IOS ---網絡異步請求

異步請求使用與同步和隊列式異步請求相同的對象，只不過又增加了另一個對象，即NSURLConnectionDelegate：
上代碼：

#import "ViewController.h"

NSInteger totalDownLoaded = 0;

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSURL *url = [NSURL URLWithString:@"http://www.example.com/test.php"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    NSURLConnection *conn = [NSURLConnection connectionWithRequest:request delegate:self];

    [conn start];
}

/*
 *如果協議處理器接收到來自服務器的重定向請求，就會調用該方法
 */
-(NSURLRequest *)connection:(NSURLConnection *)connection willSendRequest:(NSURLRequest *)request redirectResponse:(NSURLResponse *)response{
//    NSLog(@"All Headers = %@", [(NSHTTPURLResponse *) response allHeaderFields]);
    return  request;
}

/*
 *當協議處理器接收到足夠的數據來創建URL響應對象時會調用didReceiveResponse方法。如果在接收到足夠的數據來構建對象前出現了錯誤，
 *就不會調用該方法
 */

-(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
    NSLog(@"All Headers = %@", [httpResponse allHeaderFields]);
    NSLog(@"statusCode = %ld", (long)httpResponse.statusCode);
    if (httpResponse.statusCode != 200) {
        [connection cancel];
        return;
    }
}

/*
 *當協議處理器接收到部分或全部響應體時會調用該方法。該方法可能不會調用，也可能調用多次，並且調用總是跟在最初的connection:didReceiveResponse之後
 */
-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
    totalDownLoaded += [data length];
    NSLog(@"%ld", (long)totalDownLoaded);
}

/*
 *當連接失敗時會調用這個委託方法。該方法可能會在請求處理的任何階段得到調用
 */
-(void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error{
    NSLog(@"netWork connect error");
}

/*
 *當整個請求完成加載並且接收到的所有數據都被傳遞給委託後，就會調用該委託方法
 */
-(void)connectionDidFinishLoading:(NSURLConnection *)connection{
    NSLog(@"Finish");
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end