@Retention(value=CLASS)
@Target(value=METHOD)
public @interface DownloadGroup
@DownloadGroup.onPre(groupHash)
protected void onPre(DownloadGroupTask task) {
}
如果希望获取子任务的状态,可以使用onSub..类的注解
@DownloadGroup.onSubPre(groupHash)
protected void onPre(DownloadGroupTask task, String url) {
}