1.GCD
特点：易用，易读，直观，灵活，高性能
gcd其实没有线程的概念，其实gcd的多线程是由于多任务概念带来的效果
//创建子线程执行dispatch_async(dispatch_get_global_queue(0, 0), ^{ //... });//在主线程执行dispatch_async(dispatch_get_main_queue(), ^{ //... });
2.NSThead
特点：基础，易维护
NSThread* myThread = [[NSThread alloc] initWithTarget:self selector:@selector(threadInMainMethod:) object:nil];
[myThread start];
[NSThread detachNewThreadSelector:@selector(threadInMainMethod:) toTarget:self withObject:nil];
3.NSOperation
特点：系能高，安全可靠，功能强大，控制性强，线程池实现
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^(){
NSLog(@"执行第1次操作，线程：%@", [NSThread currentThread]);
}];
NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^(){
NSLog(@"执行第2次操作，线程：%@", [NSThread currentThread]);
}];
// operation1依赖于operation2
[operation1 addDependency:operation2];
[queue addOperation:operation1];
[queue addOperation:operation2];
4.performSelectorInBackground
特点：易用，清洁，同NSThead
[self performSelectorInBackground:@selector(run) withObject:nil];
