好处
1）使用线程可以把程序中占据时间长的任务放到后台去处理，如图片、视频下载
2）发挥多核处理器的优势，并发执行让系统运行的更快、更流畅，用户体验更好
缺点
1）大量的程序降低代码的可读性
2）更多的线程需要更多的空间
3）当多个线程对同一个资源出现争夺的时候需要注意线程安全问题
线程使用：
1）NSThread(两种创建方式)
[NSThread detachNewThreadSelector:@selector(threadBegin:) toTarget:self withObject:@1];
NSThread *thread1 = [[NSThread alloc] initWithTarget:self selector:@selector(threadBegin:) object:@1];
[thread1 start];
2)NSOperationQueue
__weak typeof(self) weakSelf = self;
[[NSOperationQueue new] addOperationWithBlock:^{
[weakSelf threadBegin:@1];
}];
NSBlockOperation *op = [NSBlockOperation blockOperationWithBlock:^{
[weakSelf threadBegin:@2];
}];
[op addExecutionBlock:^{
[weakSelf threadBegin:@3];
}];
[op addExecutionBlock:^{
[NSThread sleepForTimeInterval:1];
[weakSelf threadBegin:@4];
}];
[op addExecutionBlock:^{
[weakSelf threadBegin:@6];
}];
[op start];
3）GCD
dispatch_async(dispatch_get_global_queue(0, 0), ^{ //异步
[weakSelf threadBegin:@1];
});
dispatch_group_t group = dispatch_group_create(); // 异步组合
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_group_async(group, queue, ^{
[weakSelf threadBegin:@2];
});
dispatch_group_async(group, queue, ^{
[weakSelf threadBegin:@3];
});
dispatch_group_notify(group, queue, ^{
[weakSelf threadBegin:@4];
});
// 异步延时
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_global_queue(0, 0), ^{
[weakSelf threadBegin:@5];
});
