学步园

// Create the request.

NSURLRequest *theRequest=[NSURLRequest requestWithURL:[NSURL URLWithString:@"http://www.apple.com/"]

                        cachePolicy:NSURLRequestUseProtocolCachePolicy

                    timeoutInterval:60.0];

// create the connection with the request

// and start loading the data

NSURLConnection *theConnection=[[NSURLConnection alloc] initWithRequest:theRequest delegate:self];

if (theConnection) {

    // Create the NSMutableData to hold the received data.

    // receivedData is an instance variable declared elsewhere.

    receivedData = [[NSMutableData data] retain];

} else {

    // Inform the user that the connection failed.

2.当服务器准备好足够的信息时 即调用 connection:didReceiveResponse: 来返回消息，有可能会多次调用，比如由于服务器的 重定向或者有很多个MIME部分。

Listing 2  Example connection:didReceiveResponse: implementation
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response




{




    // This method is called when the server has determined that it




    // has enough information to create the NSURLResponse.




 




    // It can be called multiple times, for example in the case of a




    // redirect, so each time we reset the data.




 




    // receivedData is an instance variable declared elsewhere.




    [receivedData setLength:0];




}




3.当收到数据时，delegate周期性的发送 
connection:didReceiveData: 消息。

Listing 3  Example connection:didReceiveData: implementation





- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data




{




    // Append the new data to receivedData.




    // receivedData is an instance variable declared elsewhere.




    [receivedData appendData:data];




}




You can also use the connection:didReceiveData: method to provide an indication of the connection’s progress to the user.
可以在这里更新进度条。


4.如果在链接过程中 出现错误，则会发送 connection:didFailWithError:消息，NSError 对象作为参数用来说明具体错误内容。发生错误的URL可以通过

[[error userInfo] objectForKey:NSURLErrorFailingURLStringErrorKey]);得到。

Listing 4  Example connectionDidFailWithError: implementation
- (void)connection:(NSURLConnection *)connection




  didFailWithError:(NSError *)error




{




    // release the connection, and the data object




    [connection release];




    // receivedData is declared as a method instance elsewhere




    [receivedData release];




 




    // inform the user




    NSLog(@"Connection failed! Error - %@ %@",




          [error localizedDescription],




          [[error userInfo] objectForKey:NSURLErrorFailingURLStringErrorKey]);



5.若下载数据正常完成，则会收到



 connectionDidFinishLoading: message ，这是最后收到的消息，之后将不会收到消息。



Listing 5  Example connectionDidFinishLoading: implementation





- (void)connectionDidFinishLoading:(NSURLConnection *)connection




{




    // do something with the data




    // receivedData is declared as a method instance elsewhere




    NSLog(@"Succeeded! Received %d bytes of data",[receivedData length]);




 




    // release the connection, and the data object




    [connection release];




    [receivedData release];




 
}
 
注：一步只能收到connectionDidFinishLoading和didFailWithError消息中的一个，作为最后收到的消息。



Controlling Response Caching
By default the data for a connection is cached according to the support provided by the NSURLProtocol subclass that handles the request. An NSURLConnection delegate can further refine that behavior by implementingconnection:willCacheResponse:.
This delegate method can examine the provided NSCachedURLResponse object and change how the response is cached, for example restricting its storage to memory only or preventing it from being cached altogether. It is also possible to insert objects in an NSCachedURLResponse’s user info dictionary, causing them to be stored in the cache as part of the response.

Note: The delegate receives connection:willCacheResponse: messages only for protocols that support caching.
 

The example in Listing 6 prevents the caching of https responses. It also adds the current date to the user info dictionary for responses that are cached.

Listing 6  Example connection:withCacheResponse: implementation





-(NSCachedURLResponse *)connection:(NSURLConnection *)connection




                 willCacheResponse:(NSCachedURLResponse *)cachedResponse




{




    NSCachedURLResponse *newCachedResponse = cachedResponse;




 




    if ([[[[cachedResponse response] URL] scheme] isEqual:@"https"]) {




        newCachedResponse = nil;




    } else {




        NSDictionary *newUserInfo;




        newUserInfo = [NSDictionary dictionaryWithObject:[NSCalendarDate date]




                                                 forKey:@"Cached Date"];




        newCachedResponse = [[[NSCachedURLResponse alloc]




                                initWithResponse:[cachedResponse response]




                                    data:[cachedResponse data]




                                    userInfo:newUserInfo




                                    storagePolicy:[cachedResponse storagePolicy]]




                            autorelease];




    }




    return newCachedResponse;




}






Estimating Upload Progress
You can estimate the progress of an HTTP POST upload with the connection:didSendBodyData:totalBytesWritten:totalBytesExpectedToWrite: delegate method. Note that this is not an exact measurement of upload progress, because the connection may fail or the connection may encounter an authentication challenge.

Downloading Data Synchronously
NSURLConnection provides support for downloading the contents of an NSURLRequest in a synchronous manner using the class method sendSynchronousRequest:returningResponse:error:. Using this method is not recommended, because it has severe limitations:


The client application blocks until the data has been completely received, an error is encountered, or the request times out.


Minimal support is provided for requests that require authentication.


There is no means of modifying the default behavior of response caching or accepting server redirects.


If the download succeeds, the contents of the request are returned as an NSData object and an NSURLResponse for the request is returned by reference. If NSURLConnection is unable to download the URL, the method returns nil and any available NSError instance by reference in the appropriate parameter.
If the request requires authentication in order to make the connection, valid credentials must already be available in the NSURLCredentialStorage, or must be provided as part of the requested URL. If the credentials are not available or fail to authenticate, the URL loading system responds by sending the NSURLProtocol subclass handling the connection a continueWithoutCredentialForAuthenticationChallenge: message.
When a synchronous connection attempt encounters a server redirect, the redirect is always honored. Likewise the response data is stored in the cache according to the default support provided by the protocol implementation.
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