chinwag-android/app/src/main/java/com/keylesspalace/tusky/util/PagingRequestHelper.java

/*
 * Copyright 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 com.keylesspalace.tusky.util;

import androidx.annotation.AnyThread;
import androidx.annotation.GuardedBy;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.annotation.VisibleForTesting;
import java.util.Arrays;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.Executor;
import java.util.concurrent.atomic.AtomicBoolean;
/**
 * A helper class for {@link androidx.paging.PagedList.BoundaryCallback BoundaryCallback}s and
 * {@link androidx.paging.DataSource}s to help with tracking network requests.
 * <p>
 * It is designed to support 3 types of requests, {@link RequestType#INITIAL INITIAL},
 * {@link RequestType#BEFORE BEFORE} and {@link RequestType#AFTER AFTER} and runs only 1 request
 * for each of them via {@link #runIfNotRunning(RequestType, Request)}.
 * <p>
 * It tracks a {@link Status} and an {@code error} for each {@link RequestType}.
 * <p>
 * A sample usage of this class to limit requests looks like this:
 * <pre>
 * class PagingBoundaryCallback extends PagedList.BoundaryCallback&lt;MyItem> {
 *     // TODO replace with an executor from your application
 *     Executor executor = Executors.newSingleThreadExecutor();
 *     PagingRequestHelper helper = new PagingRequestHelper(executor);
 *     // imaginary API service, using Retrofit
 *     MyApi api;
 *
 *     {@literal @}Override
 *     public void onItemAtFrontLoaded({@literal @}NonNull MyItem itemAtFront) {
 *         helper.runIfNotRunning(PagingRequestHelper.RequestType.BEFORE,
 *                 helperCallback -> api.getTopBefore(itemAtFront.getName(), 10).enqueue(
 *                         new Callback&lt;ApiResponse>() {
 *                             {@literal @}Override
 *                             public void onResponse(Call&lt;ApiResponse> call,
 *                                     Response&lt;ApiResponse> response) {
 *                                 // TODO insert new records into database
 *                                 helperCallback.recordSuccess();
 *                             }
 *
 *                             {@literal @}Override
 *                             public void onFailure(Call&lt;ApiResponse> call, Throwable t) {
 *                                 helperCallback.recordFailure(t);
 *                             }
 *                         }));
 *     }
 *
 *     {@literal @}Override
 *     public void onItemAtEndLoaded({@literal @}NonNull MyItem itemAtEnd) {
 *         helper.runIfNotRunning(PagingRequestHelper.RequestType.AFTER,
 *                 helperCallback -> api.getTopBefore(itemAtEnd.getName(), 10).enqueue(
 *                         new Callback&lt;ApiResponse>() {
 *                             {@literal @}Override
 *                             public void onResponse(Call&lt;ApiResponse> call,
 *                                     Response&lt;ApiResponse> response) {
 *                                 // TODO insert new records into database
 *                                 helperCallback.recordSuccess();
 *                             }
 *
 *                             {@literal @}Override
 *                             public void onFailure(Call&lt;ApiResponse> call, Throwable t) {
 *                                 helperCallback.recordFailure(t);
 *                             }
 *                         }));
 *     }
 * }
 * </pre>
 * <p>
 * The helper provides an API to observe combined request status, which can be reported back to the
 * application based on your business rules.
 * <pre>
 * MutableLiveData&lt;PagingRequestHelper.Status> combined = new MutableLiveData&lt;>();
 * helper.addListener(status -> {
 *     // merge multiple states per request type into one, or dispatch separately depending on
 *     // your application logic.
