相关疑难解决方法(0)

我需要同步在主队列上调度一个块.我不知道我目前是在主线程上运行还是不运行.天真的解决方案看起来像这样:

dispatch_sync(dispatch_get_main_queue(), block);

但是如果我当前在主队列上运行的块中,则此调用会产生死锁.(同步调度等待块完成,但块甚至没有开始运行,因为我们正在等待当前块完成.)

显而易见的下一步是检查当前队列:

if (dispatch_get_current_queue() == dispatch_get_main_queue()) {
    block();
} else {
    dispatch_sync(dispatch_get_main_queue(), block);
}

这有效,但很难看.在我至少将它隐藏在一些自定义功能之前,是不是有更好的解决方案来解决这个问题？我强调我不能异步调度块 - 应用程序处于异步调度块将"执行得太晚"的情况.

如何使块同步执行,或使函数在return语句之前等待处理程序,以便数据可以从块传回？

-(id)performRequest:(id)args
{
__block NSData *data = nil;   

    [xyzclass requestAccessToAccountsWithType:accountType withCompletionHandler:^(BOOL granted, NSError *error) {
        data = [NSData dataWithData:responseData];
    }];

    return data;
}

这是我的代码:

var userRef = new Firebase("https://awesome.firebaseio.com/users/");

var tokenRef = userRef.child(key+'/tokens');
tokenRef.once('value', function(snapshot){

  var userTokenSync = $firebase(tokenRef);
  var userTokens = userTokenSync.$asArray();


  console.log(userTokens);
  console.log(userTokens[0]);

  for(var i=0, len = userTokens.length; i < len; i++) {
     console.log(userTokens[i]);
  }
  console.log('done');
})

这段代码从firebase获取用户的标记,我只想浏览标记数组.

这是控制台给我的东西:

如您所见,我无法访问该阵列.你知道我怎么能这样做吗？

提前致谢.

我知道还有另外一个类似的问题,但它适用于较旧版本的AFNetworking,并且无论如何都没有真正回答它.

我有以下代码:

AFHTTPRequestOperationManager* manager = [AFHTTPRequestOperationManager manager];
manager.securityPolicy.allowInvalidCertificates = YES;
manager.requestSerializer = [AFJSONRequestSerializer serializer];
[manager.requestSerializer setAuthorizationHeaderFieldWithUsername: currentUser() password: currentPassword()];
__block NSDictionary* response = nil;
AFHTTPRequestOperation* operation = [manager
    GET: @"https://10.20.30.40:8765/foobar"
    parameters: [NSDictionary dictionary]
    success:^(AFHTTPRequestOperation* operation, id responseObject){
        response = responseObject;
        NSLog(@"response (block): %@", response);
    }
    failure:^(AFHTTPRequestOperation* operation, NSError* error){
        NSLog(@"Error: %@", error);}
];
[operation waitUntilFinished];
NSLog(@"response: %@", response);
...

如果我运行它,我将在日志中看到的是:

2013-12-09 09:26:20.105 myValve[409:60b] response: (null)
2013-12-09 09:26:20.202 myValve[409:60b] response (block): {
    F00005 = "";
    F00008 = "";
    F00013 …

有没有办法从主线程同步查询javascript？

使用具有回调参数的异步函数从本机代码查询Javascript以处理响应:

func evaluateJavaScript(_ javaScriptString: String, completionHandler completionHandler: ((AnyObject!, NSError!) -> Void)?)

通过暂停线程并使用信号量控制执行,通常可以将异步行为转为同步:

// Executing in the main thread
let sema = dispatch_semaphore_create(0)
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
// Background thread
  self.evaluateJavaScript("navigator.userAgent", completionHandler: { (value:AnyObject!, error: NSError!) -> Void in
    if let ua = value as? String {
      userAgent = ua
    } else {
      ERROR("ERROR There was an error retrieving the default user agent, using hardcoded value \(error)")
    }
    dispatch_semaphore_signal(sema)
  })
}
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)

然而......在这种情况下,因为completionHandler是一直被称为主线程,代码死锁,因为completionHandler块永远不会执行(主线程暂停由dispatch_semaphore_wait …

我有一系列我正在执行的dispatch_async,我只想在完成后更新UI.问题是dispatch_async中的方法在一个单独的线程中调用某些内容,因此它在数据完全加载之前返回,并在加载所有内容之前调用dispatch_group_notify.

所以我引入一个无限循环让它等到标志设置.这是最好的方法吗？见下面的代码.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_group_t group = dispatch_group_create();

for (...) {
  dispatch_group_async(group, queue, ^{
    __block BOOL dataLoaded = NO;

    [thirdPartyCodeCallWithCompletion:^{
       dataLoaded = YES;
    }];

    // prevent infinite loop
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), 
    queue, ^{
          dataLoaded = YES;
    });

    // infinite loop to wait until data is loaded
    while (1) {
       if (dataLoaded) break;
    }
  }

  dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    //update UI
  });
}

我是阻止的新手,我试图弄清楚如何在执行我的操作之前等待块完成(在这种情况下是一个nslog)所以我怎么能等到块完成之后再执行以下代码中的这个nslog: NSLog(@"convertedPhotos::%@",convertedImages);

          convertedImages =  [[NSMutableArray alloc] init];
        for (NSDictionary *photo in photos) {
            // photo is a dictionary containing a "caption" and a "urlRep"
            [photoUrls addObject:photo[@"urlRep"]];
        }

        if (photoUrls.count) {
            for (id photos in photoUrls){
                NSString *urlString = photos;
                [self base64ImageAtUrlString:urlString result:^(NSString *base64) {


                    [jsonWithPhotos setObject:convertedImages forKey:@"photo64"];
                    NSError *error;
                    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:jsonWithPhotos
                                                                       options:NSJSONWritingPrettyPrinted // Pass 0 if you don't care about the readability of the generated string
                                                                         error:&error];

                    if (! jsonData) {
                        NSLog(@"Got an error: %@", error);
                    } …