如何从同一个方法的失败块中调用iOS方法？

dispatch_async(dispatch_get_main_queue(), ^{
    // 在这里调用需要执行的方法
});

@protocol MyDelegate <NSObject>
- (void)doSomething;
@end

@interface MyClass : NSObject
@property (nonatomic, weak) id<MyDelegate> delegate;
@end

if (self.delegate && [self.delegate respondsToSelector:@selector(doSomething)]) {
    [self.delegate doSomething];
}

@interface OtherClass : NSObject <MyDelegate>
@end

@implementation OtherClass
- (void)doSomething {
    // 在这里执行需要的操作
}
@end

[[NSNotificationCenter defaultCenter] postNotificationName:@"MyNotification" object:nil];

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doSomething) name:@"MyNotification" object:nil];