize;
        final int pageCount = mStorage.getPageCount();
        for (int i = 0; i < pageCount; i++) {
            int pageIndex = i + leadingNullPages;
            int updatedPages = 0;
            // count number of consecutive pages that were added since the snapshot...
            while (updatedPages < mStorage.getPageCount()
                    && mStorage.hasPage(pageSize, pageIndex + updatedPages)
                    && !snapshot.hasPage(pageSize, pageIndex + updatedPages)) {
                updatedPages++;
            }
            // and signal them all at once to the callback
            if (updatedPages > 0) {
                callback.onChanged(pageIndex * pageSize, pageSize * updatedPages);
                i += updatedPages - 1;
            }
        }
    }

    @Override
    protected void loadAroundInternal(int index) {
        mStorage.allocatePlaceholders(index, mConfig.prefetchDistance, mConfig.pageSize, this);
    }

    @Override
    public void onInitialized(int count) {
        notifyInserted(0, count);
    }

    @Override
    public void onPagePrepended(int leadingNulls, int changed, int added) {
        throw new IllegalStateException("Contiguous callback on TiledPagedList");
    }

    @Override
    public void onPageAppended(int endPosition, int changed, int added) {
        throw new IllegalStateException("Contiguous callback on TiledPagedList");
    }

    @Override
    public void onPagePlaceholderInserted(final int pageIndex) {
        // placeholder means initialize a load
        mBackgroundThreadExecutor.execute(new Runnable() {
            @Override
            public void run() {
                if (isDetached()) {
                    return;
                }
                final int pageSize = mConfig.pageSize;

                if (mDataSource.isInvalid()) {
                    detach();
                } else {
                    int startPosition = pageIndex * pageSize;
                    int count = Math.min(pageSize, mStorage.size() - startPosition);
                    mDataSource.loadRange(startPosition, count, mMainThreadExecutor, mReceiver);
                }
            }
        });
    }

    @Override
    public void onPageInserted(int start, int count) {
        notifyChanged(start, count);
    }
}