Protocol Usage and asynchronous call implementation

 #import <Foundation/Foundation.h>  
   
 @protocol DownloaderDelegate <NSObject>  
   
 -(void) downloadCompleted:(NSData*) data;  
   
 @end  
   
   
   
   
 #import <Foundation/Foundation.h>  
 #import "DownloaderDelegate.h"  
   
 typedef  void (^CallBack)(NSData *data);  
   
 @interface Downloader : NSObject  
 {  
   NSString *path;  
   CallBack callback;  
 }  
   
 @property (weak) id<DownloaderDelegate> delegate;  
   
 -(void) downloadFile:(NSString*) filePath;  
 -(void) downloadFile:(NSString*) filePath completion:(void(^)(NSData *data)) block;  
   
 @end  
   
   
 #import "Downloader.h"  
   
 @implementation Downloader  
 @synthesize delegate;  
   
   
 -(void) downloadFile:(NSString*) filePath  
 {    
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
     [self download:(NSString*) path];  
   });  
     
 }  
   
 -(void) download:(NSString *) resourcePath  
 {  
   NSURL *url = [NSURL URLWithString:resourcePath];  
   NSData *data = [NSData dataWithContentsOfURL:url];  
   
   if (callback)  
   {  
     callback(data);  
   }  
   else  
   {  
     if ([delegate conformsToProtocol:@protocol(DownloaderDelegate)])  
     {  
       [delegate downloadCompleted:data];  
     }  
   }  
       
 }  
   
 -(void) downloadFile:(NSString*) filePath completion:(void(^)(NSData *data)) block  
 {  
   callback = block;  
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
     [self download:(NSString*) path];  
   });  
 }  
   
 @end