iPhone NSURLConnection：connectionDidFinishLoading - 如何将字符串返回到调用方法

Question

我已经回顾了类似的 stackoverflow 问题/答案，但我仍然感到困惑。

我是一个初学者，我真的很挣扎。使用 iPhone，我可以从 URL 下载 XML，但无法将结果字符串存储在 NSString 变量中并从调用函数中查看它。

我在自定义类中有以下声明：

@interface comms : NSObject {
    NSString *currURL, *receivedString;
    NSMutableData *receivedData;
    NSURLConnection *conn;
}

@property (copy, readwrite) NSString *currURL;
@property (copy, readonly) NSString *receivedString;
@property (nonatomic, assign) NSMutableData *receivedData;
@property (nonatomic, assign) NSURLConnection *conn;

-(void) getContentURL;

我在通讯类中有以下方法：

-(void) getContentURL
{
    NSLog( @"Begin getContentURL." );

    // Request data related.
    NSMutableURLRequest *request = [[[NSMutableURLRequest alloc] init] 
                                    autorelease];
    [request setURL:[NSURL URLWithString: currURL]];

    // Content-Type related.
    [request setValue:@"application/x-www-form-urlencoded" 
   forHTTPHeaderField:@"Content-Type"];

    // Create Connection.
    conn = [[NSURLConnection alloc] initWithRequest:request delegate:self];

    if (conn) {
        // The connection was established.
        receivedData = [[NSMutableData data] retain];
        NSLog( @"Data will be received from URL: %@", request.URL );
    }
    else
    {
        // The download could not be made.
        NSLog( @"Data could not be received from: %@", request.URL );
    }

    // PROBLEM - receivedString is NULL here.
    NSLog( @"From getContentURL: %@", receivedString );
}

我已按照以下方式在通讯类中创建了所需的委托：

-(void)connection:(NSURLConnection *)connection didReceiveResponse:
    (NSURLResponse *)response
{
    // Discard all previously received data.
    [receivedData setLength:0];
}

-(void)connection:(NSURLConnection *)connection didReceiveData:
(NSData *)data
{
    // Append the new data to the receivedData.
[receivedData appendData:data];     
}

-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    // Connection succeeded in downloading the request.
    NSLog( @"Succeeded! Received %d bytes of data", [receivedData length] );

    // Convert received data into string.
    receivedString = [[NSString alloc] initWithData:receivedData 
        encoding:NSASCIIStringEncoding];
    NSLog( @"From connectionDidFinishLoading: %@", receivedString );

    // release the connection, and the data object
    [conn release];
    [receivedData release];
}

我可以在 connectionDidFinishLoading 委托中使用 NSLog 成功输出 receiveString 字符串。

    // Convert received data into string.
    receivedString = [[NSString alloc] initWithData:receivedData 
        encoding:NSASCIIStringEncoding];
    NSLog( @"From connectionDidFinishLoading: %@", receivedString );

但是，当我在 getContentURL 中输出 receiveString 字符串时，它为 null（因此从我调用 comms 类的 ViewController.m 类中也为 null）。

    // PROBLEM - receivedString is NULL here.
    NSLog( @"From getContentURL: %@", receivedString );

关于如何在 getContentURL 和 ViewController.m 类中查看 receiveString 的值有什么想法吗？

Answer 1

NSURLConnection 是一个异步 API。当您启动请求时，该对象将生成一个新线程，并且仅通过回调/委托方法更新您的主线程。您当前的方法很可能会在请求完成之前返回，因此结果的字符串还没有下载！

如果您想同步执行此操作，您将有两种选择：

1. 使用内置的同步下载方法。请注意，当此阻塞时，它将不允许用户与 UI 交互。
2. 使用 C 函数 CFRunLoopRun() 和 CFRunLoopStop() 在调用函数内启动运行循环，等待下载完成或失败，然后使用 CFRunLoopStop() 将控制权返回给调用方法。

Answer 2

要从委托方法返回数据，请使用代码块。

请参阅 Apple 的 Blocks 文档。