Swift object pool design sample

A generic object pool in Swift

The object pool sample is a creational design sample. The principle concept behind it’s that first you create a set of objects (a pool), then you definately purchase & launch objects from the pool, as a substitute of continually creating and releasing them. 👍

Why? Efficiency enhancements. For instance the Dispatch framework makes use of an object pool sample to present pre-created queues for the builders, as a result of making a queue (with an related thread) is an comparatively costly operation.

One other use case of the object pool sample is staff. For instance you must obtain tons of of photographs from the net, however you’d prefer to obtain solely 5 concurrently you are able to do it with a pool of 5 employee objects. In all probability it will be lots cheaper to allocate a small variety of staff (that’ll really do the obtain activity), than create a brand new one for each single picture obtain request. 🖼

What in regards to the downsides of this sample? There are some. For instance when you have staff in your pool, they could include states or delicate consumer information. It’s a must to be very cautious with them aka. reset every little thing. Additionally if you’re working in a multi-threaded atmosphere you must make your pool thread-safe.

Right here is an easy generic thread-safe object pool class:

import Basis

class Pool<T> {

    personal let lockQueue = DispatchQueue(label: "pool.lock.queue")
    personal let semaphore: DispatchSemaphore
    personal var objects = [T]()

    init(_ objects: [T]) {
        self.semaphore = DispatchSemaphore(worth: objects.depend)
        self.objects.reserveCapacity(objects.depend)
        self.objects.append(contentsOf: objects)
    }

    func purchase() -> T? {
        if self.semaphore.wait(timeout: .distantFuture) == .success, !self.objects.isEmpty {
            return self.lockQueue.sync {
                return self.objects.take away(at: 0)
            }
        }
        return nil
    }

    func launch(_ merchandise: T) {
        self.lockQueue.sync {
            self.objects.append(merchandise)
            self.semaphore.sign()
        }
    }
}


let pool = Pool<String>(["a", "b", "c"])

let a = pool.purchase()
print("(a ?? "n/a") acquired")
let b = pool.purchase()
print("(b ?? "n/a") acquired")
let c = pool.purchase()
print("(c ?? "n/a") acquired")

DispatchQueue.world(qos: .default).asyncAfter(deadline: .now() + .seconds(2)) {
    if let merchandise = b {
        pool.launch(merchandise)
    }
}

print("No extra useful resource within the pool, blocking thread till...")
let x = pool.purchase()
print("(x ?? "n/a") acquired once more")

As you’ll be able to see the implementation is only a few traces. You will have the thread protected array of the generic pool objects, a dispatch semaphore that’ll block if there are not any objects obtainable within the pool, and two strategies with a view to really use the thing pool.

Within the pattern you’ll be able to see that if there are not any extra objects left within the pool, the present queue will likely be blocked till a useful resource is being freed & prepared to make use of. So be careful & do not block the principle thread by chance! 😉