理解Service

/**
 * Constant to return from {@link #onStartCommand}: compatibility
 * version of {@link #START_STICKY} that does not guarantee that
 * {@link #onStartCommand} will be called again after being killed.
 */
public static final int START_STICKY_COMPATIBILITY = 0;

/**
 * Constant to return from {@link #onStartCommand}: if this service's
 * process is killed while it is started (after returning from
 * {@link #onStartCommand}), then leave it in the started state but
 * don't retain this delivered intent.  Later the system will try to
 * re-create the service.  Because it is in the started state, it will
 * guarantee to call {@link #onStartCommand} after creating the new
 * service instance; if there are not any pending start commands to be
 * delivered to the service, it will be called with a null intent
 * object, so you must take care to check for this.
 *
 * <p>This mode makes sense for things that will be explicitly started
 * and stopped to run for arbitrary periods of time, such as a service
 * performing background music playback.
 */
public static final int START_STICKY = 1;

/**
 * Constant to return from {@link #onStartCommand}: if this service's
 * process is killed while it is started (after returning from
 * {@link #onStartCommand}), and there are no new start intents to
 * deliver to it, then take the service out of the started state and
 * don't recreate until a future explicit call to
 * {@link Context#startService Context.startService(Intent)}.  The
 * service will not receive a {@link #onStartCommand(Intent, int, int)}
 * call with a null Intent because it will not be re-started if there
 * are no pending Intents to deliver.
 *
 * <p>This mode makes sense for things that want to do some work as a
 * result of being started, but can be stopped when under memory pressure
 * and will explicit start themselves again later to do more work.  An
 * example of such a service would be one that polls for data from
 * a server: it could schedule an alarm to poll every N minutes by having
 * the alarm start its service.  When its {@link #onStartCommand} is
 * called from the alarm, it schedules a new alarm for N minutes later,
 * and spawns a thread to do its networking.  If its process is killed
 * while doing that check, the service will not be restarted until the
 * alarm goes off.
 */
public static final int START_NOT_STICKY = 2;

/**
 * Constant to return from {@link #onStartCommand}: if this service's
 * process is killed while it is started (after returning from
 * {@link #onStartCommand}), then it will be scheduled for a restart
 * and the last delivered Intent re-delivered to it again via
 * {@link #onStartCommand}.  This Intent will remain scheduled for
 * redelivery until the service calls {@link #stopSelf(int)} with the
 * start ID provided to {@link #onStartCommand}.  The
 * service will not receive a {@link #onStartCommand(Intent, int, int)}
 * call with a null Intent because it will will only be re-started if
 * it is not finished processing all Intents sent to it (and any such
 * pending events will be delivered at the point of restart).
 */
public static final int START_REDELIVER_INTENT = 3;