[android] Add JobScheduler-based BackgroundTaskScheduler.

components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.components.background_task_scheduler;
+
+import android.annotation.TargetApi;
+import android.content.Context;
+import android.os.Build;
+import android.support.annotation.Nullable;
+
+import org.chromium.base.Log;
+import org.chromium.base.ThreadUtils;
+
+import java.lang.reflect.Constructor;
+
+/**
+ * A BackgroundTaskScheduler which is used to schedule jobs that run in the background.
+ * It is backed by system APIs ({@link android.app.job.JobScheduler}) on newer platforms
+ * and by GCM ({@link com.google.android.gms.gcm.GcmNetworkManager}) on older platforms.
+ *
+ * To get an instance of this class, use {@link BackgroundTaskSchedulerFactory#getScheduler()}.
+ */
+@TargetApi(Build.VERSION_CODES.LOLLIPOP)
+public class BackgroundTaskScheduler {
+    private static final String TAG = "BkgrdTaskScheduler";
+
+    @Nullable
+    static BackgroundTask getBackgroundTaskFromClassName(String backgroundTaskClassName) {
+        if (backgroundTaskClassName == null) return null;
+
+        Class<?> clazz;
+        try {
+            clazz = Class.forName(backgroundTaskClassName);
+        } catch (ClassNotFoundException e) {
+            Log.w(TAG, "Unable to find BackgroundTask class with name " + backgroundTaskClassName);
+            return null;
+        }
+
+        if (!BackgroundTask.class.isAssignableFrom(clazz)) {
+            Log.w(TAG, "Class " + clazz + " is not a BackgroundTask");
+            return null;
+        }
+
+        try {
+            return (BackgroundTask) clazz.newInstance();
+        } catch (InstantiationException | IllegalAccessException e) {
+            Log.w(TAG, "Unable to instantiate class " + clazz);
+            return null;
+        }
+    }
+
+    static boolean hasParameterlessPublicConstructor(Class<? extends BackgroundTask> clazz) {
+        for (Constructor<?> constructor : clazz.getConstructors()) {
+            if (constructor.getParameterTypes().length == 0) return true;
+        }
+        return false;
+    }
+
+    private final BackgroundTaskSchedulerDelegate mSchedulerDelegate;
+
+    BackgroundTaskScheduler(BackgroundTaskSchedulerDelegate schedulerDelegate) {
+        assert schedulerDelegate != null;
+        mSchedulerDelegate = schedulerDelegate;
+    }
+
+    /**
+     * Schedules a background task. See {@link TaskInfo} for information on what types of tasks that
+     * can be scheduled.
+     *
+     * @param context the current context.
+     * @param taskInfo the information about the task to be scheduled.
+     * @return true if the schedule operation succeeded, and false otherwise.
+     * @see TaskInfo
+     */
+    public boolean schedule(Context context, TaskInfo taskInfo) {
+        ThreadUtils.assertOnUiThread();
+        return mSchedulerDelegate.schedule(context, taskInfo);
+    }
+
+    /**
+     * Cancels the task specified by the task ID.
+     *
+     * @param context the current context.
+     * @param taskId the ID of the task to cancel. See {@link TaskIds} for a list.
+     */
+    public void cancel(Context context, int taskId) {
+        ThreadUtils.assertOnUiThread();
+        mSchedulerDelegate.cancel(context, taskId);
+    }
+}