Swift object pool design sample

A generic object pool in Swift

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

Why? Efficiency enhancements. For instance the Dispatch framework makes use of an object pool sample to provide 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 employees. For instance it’s important to obtain tons of of photographs from the online, however you’d prefer to obtain solely 5 concurrently you are able to do it with a pool of 5 employee objects. Most likely it’s going to be loads cheaper to allocate a small variety of employees (that’ll really do the obtain job), than create a brand new one for each single picture obtain request. 🖼

What concerning the downsides of this sample? There are some. For instance if in case you have employees in your pool, they could include states or delicate person knowledge. You need to be very cautious with them aka. reset all the things. Additionally if you’re working in a multi-threaded atmosphere it’s important to make your pool thread-safe.

Right here is a straightforward generic thread-safe object pool class:

import Basis

class Pool {

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

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

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

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


let pool = Pool(["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.international(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 just some strains. You’ve got the thread protected array of the generic pool gadgets, a dispatch semaphore that’ll block if there aren’t any objects out there within the pool, and two strategies with a purpose to really use the thing pool.

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