iPhone NSURLConnection：connectionDidFinishLoading - 如何将字符串返回给调用方法

Posted 2023-02-22

【中文标题】iPhone NSURLConnection：connectionDidFinishLoading - 如何将字符串返回给调用方法

【英文标题】：iPhone NSURLConnection: connectionDidFinishLoading - how to return a string to the calling method

【发布时间】：2011-01-16 00:16:56

【问题描述】：

我已经查看了类似的 *** 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。

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 comms 类中有如下方法：

-(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 );

我已按照以下内容在 comms 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 receivedString 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 receivedString 字符串时，它为空（因此在我调用 comms 类的 ViewController.m 类中也为空）。

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

关于如何在 getContentURL 和 ViewController.m 类中查看 receivedString 值的任何想法？

【参考方案1】：

要从委托方法返回数据，请使用代码块。

见Apple's Blocks documentation。

【讨论】：

关于 OP 问题的代码很多，注意使用一些代码并显示示例吗？检查元how to provide a good answer 和有用的Jon Skeet blog

【参考方案2】：

NSURLConnection 是一个异步 API。当您启动请求时，该对象将产生一个新线程，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：

使用内置的同步下载方法。请注意，由于这会阻塞，因此它不允许用户与 UI 交互。 使用 C 函数 CFRunLoopRun() 和 CFRunLoopStop() 在调用函数中启动运行循环，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。

【讨论】：

谢谢 chpwn，太好了！我现在已经实现了“同步下载”方法，因为它仍然是我们正在构建的原型。下周，我一定会实现“异步循环”方法。如果我在 getContentURL 方法中启动运行循环，这实际上可以让我将下载的数据保存到 receievedString 属性中吗？

基本上，它会做的是允许在不先退出该函数/消息的情况下调用回调。