 *     if (status.hasRunning()) {
 *         combined.postValue(PagingRequestHelper.Status.RUNNING);
 *     } else if (status.hasError()) {
 *         // can also obtain the error via {@link StatusReport#getErrorFor(RequestType)}
 *         combined.postValue(PagingRequestHelper.Status.FAILED);
 *     } else {
 *         combined.postValue(PagingRequestHelper.Status.SUCCESS);
 *     }
 * });
 * </pre>
 */
// THIS class is likely to be moved into the library in a future release. Feel free to copy it
// from this sample.
public class PagingRequestHelper {
    private final Object mLock = new Object();
    private final Executor mRetryService;
    @GuardedBy("mLock")
    private final RequestQueue[] mRequestQueues = new RequestQueue[]
            {new RequestQueue(RequestType.INITIAL),
                    new RequestQueue(RequestType.BEFORE),
                    new RequestQueue(RequestType.AFTER)};
    @NonNull
    final CopyOnWriteArrayList<Listener> mListeners = new CopyOnWriteArrayList<>();
    /**
     * Creates a new PagingRequestHelper with the given {@link Executor} which is used to run
     * retry actions.
     *
     * @param retryService The {@link Executor} that can run the retry actions.
     */
    public PagingRequestHelper(@NonNull Executor retryService) {
        mRetryService = retryService;
    }
    /**
     * Adds a new listener that will be notified when any request changes {@link Status state}.
     *
     * @param listener The listener that will be notified each time a request's status changes.
     * @return True if it is added, false otherwise (e.g. it already exists in the list).
     */
    @AnyThread
    public boolean addListener(@NonNull Listener listener) {
        return mListeners.add(listener);
    }
    /**
     * Removes the given listener from the listeners list.
     *
     * @param listener The listener that will be removed.
     * @return True if the listener is removed, false otherwise (e.g. it never existed)
     */
    public boolean removeListener(@NonNull Listener listener) {
        return mListeners.remove(listener);
    }
    /**
     * Runs the given {@link Request} if no other requests in the given request type is already
     * running.
     * <p>
     * If run, the request will be run in the current thread.
     *
     * @param type    The type of the request.
     * @param request The request to run.
     * @return True if the request is run, false otherwise.
     */
    @SuppressWarnings("WeakerAccess")
    @AnyThread
    public boolean runIfNotRunning(@NonNull RequestType type, @NonNull Request request) {
        boolean hasListeners = !mListeners.isEmpty();
        StatusReport report = null;
        synchronized (mLock) {
            RequestQueue queue = mRequestQueues[type.ordinal()];
            if (queue.mRunning != null) {
                return false;
            }
            queue.mRunning = request;
            queue.mStatus = Status.RUNNING;
            queue.mFailed = null;
            queue.mLastError = null;
            if (hasListeners) {
                report = prepareStatusReportLocked();
            }
        }
        if (report != null) {
            dispatchReport(report);
        }
        final RequestWrapper wrapper = new RequestWrapper(request, this, type);
        wrapper.run();
        return true;
    }
    @GuardedBy("mLock")
    private StatusReport prepareStatusReportLocked() {
        Throwable[] errors = new Throwable[]{
                mRequestQueues[0].mLastError,
                mRequestQueues[1].mLastError,
                mRequestQueues[2].mLastError
        };
        return new StatusReport(
                getStatusForLocked(RequestType.INITIAL),
                getStatusForLocked(RequestType.BEFORE),
                getStatusForLocked(RequestType.AFTER),
                errors
        );
    }
    @GuardedBy("mLock")
    private Status getStatusForLocked(RequestType type) {
        return mRequestQueues[type.ordinal()].mStatus;
    }
    @AnyThread
    @VisibleForTesting
    void recordResult(@NonNull RequestWrapper wrapper, @Nullable Throwable throwable) {
        StatusReport report = null;
        final boolean success = throwable == null;
        boolean hasListeners = !mListeners.isEmpty();
        synchronized (mLock) {
            RequestQueue queue = mRequestQueues[wrapper.mType.ordinal()];
            queue.mRunning = null;
            queue.mLastError = throwable;
            if (success) {
                queue.mFailed = null;
                queue.mStatus = Status.SUCCESS;
            } else {
                queue.mFailed = wrapper;
                queue.mStatus = Status.FAILED;
            }
            if (hasListeners) {
                report = prepareStatusReportLocked();
            }
        }
        if (report != null) {
            dispatchReport(report);
        }
    }
    private void dispatchReport(StatusReport report) {
        for (Listener listener : mListeners) {
            listener.onStatusChange(report);
        }
    }
    /**
     * Retries all failed requests.
