为什么在NSObject协议中没有performSelectorOnMainThread :？

@protocol MyOperationDelegate;

@interface MyOperation : NSOperation {
     id <MyOperationDelegate> delegate;
}
@property (nonatomic, assign) id <MyOperationDelegate> delegate;
@end

@protocol MyOperationDelegate <NSObject>
@optional
- (void) didFinishHandleWithResult:(NSDictionary *)result;
@end

@implementation MyOperation
@synthesize delegate;
- (void) main {
     NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

     NSDictionary *aDict = [[MySingleton sharedObject] fetchSomethingMeaningful];

     //do something and call delegate
     if ([delegate respondsToSelector:@selector(didFinishHandleWithResult:)]) {
          [delegate performSelector:@selector(didFinishHandleWithResult:) withObject:
     }

     [pool release];
}
@end


@interface MyViewCon : UIViewController <MyOperationDelegate> {
    NSOperationQueue *queue;
}
@end

@implementation MyViewCon
- (void) viewDidLoad {
    MyOperation *op = [[MyOperation alloc] init];
    op.delegate = self;
    [queue addOperation:op];
    [op release];
}

- (void) reloadUserInterface:(NSDictionary *)dict {
    // Do reload data on User Interfaces.
}

- (void) didFinishHandleWithResult:(NSDictionary *)myDict {
    // Couldn\'t execute method that\'s handling UI, maybe could but very slow...
    // So it must run on main thread.
    [self performSelectorOnMainThread:@selector(reloadUserInterface:) withObject:myDict];
}
@end