ModernAsyncTask.java

/*
 * Copyright 2018 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 androidx.loader.content;

import android.os.Binder;
import android.os.Handler;
import android.os.Looper;
import android.os.Process;

import androidx.annotation.NonNull;

import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.FutureTask;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * Copy of the required parts of {@link android.os.AsyncTask} from Android 3.0 that is
 * needed to support AsyncTaskLoader.  We use this rather than the one from the platform
 * because we rely on some subtle behavior of AsyncTask that is not reliable on
 * older platforms.
 *
 * <p>Note that for now this is not publicly available because it is not a
 * complete implementation, only sufficient for the needs of
 * {@link android.content.AsyncTaskLoader}.
 */
abstract class ModernAsyncTask<Result> {
    private static final String LOG_TAG = "AsyncTask";

    private static Handler sHandler;

    private final FutureTask<Result> mFuture;

    private volatile Status mStatus = Status.PENDING;

    @SuppressWarnings("WeakerAccess") /* synthetic access */
    final AtomicBoolean mCancelled = new AtomicBoolean();
    @SuppressWarnings("WeakerAccess") /* synthetic access */
    final AtomicBoolean mTaskInvoked = new AtomicBoolean();

    /**
     * Indicates the current status of the task. Each status will be set only once
     * during the lifetime of a task.
     */
    public enum Status {
        /**
         * Indicates that the task has not been executed yet.
         */
        PENDING,
        /**
         * Indicates that the task is running.
         */
        RUNNING,
        /**
         * Indicates that {@link android.os.AsyncTask#onPostExecute(Object)} has finished.
         */
        FINISHED,
    }

    private static Handler getHandler() {
        synchronized (ModernAsyncTask.class) {
            if (sHandler == null) {
                sHandler = new Handler(Looper.getMainLooper());
            }
            return sHandler;
        }
    }

    /**
     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
     */
    ModernAsyncTask() {
        Callable<Result> worker = new Callable<Result>() {
            @Override
            public Result call() {
                mTaskInvoked.set(true);
                Result result = null;
                try {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    result = doInBackground();
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                return result;
            }
        };

        mFuture = new FutureTask<Result>(worker) {
            @Override
            protected void done() {
                try {
                    final Result result = get();

                    postResultIfNotInvoked(result);
                } catch (InterruptedException e) {
                    android.util.Log.w(LOG_TAG, e);
                } catch (ExecutionException e) {
                    throw new RuntimeException(
                            "An error occurred while executing doInBackground()", e.getCause());
                } catch (CancellationException e) {
                    postResultIfNotInvoked(null);
                } catch (Throwable t) {
                    throw new RuntimeException(
                            "An error occurred while executing doInBackground()", t);
                }
            }
        };
    }

    @SuppressWarnings("WeakerAccess") /* synthetic access */
    void postResultIfNotInvoked(Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }

    @SuppressWarnings("WeakerAccess") /* synthetic access */
    void postResult(final Result result) {
        getHandler().post(new Runnable() {
            @Override
            public void run() {
                finish(result);
            }
        });
    }

    /**
     * Override this method to perform a computation on a background thread.
     *
     * @return A result, defined by the subclass of this task.
     *
     * @see #onPostExecute
     */
    protected abstract Result doInBackground();

    /**
     * <p>Runs on the UI thread after {@link #doInBackground}. The
     * specified result is the value returned by {@link #doInBackground}.</p>
     *
     * <p>This method won't be invoked if the task was cancelled.</p>
     *
     * @param result The result of the operation computed by {@link #doInBackground}.
     *
     * @see #doInBackground
     * @see #onCancelled(Object)
     */
    protected void onPostExecute(Result result) {
    }

    /**
     * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
     * {@link #doInBackground()} has finished.</p>
     *
     * @param result The result, if any, computed in
     *               {@link #doInBackground()}, can be null
     *
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    protected void onCancelled(Result result) {
    }

    /**
     * Returns <tt>true</tt> if this task was cancelled before it completed
     * normally. If you are calling {@link #cancel(boolean)} on the task,
     * the value returned by this method should be checked periodically from
     * {@link #doInBackground()} to end the task as soon as possible.
     *
     * @return <tt>true</tt> if task was cancelled before it completed
     *
     * @see #cancel(boolean)
     */
    public final boolean isCancelled() {
        return mCancelled.get();
    }

    /**
     * <p>Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when <tt>cancel</tt> is called,
     * this task should never run. If the task has already started,
     * then the <tt>mayInterruptIfRunning</tt> parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.</p>
     *
     * <p>Calling this method will result in {@link #onCancelled(Object)} being
     * invoked on the UI thread after {@link #doInBackground()}
     * returns. Calling this method guarantees that {@link #onPostExecute(Object)}
     * is never invoked. After invoking this method, you should check the
     * value returned by {@link #isCancelled()} periodically from
     * {@link #doInBackground()} to finish the task as early as
     * possible.</p>
     *
     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
     *        task should be interrupted; otherwise, in-progress tasks are allowed
     *        to complete.
     *
     * @return <tt>false</tt> if the task could not be cancelled,
     *         typically because it has already completed normally;
     *         <tt>true</tt> otherwise
     *
     * @see #isCancelled()
     * @see #onCancelled(Object)
     */
    public final boolean cancel(boolean mayInterruptIfRunning) {
        mCancelled.set(true);
        return mFuture.cancel(mayInterruptIfRunning);
    }

    /**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     *
     * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from
     * a thread pool is generally <em>not</em> what one wants, because the order
     * of their operation is not defined.  For example, if these tasks are used
     * to modify any state in common (such as writing a file due to a button click),
     * there are no guarantees on the order of the modifications.
     * Without careful work it is possible in rare cases for the newer version
     * of the data to be over-written by an older one, leading to obscure data
     * loss and stability issues.
     *
     * <p>This method must be invoked on the UI thread.
     *
     * @param exec The executor to use.
     *
     * @throws IllegalStateException If already running or finished.
     */
    public final void executeOnExecutor(@NonNull Executor exec) {
        if (mStatus != Status.PENDING) {
            switch (mStatus) {
                case RUNNING:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task is already running.");
                case FINISHED:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task has already been executed "
                            + "(a task can be executed only once)");
                default:
                    throw new IllegalStateException("We should never reach this state");
            }
        }

        mStatus = Status.RUNNING;

        exec.execute(mFuture);
    }

    void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }
}