在实际开发中，经常需要异步处理数据，然后刷新UI只能在主线程，以下是几种主线程操作的方法，gcd方法是大家最熟悉的。
1. GCD方法（最常见，使用简单方便，苹果封装）
dispatch_async(dispatch_get_main_queue(), ^{
//do something
});
2.NSOperation方法
NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];主队列
NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
//do something
}];
[mainQueue addOperation:operation];//不要忘记加这句
3. NSThread方法
[selfperformSelector:@selector(method) onThread:[NSThread mainThread] withObject:nilwaitUntilDone:YESmodes:nil];
[selfperformSelectorOnMainThread:@selector(method) withObject:nilwaitUntilDone:YES];
[[NSThread mainThread] performSelector:@selector(method) withObject:nil];
4. RunLoop方法
[[NSRunLoop mainRunLoop] performSelector:@selector(method) withObject:nil];
