Run Loop Source

Readme.markdown

CFRunLoopSource is cool. It lets you build behavior similar to the mechanisms that drive setNeedsLayout and setNeedsDisplay in UIKit.

I found myself in need of something like this a couple of times. It's great to know that no matter how many times I say I need to update something, I will get a single callback at the end of the run loop that gives me a chance to perform my work.

Here is a little Swift wrapper that makes the API easier to deal with.

RunLoopSource.swift

import CoreFoundation

public final class RunLoopSource {

    // MARK: - Properties

    private lazy var source: CFRunLoopSource = {
        var context = CFRunLoopSourceContext()
        context.info = UnsafeMutablePointer(unsafeAddressOf(self))
        context.perform = _perform

        return CFRunLoopSourceCreate(nil, self.order, &context)
    }()

    private let subscription: (CFRunLoop, CFString)

    private let order: CFIndex

    private let perform: () -> ()


    // MARK: - Initializers

    public init(loop: CFRunLoop = CFRunLoopGetCurrent(), mode: CFString = kCFRunLoopCommonModes, order: CFIndex = 0, perform: () -> ()) {
        self.subscription = (loop, mode)
        self.perform = perform
        self.order = order
        CFRunLoopAddSource(loop, source, mode)
    }

    deinit {
        let (loop, mode) = subscription
        CFRunLoopRemoveSource(loop, source, mode)
    }


    // MARK: - Public

    public func signal() {
        CFRunLoopSourceSignal(source)
    }
}

private func _perform(info: UnsafeMutablePointer<Void>) {
    unsafeBitCast(info, RunLoopSource.self).perform()
}

View.swift

///
/// Here is an example class that demonstrates how you might use
/// `RunLoopSource`. I know this isn't really how UIView works.
///
class View {
    private var needsLayout = false
    
    private lazy var layoutSource: RunLoopSource = {
        RunLoopSource(perform: { [weak self] in
            self?.layoutIfNeeded()
        })
    }()
    
    final func setNeedsLayout() {
        needsLayout = true
        layoutSource.signal()
    }
    
    final func layoutIfNeeded() {
        if needsLayout {
            layoutSubviews()
        }
        needsLayout = false
    }
    
    func layoutSubviews() {
        
    }
}