java - AsyncTask onCancelled(Object) 在 asyncTask.cancel(true) 之后从未调用过；

Question

如文档所述，onCancelled(Object) 应在以下时间之后调用:

cancel(boolean) 被调用并且 doInBackground(Object[]) 已完成。

在我的 AsyncTask 中，我调用了 this.cancel(true); 但从未调用 onCancelled(Object) 方法。

此处仅贴出相关代码。

MainActivity.java:

 AsyncTask.Status asyncTaskStatus = new GetHtmlDocument(urlString).execute().getStatus();

异步任务:

private class GetHtmlDocument extends AsyncTask<String,Void,HtmlPage>
    {
        private String url;

        /**
         * Constructor.
         *
         * @param url Url to parse from in the web.
         */
        public GetHtmlDocument(String url)
        {
            this.url = url;
        }

        @Override
        protected void onPreExecute()
        {
            Log.d(MainActivity.ASYNC_TASK_TAG, "onPreExecute() called");
        }

        @Override
        protected HtmlPage doInBackground(String... params)
        {
            //android.os.Debug.waitForDebugger();
            Log.d(MainActivity.ASYNC_TASK_TAG, "doInBackground() called");

            if (this.isCancelled())
            {
                return null;
            }
            else
            {


                HtmlPage htmlPage = new HtmlPage(getParsedDocument(this.url));

                return htmlPage;
            }
        }

        /**
         * Runs on the UI thread after doInBackground().
         * The specified result is the value returned by doInBackground().
         * This method won't be invoked if the asynchronous task was cancelled.
         *
         * @param htmlPage
         */
        @Override
        protected void onPostExecute(HtmlPage htmlPage)
        {
            Log.d(MainActivity.ASYNC_TASK_TAG, "onPostExecute() called");

            if (htmlPage.getHtmlDocument() != null)
            {
                this.cancel(true);
            }

            setHtmlPage(htmlPage);
        }

        /**
         * A task can be cancelled at any time by invoking cancel(boolean).
         * Invoking this method will cause subsequent calls to isCancelled() to return true.
         * After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns.
         * To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically from doInBackground(Object[]), if possible (inside a loop for instance.)
         *
         * @param htmlPage
         *
         */
        @Override
        protected void onCancelled(HtmlPage htmlPage)
        {
            Log.d(MainActivity.ASYNC_TASK_TAG, "onCancelled() called");
        }


    }

我调试了应用程序，我确定 this.cancel(true); 在 AsyncTask onPostExecute() 方法中被调用.

Answer 1

这只是因为您在 AsyncTask 完成其工作后调用了 this.cancel(true)!

意思是，当 AsyncTask 在 doInBackground 中完成它的工作时，您可以取消它，并且在它的状态为“完成”之后

来自文档，

A task can be cancelled at any time by invoking cancel(boolean). Invoking this method will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically from doInBackground(Object[]), if possible (inside a loop for instance.)

很明显，onCancelled 方法是在 AsyncTask 执行任务时调用的，在 doInBackground 完成之前，onCancelled(Object) 显然意味着在取消事件时调用 onPostExecute 的“INSTEAD”。 cancel(true)方法旨在“中断”AsyncTask 的操作。

还有，我想知道你为什么要取消任务？您可以简单地检查结果是否为 null 并仅在其不为 null 时才执行 onPostExecute，如下所示:

@Override
protected void onPostExecute(HtmlPage htmlPage)
{
    if (htmlPage.getHtmlDocument() != null){
       Log.d(MainActivity.ASYNC_TASK_TAG, "onPostExecute() called");
       setHtmlPage(htmlPage);
    }
}

或

@Override
protected void onPostExecute(HtmlPage htmlPage)
{
    Log.d(MainActivity.ASYNC_TASK_TAG, "onPostExecute() called");
    if (htmlPage.getHtmlDocument() != null){
       return;
    }

    setHtmlPage(htmlPage);
}