ios - 如何检查 connectionDidFinishLoading 何时完成

Question

我在我的 iOS 应用程序中设置了一个简单的 Objective-C 类，它有一个简单的任务，即下载一个 JSON 文件，解析它，然后传回一个 NSString，其中包含一个从下载的 JSON 文件解析的变量。

我遇到的问题是我从另一个类调用这个类，这一切都很好，但是我需要将 NSString 传回我调用它的类。

问题是该方法在 connectionDidFinishLoading 发生之前传回空的 NSString ....因此 NSString 永远不会被分配一个字符串......

我在我的方法中设置了一个 while 循环，但它并没有真正起作用......

这是我的代码:

-(NSString *)get_user_icon:(NSString *)YT_ID {

// Set BOOL to 0 for initial setup.
icon_check = 0;

NSString *url_YT = [NSString stringWithFormat:YOUT_profile_part_2, YT_ID];

dispatch_queue_t downloadQueue = dispatch_queue_create("Icon downloader YouTube", NULL);
dispatch_async(downloadQueue, ^{

    dispatch_async(dispatch_get_main_queue(), ^{

        NSURLRequest *theRequest_YT = [NSURLRequest requestWithURL:[NSURL URLWithString:url_YT]];
        NSURLConnection *theConnection_YT = [[NSURLConnection alloc] initWithRequest:theRequest_YT delegate:self];

        if (theConnection_YT) {
            YT_JSON_FEED = [[NSMutableData alloc] init];
            NSLog(@"Respoce happening...");
        }

        else {
            NSLog(@"failed");
        }
    });
});

while (icon_check == 0) {
    NSLog(@"Wait");
}

return icon_url;
}

/// Data loading ///

-(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    [YT_JSON_FEED setLength:0];
}

-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    [YT_JSON_FEED appendData:data];
}

-(void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    NSString *msg = [NSString stringWithFormat:@"Failed: %@", [error description]];
    NSLog(@"%@",msg);
}

-(void)connectionDidFinishLoading:(NSURLConnection *)connection {

NSError *myError = nil;
NSDictionary *feed = [NSJSONSerialization JSONObjectWithData:YT_JSON_FEED options:NSJSONReadingMutableLeaves error:&myError];

icon_url = [[[[[feed objectForKey:@"items"] valueForKey:@"snippet"] valueForKey:@"thumbnails"] valueForKey:@"default"] valueForKey:@"url"];

icon_check = 1;
}

Answer 1

对于同步请求(阻塞直到有东西返回)，使用 NSURLConnection 的 sendSynchronousRequest:returningResponse:error: 代替。像这样:

-(NSString *)get_user_icon:(NSString *)YT_ID {

    NSString *url_YT = [NSString stringWithFormat:YOUT_profile_part_2, YT_ID];

    NSURLRequest *theRequest_YT = [NSURLRequest requestWithURL:[NSURL URLWithString:url_YT]];

    NSURLResponse* response = nil;
    NSError* error = nil;
    NSData* data = [NSURLConnection sendSynchronousRequest:theRequest_YT returningResponse:&response error:&error];

    //Check response and error for possible errors here.

    //If no errors.
    NSDictionary *feed = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves error:&myError];

    icon_url = [[[[[feed objectForKey:@"items"] valueForKey:@"snippet"] valueForKey:@"thumbnails"] valueForKey:@"default"] valueForKey:@"url"];

    return icon_url;
}

但是不推荐这样做。您需要将 API 更改为异步的。基于委托(delegate)，但更优选地，使用基于 block 的 API。