android - 重复异步任务

Question

我怀疑在 Android 应用程序中重复 AsyncTask 的可能性。我想重复一些操作，例如从服务器下载文件，如果由于某些原因无法下载文件，则重复 n 次。有快速的方法吗？

Answer 1

您不能重复 AsyncTask 但您可以重复它执行的操作。

我制作了这个您可能想要扩展的小助手类来代替 AsyncTask，唯一的大区别是您将使用 repeatInBackground 而不是 doInBackground，并且 onPostExecute 将有一个新参数，即最终抛出的异常。

repeatInBackground 中的任何内容都将自动重复，直到结果不同于 null/不抛出异常并且次数少于 maxTries。

循环中抛出的最后一个异常将在 onPostExecute(Result, Exception) 中返回。

您可以使用 RepeatableAsyncTask(int retries) 构造函数设置最大尝试次数。

public abstract class RepeatableAsyncTask<A, B, C> extends AsyncTask<A, B, C> {
    private static final String TAG = "RepeatableAsyncTask";
    public static final int DEFAULT_MAX_RETRY = 5;

    private int mMaxRetries = DEFAULT_MAX_RETRY;
    private Exception mException = null;

    /**
     * Default constructor
     */
    public RepeatableAsyncTask() {
        super();
    }

    /**
     * Constructs an AsyncTask that will repeate itself for max Retries
     * @param retries Max Retries.
     */
    public RepeatableAsyncTask(int retries) {
        super();
        mMaxRetries = retries;
    }

    /**
     * Will be repeated for max retries while the result is null or an exception is thrown.
     * @param inputs Same as AsyncTask's
     * @return Same as AsyncTask's
     */
    protected abstract C repeatInBackground(A...inputs);

    @Override
    protected final C doInBackground(A...inputs) {
        int tries = 0;
        C result = null;

        /* This is the main loop, repeatInBackground will be repeated until result will not be null */
        while(tries++ < mMaxRetries && result == null) {
            try {
                result = repeatInBackground(inputs);
            } catch (Exception exception) {
                /* You might want to log the exception everytime, do it here. */
                mException = exception;
            }
        }
        return result;
    }

    /**
     * Like onPostExecute but will return an eventual Exception
     * @param c Result same as AsyncTask
     * @param exception Exception thrown in the loop, even if the result is not null.
     */
    protected abstract void onPostExecute(C c, Exception exception);

    @Override
    protected final void onPostExecute(C c) {
        super.onPostExecute(c);
        onPostExecute(c, mException);
    }
}