Swift 5.6 how to put async Task into a queue

user_10317684 0
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Let say I have this code

class Duck{
    
    func walk() async {
        //do something
        print("walk start")
        try? await Task.sleep(nanoseconds: UInt64(2e9))
        print("walk end")
    }
    
    func quack() async {
        //do something...
        print("quack start")
        try? await Task.sleep(nanoseconds: UInt64(2e9))
        print("quack end")
    }
    
    func fly() async{
        //do something
        print("fly start")
        try? await Task.sleep(nanoseconds: UInt64(2e9))
        print("fly end")
    }
    
}

let duck = Duck()

Task{
    await duck.walk()
}

Task{
    await duck.quack()
}

Task{
    await duck.fly()
}

This would print

walk start
quack start
fly start
walk end
quack end
fly end

which I understand and expected. But what if I want those 3 Tasks run sequentially? Let say each Task is created by user pressing a button. I want the tasks to queue up in the background and run one by one. Is there any thing like you can queue up DispatchWorkItem in a DispatchQueue, but a Task version?

Edit:

I came up with a solution, but I am not sure if this is a good way to implement it. As this implementation potentially create many layer of cascaded Task, I wonder if there would be risk of stack overflow or memory leaks?

class TaskQueue{
    private var currentTask : Task<Void,Never> = Task{}
    
    func dispatch(block:@escaping () async ->Void){
        
        let oldTask = currentTask
        currentTask = Task{
            _ = await oldTask.value
            await block()
        }
    }
}

taskQueue.dispatch {
    await duck.walk()
}
taskQueue.dispatch {
    await duck.quack()
}
taskQueue.dispatch {
    await duck.fly()
}
async-await swift concurrency asynchronous ios
1 Answers

Here is my updated implementation which I think is safer than the one I posted in the question. The TaskQueueActor part does all of the job, I wrap it with an outer class just to make it cleaner when calling from a non-async context.

class TaskQueue{
    
    private actor TaskQueueActor{
        private var blocks : [() async -> Void] = []
        private var currentTask : Task<Void,Never>? = nil
        
        func addBlock(block:@escaping () async -> Void){
            blocks.append(block)
            next()
        }
        
        func next()
        {
            if(currentTask != nil) {
                return
            }
            if(!blocks.isEmpty)
            {
                let block = blocks.removeFirst()
                currentTask = Task{
                    await block()
                    currentTask = nil
                    next()
                }
            }
        }
    }
    private let taskQueueActor = TaskQueueActor()
    
    func dispatch(block:@escaping () async ->Void){
        Task{
            await taskQueueActor.addBlock(block: block)
        }
    }
}
user_103176840
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