How can I cancel an asynchronous call through NSUR

问题:

I've got a web service call performing some validation on user input in real time. I'd like to use [NSURLConnection sendAsynchronousRequest] on the validation (which was introduced in iOS 5), but cancel it if the user changes the input field content in the mean time. What is the best way to cancel a current request?

回答1:

It doesn't appear that there is a good way to do this. The solution seems to be to not use the new [NSURLConnection sendAsynchronousRequest] in situations in which you need to cancel the request.

回答2:

I've managed to do this by placing the sendAsynchronousRequest method in a separate DownloadWrapper class, as follows:

//
//  DownloadWrapper.h
//
//  Created by Ahmed Khalaf on 16/12/11.
//  Copyright (c) 2011 arkuana. All rights reserved.
//

#import <Foundation/Foundation.h>

@protocol DownloadWrapperDelegate
- (void)receivedData:(NSData *)data;
- (void)emptyReply;
- (void)timedOut;
- (void)downloadError:(NSError *)error;
@end

@interface DownloadWrapper : NSObject {
    id<DownloadWrapperDelegate> delegate;
}
@property(nonatomic, retain) id<DownloadWrapperDelegate> delegate;
- (void)downloadContentsOfURL:(NSString *)urlString;
@end

@implementation DownloadWrapper
@synthesize delegate;

- (void)downloadContentsOfURL:(NSString *)urlString
{
    NSURL *url = [NSURL URLWithString:urlString];

    NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringLocalAndRemoteCacheData timeoutInterval:TIMEOUT_INTERVAL];
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];

    [NSURLConnection sendAsynchronousRequest:urlRequest queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
    {
        if ([data length] > 0 && error == nil)
            [delegate receivedData:data];
        else if ([data length] == 0 && error == nil)
            [delegate emptyReply];
        else if (error != nil && error.code == ERROR_CODE_TIMEOUT)
            [delegate timedOut];
        else if (error != nil)
            [delegate downloadError:error];
    }];
}
@end

To utilise this class, I do the following, in addition to declaring the DownloadWrapper *downloadWrapper variable (in the interface declaration) and implementing the protocol methods which handles the response or a lack of one:

NSString *urlString = @"http://yoursite.com/page/to/download.html";
downloadWrapper = [DownloadWrapper alloc];
downloadWrapper.delegate = self;
[downloadWrapper downloadContentsOfURL:urlString];

Then I simply do the following to 'cancel' the connection when the view is about to disappear:

- (void)viewDidUnload
{
    [super viewDidUnload];
    downloadWrapper = nil;
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    [downloadWrapper setDelegate:nil];
}

It's as simple as that. This would hopefully mimic the documented cancel method, which states that it does the following:

Once this method is called, the receiver’s delegate will no longer receive any messages for this NSURLConnection.

I was concerned that this (somewhat naive) method means that the packets of data would still come through in response to our URL request - only that we're no longer 'listening in' as the delegate. But then I realised that once the URL request was sent through, there's really no way of stopping the response from coming back to us - we can only disregard it (if not at this level, then still at some lower level in the network hierarchy). Please correct me if I'm wrong.

Either way, hope this helps.