IntentService 组件类的扩展 按需 Service 处理异步请求

Question

IntentService 本质 = Handler + HandlerThread

• 按顺序、在后台执行
• 若启动 IntentService 多次，那么 每个耗时操作 则 以队列的方式 在 IntentService 的 onHandleIntent 回调方法中依次执行，执行完自动结束。但 onStartCommand 也会调用多次
• 为什么不能 bindService 去执行任务？？因为并不会调用 onStart() 或 onStartcommand()，故不会将消息发送到消息队列，那么 onHandleIntent() 将不会回调，即无法实现多线程的操作，失去了它本身的意义

@Override
public void onStart(@Nullable Intent intent, int startId) {
    Message msg = mServiceHandler.obtainMessage();
    msg.arg1 = startId;
    msg.obj = intent;
    mServiceHandler.sendMessage(msg);
}

可以看到，start 之后把 intent 包装在 message 里入队

/**
    * Sets intent redelivery preferences.  Usually called from the constructor
    * with your preferred semantics.
    *
    * <p>If enabled is true,
    * {@link #onStartCommand(Intent, int, int)} will return
    * {@link Service#START_REDELIVER_INTENT}, so if this process dies before
    * {@link #onHandleIntent(Intent)} returns, the process will be restarted
    * and the intent redelivered.  If multiple Intents have been sent, only
    * the most recent one is guaranteed to be redelivered.
    *
    * <p>If enabled is false (the default),
    * {@link #onStartCommand(Intent, int, int)} will return
    * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
    * dies along with it.
    */
public void setIntentRedelivery(boolean enabled) {
    mRedelivery = enabled;
}

还有个 redelivery 功能：如果任务执行完成之前 Service 挂了会自动 restart，然后处理最近的一个 intent