Convert synchronous to async Objective-C - objective-c

Question

I’m working in a new codebase and I don’t have many people who understand it, so I’m hoping I can get some help. I am updating an interface and some of the synchronous methods are now async which is making it difficult to fit into the current architecture for resolving data.
Currently we have a function map which stores these synchronous methods, then when we want the data we do “call” which executes the block/method and returns the value.
Some code below shows how it currently is.
fnMap[#“vid”] = [[Callback alloc] initWithBlock:^id(id param) {
return #([services getVisitorID]);
}];
… later, to resolve the data
id fnMapVal = [fnMap[key] call:nil];
Here is how a callback and callback block are defined.
typedef id (^CallbackBlock)(id);
#interface Callback : NSObject
#property(copy, nonatomic, readonly) CallbackBlock block;
- (instancetype)initWithBlock:(CallbackBlock)block
- (id)call:(id)param
{
return self.block(param);
}
Now the service needs to call an async method to get the ID so I had to change it to:
- (void)getVisitorID: (nullable void (^) (NSString* __nullable visitorIdentifier)) callback
{
[SDK getUserIdentifier:^(NSString * _Nullable userIdentifier) {
callback(userIdentifier);
}];
}
So the call is:
[services getVisitorID:^(NSString * _Nullable visitorIdentifier) {
}];
I haven’t been able to find a way to fit this into the current architecture. Some options I’ve explored is using a run loop to wait for the async method to finish and keep my interface synchronous but this sounds like a bad idea. I’m for some suggestions on how to fit this in as I’ve never seen something like this before.

Answer 1

You need to use dispatch_queues or NSOperationQueue to run your code off the main thread. Dispatch queues are very low level and good to just fire off async tasks:
// we're going to run the getVisitorID method on a background queue
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[services getVisitorID:^(NSString * _Nullable visitorIdentifier) {
dispatch_async(dispatch_get_main_queue(), ^(void){
// update the user interface on the main queue
});
}];
});
I prefer using NSOperationQueue because the API is cleaner, and they allow you to do more advanced things like make asynchronous task cancellable:
// create a background queue
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[queue addOperationWithBlock:^{
[services getVisitorID:^(NSString * _Nullable visitorIdentifier) {
// get the main queue and add your UI update code to it
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
// update the UI from here
}];
}];
}];
Queues are much easier to manage than messing around with run loops. For more info see here: https://developer.apple.com/library/archive/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html

Answer 2

What you need is the Future/Promise concept. Please don't reinvent the wheel, you will have to cover marathon distance trying to achieve the same functionality of requesting value and asynchronously consuming it.
As one of the favourites please consider PromiseKit. However don't be shy to explore the alternatives
You will quickly find it super delightful building chains of async tasks, mapping their results to different values, combining futures together to get a tuple of multiple resolved values once all are available - all that in a concise, well designed form, live tested in millions of applications.