Objective-C URL加载系统

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要访问互联网的URL，即项目是有用的URL加载。它提供的下面的类的帮助：

NSMutableURLRequest

NSURLConnection

NSURLCache

NSURLAuthenticationChallenge

NSURLCredential

NSURLProtectionSpace

NSURLResponse

NSURLDownload

NSURLSession

下面是一个简单的URL加载的例子。这是不能在命令行中运行。我们需要创建Cocoa应用程序。

appdelegate.h 文件内容如下：#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

现在，当我们编译并运行程序，我们会得到以下的结果。

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