要訪問互聯(lián)網的URL，即項目是有用的URL加載。它提供的下面的類的幫助：

• NSMutableURLRequest

• NSURLConnection

• NSURLCache

• NSURLAuthenticationChallenge

• NSURLCredential

• NSURLProtectionSpace

• NSURLResponse

• NSURLDownload

• NSURLSession

下面是一個簡單的URL加載的例子。這是不能在命令行中運行。我們需要創(chuàng)建Cocoa應用程序。

這可以通過選擇新，然后在XCode項目下的OS X應用程序部分出現(xiàn)的窗口中選擇Cocoa應用程序。

完成步驟的順序點擊下一步，會被要求提供一個項目名稱，給它一個名字。

appdelegate.h 文件內容如下：

#import <Cocoa/Cocoa.h>

@interface AppDelegate : NSObject <NSApplicationDelegate>

@property (assign) IBOutlet NSWindow *window;

@end

更新以下AppDelegate.m文件：

#import "AppDelegate.h"

@interface SampleClass:NSObject<NSURLConnectionDelegate>
{
    NSMutableData *_responseData;
}
- (void)initiateURLConnection;

@end


@implementation SampleClass

- (void)initiateURLConnection
{
    // Create the request.
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:@"http://date.jsontest.com"]];
    
    // Create url connection and fire request
    NSURLConnection *conn = [[NSURLConnection alloc] initWithRequest:request delegate:self];
    [conn start];
}
#pragma mark NSURLConnection Delegate Methods

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    // A response has been received, this is where we initialize the instance var you created
    // so that we can append data to it in the didReceiveData method
    // Furthermore, this method is called each time there is a redirect so reinitializing it
    // also serves to clear it
    _responseData = [[NSMutableData alloc] init];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    // Append the new data to the instance variable you declared
    [_responseData appendData:data];
}

- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
                  willCacheResponse:(NSCachedURLResponse*)cachedResponse {
    // Return nil to indicate not necessary to store a cached response for this connection
    return nil;
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    // The request is complete and data has been received
    // You can parse the stuff in your instance variable now
    NSLog(@"%@",[[NSString alloc]initWithData:_responseData encoding:NSUTF8StringEncoding]);
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    // The request has failed for some reason!
    // Check the error var
}

@end

@implementation AppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    SampleClass *sampleClass = [[SampleClass alloc]init];
    [sampleClass initiateURLConnection];
    // Insert code here to initialize your application
}
@end

現(xiàn)在，當我們編譯并運行程序，我們會得到以下的結果。

2013-09-29 16:50:31.953 NSURLConnectionSample[1444:303] {
   "time": "11:20:31 AM",
   "milliseconds_since_epoch": 1380453631948,
   "date": "09-29-2013"
}

在上述程序中，我們已經創(chuàng)建了一個簡單的JSON格式的，帶有時間和顯示時間的URL連接。