Question

Apple提供this of background execution：

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    bgTask = [application beginBackgroundTaskWithName:@"MyTask" 
    expirationHandler:^{
    // Clean up any unfinished task business by marking where you
    // stopped or ending the task outright.
    [application endBackgroundTask:bgTask];
    bgTask = UIBackgroundTaskInvalid;
    }];

// Start the long-running task and return immediately.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    // Do the work associated with the task, preferably in chunks.

    [application endBackgroundTask:bgTask];
    bgTask = UIBackgroundTaskInvalid;
    });
}

这个例子从来没有对我有任何意义，我已经看到它复制到许多背景应用程序示例中。

第一件没有意义的是expirationHandler中的这两行：

[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;

似乎bgTask在块中捕获时不会有值。编译器就这样抱怨。然后在dispatch_async下面，示例显示相同的两行。我希望它在dispatch_async中，但不在块中。谁能解释为什么我们在这个区块中有这些线？

每个beginBackgroundTaskWithName的文档也说＆＃34;标记新的长时间运行的后台任务的开始。＆＃34;这究竟是怎么做到的？什么定义了任务？是块范围内的任何代码吗？

Answer 1

  printf("\nGetting Card IDs!\n");

  // Jmd: Okay, this loop is going to fill up the IDs[] array which has
  // 612,967 slots. as this loops through and find new combinations it
  // adds them to the end. I need this list to be stable when I set the
  // handranks (next set)  (I do the insertion sort on new IDs these)
  // so I had to get the IDs first and then set the handranks
  for (IDnum = 0; IDs[IDnum] || IDnum == 0; IDnum++) {
    // start at 1 so I have a zero catching entry (just in case)
    for (card = 1; card < 53; card++) {
      // the ids above contain cards upto the current card.  Now add a new card
      ID = MakeID(IDs[IDnum], card);   // get the new ID for it
      // and save it in the list if I am not on the 7th card
      if (numcards < 7) holdid = SaveID(ID);
    }
    printf("\rID - %d", IDnum);   // show progress -- this counts up to 612976
  }

// main()
  printf("\nSetting HandRanks!\n");

  // this is as above, but will not add anything to the ID list, so it is stable
  for (IDnum = 0; IDs[IDnum] || IDnum == 0; IDnum++) {
    // start at 1 so I have a zero catching entry (just in case)
    for (card = 1; card < 53; card++) {
      ID = MakeID(IDs[IDnum], card);

      if (numcards < 7) {
    // when in the index mode (< 7 cards) get the id to save
    IDslot = SaveID(ID) * 53 + 53;
      } else {
    // if I am at the 7th card, get the equivalence class ("hand rank") to save
    IDslot = DoEval(ID);
      }

      maxHR = IDnum * 53 + card + 53;   // find where to put it
      HR[maxHR] = IDslot; // and save the pointer to the next card or the handrank
    }

    if (numcards == 6 || numcards == 7) {
      // an extra, If you want to know what the handrank when there is 5 or 6 cards
      // you can just do HR[u3] or HR[u4] from below code for Handrank of the 5 or
      // 6 card hand
      // this puts the above handrank into the array
      HR[IDnum * 53 + 53] = DoEval(IDs[IDnum]);
    }

    printf("\rID - %d", IDnum); // show the progress -- counts to 612976 again
}

告诉iOS您的应用程序正在启动新的后台任务。 iOS并不关心哪些代码包含任务，它只知道它需要为您的应用提供更多时间在后台执行。

执行此行后，bgTask = [application beginBackgroundTaskWithName:@"MyTask" expirationHandler:]将包含新的后台任务标识符。

当您的后台任务完成后，您可以调用bgTask并且iOS知道您的应用已完成指定的后台任务，并且可能不再需要任何后台执行时间（您可能还有其他后台任务由{ {1}}杰出的。）

该行：

[application endBackgroundTask:bgTask];

只是家务;如果省略此行，则不会发生任何错误，但beginBackgroundTaskWithName:expirationHandler将包含无效的标识符。

如果您在应用的后台时间到期之前没有拨打bgTask = UIBackgroundTaskInvalid;，那么将调用到期处理程序块。

在到期处理程序中bgTask将具有您在调用endBackgroundTask时分配的值，因此这是传递给bgTask并再次分配beginBackgroundTaskWithName:expirationHandler的内容持家

Apple的ios后台执行示例

1 个答案: