iOS wait block until complete

Question

I have created a block void function wich is using AFNetworking library. This function is used for GET request and uses like so.

[Mics getRequestBLOCK:clientsUrlStr BlockRequest:^(NSString *response) {
    //use response here
}];

Everything work fine, but I really get a headache when I use this block in a function that need return a value, for instance.

+ (NSArray*)OnlineClients
{
    NSString *clientsUrlStr = [NSString stringWithFormat:@"%@/sinch/clients", ROOT_URL];
    __block NSArray *clients = [[NSArray alloc] init];
    [Mics getRequestBLOCK:clientsUrlStr BlockRequest:^(NSString *response) {
        clients = [response componentsSeparatedByString: @","];
    }];

    return clients;
}

OnlineClients array function always return 0 item because request block is not finished with completion.

So, how can I wait for block completion to get response request for return in a function?

I have been searching various topics in StackOverflow and google but can't found any solution.

Please help.

Then you haven't searched very well. This is probably the most commonly asked and answered question in all of iOS programming on Stack Overflow. — matt, Sep 08 '16 at 02:45
Possible duplicate of [iOS: Waiting for API Completion Block and returning the result](http://stackoverflow.com/questions/23507331/ios-waiting-for-api-completion-block-and-returning-the-result) — HAS, Sep 08 '16 at 07:05

HSG · Answer 1 · 2016-09-08T02:33:28.763

3

There are several possibilities to deal with your problem. You are able to either use dispatch_group or dispatch_semaphore to wait for the response from async call before return value or modify your method to have a block to callback the response. Please take a look on codes below. To be NOTICED that dispatch_group and dispatch_semaphore are going to block the current thread to wait until another one triggers a signal, therefore, should not use it on the main thread. In my opinion, I would suggest the 3rd solution. I hope it would help you.

//1
+ (NSArray*)OnlineClients
{
    dispatch_group_t serviceGroup = dispatch_group_create();

    NSString *clientsUrlStr = [NSString stringWithFormat:@"%@/sinch/clients", ROOT_URL];
    __block NSArray *clients = [[NSArray alloc] init];
    [Mics getRequestBLOCK:clientsUrlStr BlockRequest:^(NSString *response) {
        clients = [response componentsSeparatedByString: @","];
        dispatch_group_leave(serviceGroup);
    }];

    dispatch_group_enter(serviceGroup);

    return clients;
}

//2
+ (NSArray*)OnlineClients
{
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);

    NSString *clientsUrlStr = [NSString stringWithFormat:@"%@/sinch/clients", ROOT_URL];
    __block NSArray *clients = [[NSArray alloc] init];
    [Mics getRequestBLOCK:clientsUrlStr BlockRequest:^(NSString *response) {
        clients = [response componentsSeparatedByString: @","];
        dispatch_semaphore_signal(sema);
    }];

    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);

    return clients;
}

//3
+ (void)getOnlineClientsWithCompletionBlock:(void (^) (NSArray *clients))completionBlock
{   
    NSString *clientsUrlStr = [NSString stringWithFormat:@"%@/sinch/clients", ROOT_URL];
    __block NSArray *clients = [[NSArray alloc] init];
    [Mics getRequestBLOCK:clientsUrlStr BlockRequest:^(NSString *response) {
        clients = [response componentsSeparatedByString: @","];
        if (completionBlock) {
            completionBlock(clients);
        }
    }];

}

edited Sep 08 '16 at 02:33

answered Sep 08 '16 at 02:03

HSG

1,224
11
17

1 & 2 are going to block the thread right, which if on the main thread is going to block any UI interactions. Might be useful to mention that. – sbarow Sep 08 '16 at 02:14
@sbarow exactly. Thank you for your point, should be careful in term of thread blocking. – HSG Sep 08 '16 at 02:17
First, thanks for help me. Solution 1: I don't see this waiting for anything, array still return `0 elements`, honestly I have tried this before but it does not work. Solution 2: When I apply this code like I have done this before, this is waiting forever and never goes to return. – Viet Nguyen Sep 08 '16 at 02:30
@vietnguyen09 could you get rid of the main thread when trying to do it? – HSG Sep 08 '16 at 02:34
@HDT Yes for the first one, but none for the second one. – Viet Nguyen Sep 08 '16 at 02:35
1

@vietnguyen09 maybe I missed `dispatch_group_wait(downloadGroup, DISPATCH_TIME_FOREVER);` this at the next line of `dispatch_group_enter(serviceGroup);`, I used them and it worked from my side, why don't you refer the 3rd one which is really simple? – HSG Sep 08 '16 at 02:39
1

Option 1 is wrong as written. You should call `dispatch_group_enter` before calling `getRequestBLOCK` and you should call `dispatch_group_wait` where you now have `dispatch_group_enter`. – rmaddy Sep 08 '16 at 02:45
@HDT My project is using `ASIHTTPRequest` before and I have created a get request function that used in the whole project, but then `ASIHTTPRequest` can't work well when my API server using `SSL Client certificate authentication` so I have to use AFNetworking. And now, I trying to make that request function work in my whole project without change too many places :( – Viet Nguyen Sep 08 '16 at 02:45

score 0 · Answer 2 · answered Oct 19 '18 at 08:16

An UGLY solution but it's works!

__block BOOL isChecked = NO;
YourService *yourService = [[YourService alloc] init];
[yourService checkStatus:string completionHandler:^(BOOL isSuccessful, Error *err) {
    isValid = err ? NO : isSuccessful;
    isChecked = YES;
}];
while (!isChecked) {
    [NSThread 0.6];
}

iOS wait block until complete

2 Answers2