     *
     * @return True if any request is retried, false otherwise.
     */
    public boolean retryAllFailed() {
        final RequestWrapper[] toBeRetried = new RequestWrapper[RequestType.values().length];
        boolean retried = false;
        synchronized (mLock) {
            for (int i = 0; i < RequestType.values().length; i++) {
                toBeRetried[i] = mRequestQueues[i].mFailed;
                mRequestQueues[i].mFailed = null;
            }
        }
        for (RequestWrapper failed : toBeRetried) {
            if (failed != null) {
                failed.retry(mRetryService);
                retried = true;
            }
        }
        return retried;
    }
    static class RequestWrapper implements Runnable {
        @NonNull
        final Request mRequest;
        @NonNull
        final PagingRequestHelper mHelper;
        @NonNull
        final RequestType mType;
        RequestWrapper(@NonNull Request request, @NonNull PagingRequestHelper helper,
                @NonNull RequestType type) {
            mRequest = request;
            mHelper = helper;
            mType = type;
        }
        @Override
        public void run() {
            mRequest.run(new Request.Callback(this, mHelper));
        }
        void retry(Executor service) {
            service.execute(new Runnable() {
                @Override
                public void run() {
                    mHelper.runIfNotRunning(mType, mRequest);
                }
            });
        }
    }
    /**
     * Runner class that runs a request tracked by the {@link PagingRequestHelper}.
     * <p>
     * When a request is invoked, it must call one of {@link Callback#recordFailure(Throwable)}
     * or {@link Callback#recordSuccess()} once and only once. This call
     * can be made any time. Until that method call is made, {@link PagingRequestHelper} will
     * consider the request is running.
     */
    @FunctionalInterface
    public interface Request {
        /**
         * Should run the request and call the given {@link Callback} with the result of the
         * request.
         *
         * @param callback The callback that should be invoked with the result.
         */
        void run(Callback callback);
        /**
         * Callback class provided to the {@link #run(Callback)} method to report the result.
         */
        class Callback {
            private final AtomicBoolean mCalled = new AtomicBoolean();
            private final RequestWrapper mWrapper;
            private final PagingRequestHelper mHelper;
            Callback(RequestWrapper wrapper, PagingRequestHelper helper) {
                mWrapper = wrapper;
                mHelper = helper;
            }
            /**
             * Call this method when the request succeeds and new data is fetched.
             */
            @SuppressWarnings("unused")
            public final void recordSuccess() {
                if (mCalled.compareAndSet(false, true)) {
                    mHelper.recordResult(mWrapper, null);
                } else {
                    throw new IllegalStateException(
                            "already called recordSuccess or recordFailure");
                }
            }
            /**
             * Call this method with the failure message and the request can be retried via
             * {@link #retryAllFailed()}.
             *
             * @param throwable The error that occured while carrying out the request.
             */
            @SuppressWarnings("unused")
            public final void recordFailure(@NonNull Throwable throwable) {
                //noinspection ConstantConditions
                if (throwable == null) {
                    throw new IllegalArgumentException("You must provide a throwable describing"
                            + " the error to record the failure");
                }
                if (mCalled.compareAndSet(false, true)) {
                    mHelper.recordResult(mWrapper, throwable);
                } else {
                    throw new IllegalStateException(
                            "already called recordSuccess or recordFailure");
                }
            }
        }
    }
    /**
     * Data class that holds the information about the current status of the ongoing requests
     * using this helper.
     */
    public static final class StatusReport {
        /**
         * Status of the latest request that were submitted with {@link RequestType#INITIAL}.
         */
        @NonNull
        public final Status initial;
        /**
         * Status of the latest request that were submitted with {@link RequestType#BEFORE}.
