ios - 从嵌套完成 block 返回值

Question

我有一个完成 block ，它采用一个基于用户选择的动态参数，如下所示:

NSString *data = <some value based on user selection>;
[avc activity:data withBlock:^id _Nonnull{

}

此 block 的返回类型为 id。

我有另一个像这样的 block :

[self createItem:data completion:^(NSString * _Nullable item) {
    //value of item is received here
}];

我想要实现的是:

NSString *data = <some value based on user selection>;
//This data is used in both the blocks
[avc activity:data withBlock:^id _Nonnull{
    [self createItem:data completion:^(NSString * _Nullable item) {
        return item; //Error here
    }];
}

返回项目会抛出错误，因为内部 block 的返回类型是 void。我想要实现的是，当内部 block 收到 item 中的值时，它应该将它返回到外部 block 。

我试过像这样使用调度组:

NSString *data = <some value based on user selection>;
//This data is used in both the blocks
[avc activity:data withBlock:^id _Nonnull{
    __block NSString *url = nil;
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_enter(group);
    [self createItem:data completion:^(NSString * _Nullable item) {
        url = item;
        dispatch_group_leave(group);
    }];
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    return url
}

这是行不通的。应用程序在等待语句后无限期卡住。

有什么方法可以实现我的目标吗？

Answer 1

[[NSOperationQueue new] addOperationWithBlock:^{
  NSString *data; // <some value based on user selection>;
  [avc activity:data withBlock:^id _Nonnull{
      __block NSString *url = nil;
      dispatch_semaphore_t finished = dispatch_semaphore_create(0);
      [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        [self createItem:data completion:^(NSString * _Nullable item) {
            url = item;
            dispatch_semaphore_signal(finished);
        }];
      }];
      dispatch_semaphore_wait(finished, DISPATCH_TIME_FOREVER);
      return url
  }];
}];