I have been trying to use dispatch_async
in a method that returns a result. However, I observed that the method returns before executing the dispatch_async
block. Due to this I'm not getting the results I expect. Here is some code that demonstrates my problem.
-(BOOL) isContactExists {
BOOL isContactExistsInXYZ = YES;
UserId *userId = contact.userId;
dispatch_async(dispatch_get_main_queue(), ^
{
iOSContact *contact = [iOSContact contactForUserId:userId];
if (nil == contact)
{
isContactExistsInXYZ = NO;
}
});
return isContactExistsInXYZ;
}
This method isContactExists
is called somewhere else and based on the response from that method I have to do some stuff. But every time, the value of isContactExistsInXYZ
is not what I expect. How do I handle dispatch_async
in this situation?
If your going the block route your method needs to look something like this.
- (void)isContactExistsWithCompletionHandler:(void(^)(BOOL exists)) completion
{
dispatch_async(dispatch_get_main_queue(), ^
{
BOOL isContactExistsInXYZ = YES;
UserId *userId = contact.userId;
iOSContact *contact = [iOSContact contactForUserId:userId];
if (nil == contact)
{
isContactExistsInXYZ = NO;
}
completion(isContactExistsInXYZ);
});
}
And where you are calling it something like this.
[someObject isContactExistsWithCompletionHandler:^(BOOL exists) {
// do something with your BOOL
}];
You should also consider placing your heavy operations in a other que than main. Like this.
- (void)isContactExistsWithCompletionHandler:(void(^)(BOOL exists)) completion
{
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, NULL);
dispatch_async(queue, ^
{
BOOL isContactExistsInXYZ = YES;
UserId *userId = contact.userId;
iOSContact *contact = [iOSContact contactForUserId:userId];
if (nil == contact)
{
isContactExistsInXYZ = NO;
}
dispatch_async(dispatch_get_main_queue(), ^
{
completion(isContactExistsInXYZ);
});
});
}
You need to respect that what you are trying to do is asynchronous and embrace that. This means not using a return value. Instead you can write your method to take a callback block as a parameter. Then, when your asynchronous check is complete you can call the block with the result.
So your method signature would become:
- (void)checkIfContactExistsWithCompletion:(ContactExistsBlock)completion {
Where ContactExistsBlock
is a block definition with no return and probably a single BOOL
parameter.
typedef void (^ContactExistsBlock) (BOOL exists);
原因是dispatch_async(dispatch_get_main_queue(), ^
不等到执行完成。你可能搞乱那里的东西。通常,这用于异步更新UI以及其他服务器内容下载到其他一些线程。尝试使用dispatch_sync
代替。
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.