         */
        @NonNull
        public final Status before;
        /**
         * Status of the latest request that were submitted with {@link RequestType#AFTER}.
         */
        @NonNull
        public final Status after;
        @NonNull
        private final Throwable[] mErrors;
        StatusReport(@NonNull Status initial, @NonNull Status before, @NonNull Status after,
                @NonNull Throwable[] errors) {
            this.initial = initial;
            this.before = before;
            this.after = after;
            this.mErrors = errors;
        }
        /**
         * Convenience method to check if there are any running requests.
         *
         * @return True if there are any running requests, false otherwise.
         */
        public boolean hasRunning() {
            return initial == Status.RUNNING
                    || before == Status.RUNNING
                    || after == Status.RUNNING;
        }
        /**
         * Convenience method to check if there are any requests that resulted in an error.
         *
         * @return True if there are any requests that finished with error, false otherwise.
         */
        public boolean hasError() {
            return initial == Status.FAILED
                    || before == Status.FAILED
                    || after == Status.FAILED;
        }
        /**
         * Returns the error for the given request type.
         *
         * @param type The request type for which the error should be returned.
         * @return The {@link Throwable} returned by the failing request with the given type or
         * {@code null} if the request for the given type did not fail.
         */
        @Nullable
        public Throwable getErrorFor(@NonNull RequestType type) {
            return mErrors[type.ordinal()];
        }
        @Override
        public String toString() {
            return "StatusReport{"
                    + "initial=" + initial
                    + ", before=" + before
                    + ", after=" + after
                    + ", mErrors=" + Arrays.toString(mErrors)
                    + '}';
        }
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            StatusReport that = (StatusReport) o;
            if (initial != that.initial) return false;
            if (before != that.before) return false;
            if (after != that.after) return false;
            // Probably incorrect - comparing Object[] arrays with Arrays.equals
            return Arrays.equals(mErrors, that.mErrors);
        }
        @Override
        public int hashCode() {
            int result = initial.hashCode();
            result = 31 * result + before.hashCode();
            result = 31 * result + after.hashCode();
            result = 31 * result + Arrays.hashCode(mErrors);
            return result;
        }
    }
    /**
     * Listener interface to get notified by request status changes.
     */
    public interface Listener {
        /**
         * Called when the status for any of the requests has changed.
         *
         * @param report The current status report that has all the information about the requests.
         */
        void onStatusChange(@NonNull StatusReport report);
    }
    /**
     * Represents the status of a Request for each {@link RequestType}.
     */
    public enum Status {
        /**
         * There is current a running request.
         */
        RUNNING,
        /**
         * The last request has succeeded or no such requests have ever been run.
         */
        SUCCESS,
        /**
         * The last request has failed.
         */
        FAILED
    }
    /**
     * Available request types.
     */
    public enum RequestType {
        /**
         * Corresponds to an initial request made to a {@link androidx.paging.DataSource} or the empty state for
         * a {@link androidx.paging.PagedList.BoundaryCallback BoundaryCallback}.
         */
        INITIAL,
        /**
         * Corresponds to the {@code loadBefore} calls in {@link androidx.paging.DataSource} or
         * {@code onItemAtFrontLoaded} in
         * {@link androidx.paging.PagedList.BoundaryCallback BoundaryCallback}.
         */
        BEFORE,
        /**
         * Corresponds to the {@code loadAfter} calls in {@link androidx.paging.DataSource} or
         * {@code onItemAtEndLoaded} in
         * {@link androidx.paging.PagedList.BoundaryCallback BoundaryCallback}.
         */
        AFTER
    }
    class RequestQueue {
        @NonNull
        final RequestType mRequestType;
        @Nullable
        RequestWrapper mFailed;
        @Nullable
        Request mRunning;
        @Nullable
        Throwable mLastError;
        @NonNull
        Status mStatus = Status.SUCCESS;
        RequestQueue(@NonNull RequestType requestType) {
            mRequestType = requestType;
        }
    }
}