Objective-C中的URL加载系统

URL加载用于访问URL，即互联网上的资源。它借助以下类实现：

• NSMutableURLRequest
• NSURLConnection
• NSURLCache
• NSURLAuthenticationChallenge
• NSURLCredential
• NSURLProtectionSpace
• NSURLResponse
• NSURLDownload
• NSURLSession

这是一个简单的URL加载示例。它无法在命令行运行，需要创建一个Cocoa应用程序。

这可以通过在XCode中选择“新建”>“项目”，然后在出现的窗口的OS X应用程序部分选择“Cocoa应用程序”来完成。

点击下一步完成后续步骤，系统会要求您提供项目名称，您可以随意命名。

appdelegate.h文件内容如下：

#import <Cocoa/Cocoa.h>

@interface AppDelegate : NSObject <NSApplicationDelegate>

@property (assign) IBOutlet NSWindow *window;

@end

将AppDelegate.m文件更新为：

#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

现在，当我们编译并运行程序时，我们将得到以下结果。

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

在上面的程序中，我们创建了一个简单的URL连接，它以JSON格式获取时间并显示时间。