模拟异步block：
- (NSString *)blockReturn {
__block NSString *str = @"sst";
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
str = @"Sunstataa";
});
return str;
}
此时返回的str = @"sst"，原理大家都懂，问题是怎么实现异步return正确值。
最简单方便的方法就是使用信号量，初始化信号量为0，等待异步执行完毕再增加信号量：
- (NSString *)blockReturn {
// 创建信号量
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
__block NSString *str = @"sst";
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
str = @"Sunstataa";
//信号量+1
dispatch_semaphore_signal(semaphore);
});
//信号量等等
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
return str;
}