问题描述

处理 CursorAdapter getView() 中的查询问题

上面是getView()里的代码，db.isRegistered(contactId_text)方法是一个查询语句，需要一些时间来获取结果，所以就落后于列表视图滚动屏幕，有什么方法来处理这个问题呢？我使用AsyncTask，但是当列表太长时，就会获得 AsyncTask limit error。

public View getView(int position View convertView ViewGroup parent) {...        boolean flag = db.isRegistered(contactId_text);            ImageView iv = (ImageView) vi.findViewById(R.id.typeImage);        if (flag) {                holder.image1.setImageResource(R.drawable.registered);        }...}

解决方案

http://stackoverflow.com/questions/9654148/android-asynctask-threads-limits
All AsyncTasks are controlled internally by a shared (static) ThreadPoolExecutor and a LinkedBlockingQueue. When you call execute on an AsyncTask the ThreadPoolExecutor will execute it when it is ready some time in the future.

The 'when am I ready?' behaviour of a ThreadPoolExecutor is controlled by two parameters the core pool size and the maximum pool size. If there are less than core pool size threads currently active and a new job comes in the executor will create a new thread and execute it immediately. If there are at least core pool size threads running it will try to queue the job and wait until there is an idle thread available (i.e. until another job is completed). If it is not possible to queue the job (the queue can have a max capacity) it will create a new thread (upto maximum pool size threads) for the jobs to run in. Non-core idle threads can eventually be decommissioned according to a keep-alive timeout parameter.

Before Android 1.6 the core pool size was 1 and the maximum pool size was 10. Since Android 1.6 the core pore size is 5 and the maximum pool size is 128. The size of the queue is 10 in both cases. The keep-alive timeout was 10 seconds before 2.3 and 1 second since then.

With all of this in mind it now becomes clear why the AsyncTask will only appear to execute 5/6 of your tasks. The 6th task is being queued up until one of the other tasks complete. This is a very good reason why you should not use AsyncTasks for long-running operations - it will prevent other AsyncTasks from ever running.

For completeness if you repeated your exercise with more than 6 tasks (e.g. 30) you will see that more than 6 will enter doInBackground as the queue will become full and the executor is pushed to create more worker threads. If you kept with the long-running task you should see that 20/30 become active with 10 still in the queue.