Adding condition based on previous result on DispatchQueue - ios

Question

Is it possible to set a condition on the next queue of DispatchQueue? Supposed there are 2 API calls that should be executed synchronously, callAPI1 -> callAPI2. But, callAPI2 should be only executed if callAPI1 returning true. Please check code below for more clear situation:
let dispatchQueue: DispatchQueue = DispatchQueue(label: "queue")
let dispatchGroup = DispatchGroup()
var isSuccess: Bool = false
dispatchGroup.enter()
dispatchQueue.sync {
self.callAPI1(completion: { (result) in
isSuccess = result
dispatchGroup.leave()
}
}
dispatchGroup.enter()
dispatchQueue.sync {
if isSuccess { //--> This one always get false
self.callAPI2(completion: { (result) in
isSuccess = result
dispatchGroup.leave()
})
} else {
dispatchGroup.leave()
}
}
dispatchGroup.notify(queue: DispatchQueue.main, execute: {
completion(isSuccess) //--> This one always get false
})
Currently above code always returning isSuccess as false despite on callAPI1's call returning true, which cause only callAPI1's is called.

Answer 1

All non-playground code typed directly into answer, expect little errors.
It appears that you are trying to make an asynchronous call into a synchronous one, and the way you are attempting this simply will not work. Assuming callAPI1 is asynchronous then after:
self.callAPI1(completion: { (result) in
isSuccess = result
}
the completion block has (in all probability) not yet been run, you cannot test isSuccess immediately, as in:
self.callAPI1(completion: { (result) in
isSuccess = result
}
if isSuccess
{
// in all probability this will never be reached
}
Wrapping the code into a synchronous block will have no effect whatsoever:
dispatchQueue.sync
{
self.callAPI1(completion: { (result) in
isSuccess = result
}
// at this point, in all probability, the completion block
// has not yet run, therefore...
}
// at this point it has also not run
A sync dispatch just runs its block on a different queue and waits for it to complete; if that block contains asynchronous code, as yours does, then it is not magically made synchronous - it executes asynchronously as normal, the synchronously dispatched block terminates, the sync dispatch returns, and your code continues. The sync dispatch has no real effect (apart from running the block on a different queue while blocking the current one).
If you need to sequence a number of asynchronous calls you can do it a number of ways. One method is to simply chain the calls through the completion blocks. Using this approach your code becomes:
self.callAPI1(completion: { (result) in
if !result { completion(false) }
else
{
self.callAPI2(completion: { (result) in
completion(result)
}
}
}
Using Semaphores
If you have a long sequence of such calls using the above pattern then the code can become very nested, in such a case instead of nesting you can use semaphores to sequence the calls. A simple semaphore can be used to block (thread) execution, using wait(), until it is signalled (by an unblocked thread), using signal().
Notice the emphasis here on blocking, once you introduce the ability to block execution all sorts of issues have to be considered: among them are UI responsiveness - blocking the UI thread is not good; deadlock - for example if the code that will issue semaphore wait and signal operations is executing on the same thread then after a wait there will be no signal...
Here is a sample Swift Playground script to demonstrate using semaphores. The pattern follows your original code but uses a semaphore in addition to your boolean.
import Cocoa
// some convenience functions for our dummy callAPI1 & callAPI2
func random(_ range : CountableClosedRange<UInt32>) -> UInt32
{
let lower = range.lowerBound
let upper = range.upperBound
return lower + arc4random_uniform(upper - lower + 1)
}
func randomBool() -> Bool
{
return random(0...1) == 1
}
class Demo
{
// grab the global concurrent utility queue to schedule our work on
let workerQueue = DispatchQueue.global(qos : .utility)
// dummy callAPI1, just pauses and then randomly return success or failure
func callAPI1(_ completion : #escaping (Bool) -> Void) -> Void
{
// do the "work" on workerQueue, which is concurrent so other work
// can be executing, or *blocked*, on the same queue
let pause = random(1...2)
workerQueue.asyncAfter(deadline: .now() + Double(pause))
{
// produce a random success result
let success = randomBool()
print("callAPI1 after \(pause) -> \(success)")
completion(success)
}
}
func callAPI2(_ completion : #escaping (Bool) -> Void) -> Void
{
let pause = random(1...2)
workerQueue.asyncAfter(deadline: .now() + Double(pause))
{
let success = randomBool()
print("callAPI2 after \(pause) -> \(success)")
completion(success)
}
}
func runDemo(_ completion : #escaping (Bool) -> Void) -> Void
{
// We run the demo as a standard async function
// which doesn't block the main thread
workerQueue.async
{
print("Demo starting...")
var isSuccess: Bool = false
let semaphore = DispatchSemaphore(value: 0)
// do the first call
// this will asynchronously execute on a different thread
// *including* its completion block
self.callAPI1
{ (result) in
isSuccess = result
semaphore.signal() // signal completion
}
// we can safely wait for the semaphore to be
// signalled as callAPI1 is executing on a different
// thread so we will not deadlock
semaphore.wait()
if isSuccess
{
self.callAPI2
{ (result) in
isSuccess = result
semaphore.signal() // signal completion
}
semaphore.wait() // wait for completion
}
completion(isSuccess)
}
}
}
Demo().runDemo { (result) in print("Demo result: \(result)") }
// For the Playground
// ==================
// The Playground can terminate a program run once the main thread is done
// and before all async work is finished. This can result in incomplete execution
// and/or errors. To avoid this we sleep the main thread for a few seconds.
sleep(6)
print("All done")
// Run the Playground multiple times, the results should vary
// (different wait times, callAPI2 may not run). Wait until
// the "All done"" before starting next run
// (i.e. don't push stop, it confuses the Playground)
Or...
Another approach to avoid the nesting is to design functions (or operators) which take two async methods and produce a single one by implementing the nesting pattern. Long nested sequences can then be reduce to more linear sequences. This approach is left as an exercise.
HTH