如何在Swift中设置目标与CADisplayLink实例间弱引用的CADisplayLink

Question

在Objective中，我们可以使用代理模式插入CADisplayLink以打破强引用：

WeakProxy *weakProxy = [WeakProxy weakProxyForObject:self];
self.displayLink = [CADisplayLink displayLinkWithTarget:weakProxy selector:@selector(displayDidRefresh:)];

然后，只需使displayLink在dealloc中失效。

- (void)dealloc
{
    [_displayLink invalidate];
}

然而，NSProxy似乎不能在Swift：https://bugs.swift.org/browse/SR-1715中被继承

我试着这样写：

weak var weakSelf = self    
displayLink = CADisplayLink(target: weakSelf!, selector: #selector(displayDidRefresh(dpLink:)))

它没有起作用。

我想知道在目标C中是否有实现这一目标的方法。

Answer 1

更好的方法可能是使viewWill/DidDisappear中的显示链接失效，请参见

• Correct handling / cleanup / etc of CADisplayLink in Swift custom animation?

为了获取有用的信息。

如果这不是一个选项:让代理对象从NSObject而不是NSProxy继承。例如，给出了一个目标-C的解决方案。

• CADisplayLink at iOS 6.0 not retaining target

这很容易被翻译成Swift 3：

class JAWeakProxy: NSObject {
    weak var target: NSObjectProtocol?

    init(target: NSObjectProtocol) {
        self.target = target
        super.init()
    }

    override func responds(to aSelector: Selector!) -> Bool {
        return (target?.responds(to: aSelector) ?? false) || super.responds(to: aSelector)
    }

    override func forwardingTarget(for aSelector: Selector!) -> Any? {
        return target
    }
}

然后它可以用作

displayLink = CADisplayLink(target: JAWeakProxy(target: self),
                            selector: #selector(didRefresh(dpLink:)))

你的方法

weak var weakSelf = self    
displayLink = CADisplayLink(target: weakSelf!, selector: #selector(displayDidRefresh(dpLink:)))

不工作，因为它在初始化weakSelf并将强引用传递给作为目标的self时对CADisplayLink进行解压缩。

Answer 2

这个代理类应该可以正常工作。别忘了在脱欧之前失效。

import UIKit

class CADisplayLinkProxy {

    var displaylink: CADisplayLink?
    var handle: (() -> Void)?

    init(handle: (() -> Void)?) {
        self.handle = handle
        displaylink = CADisplayLink(target: self, selector: #selector(updateHandle))
        displaylink?.add(to: RunLoop.current, forMode: .common)
    }

    @objc func updateHandle() {
        handle?()
    }

    func invalidate() {
        displaylink?.remove(from: RunLoop.current, forMode: .common)
        displaylink?.invalidate()
        displaylink = nil
    }
}

用法：

class ViewController: UIViewController {

    var displaylinkProxy: CADisplayLinkProxy?

    override func viewDidLoad() {
        super.viewDidLoad()
        displaylinkProxy = CADisplayLinkProxy(handle: { [weak self] in
            self?.updateAnything()
        })
    }

    @objc func updateAnything() {
        print(Date())
    }
}

Answer 3

另一种解决方案是，该解决方案将代理/ objc运行时隐藏在其外部API中。只要DisplayLink被变量引用，它就能存活多久。一旦变量超出作用域或设置为零，CADisplayLink将失效，以便也可以对目标进行删除。

import Foundation
import UIKit

/// DisplayLink provides a block based interface for CADisplayLink.
/// The CADisplayLink is invalidated upon DisplayLink deinit.
///
/// Usage:
/// ```
/// let displayLink = DisplayLink { caDisplayLink in print("Next frame scheduled \(caDisplayLink.targetTimestamp)") }
/// ```
///
/// Note: Keep a reference to the DisplayLink.
final class DisplayLink {
    let displayLink: CADisplayLink

    init(runloop: RunLoop? = .main, prepareNextFrame: @escaping (CADisplayLink) -> ()) {
        displayLink = CADisplayLink(
            target: DisplayLinkTarget(prepareNextFrame),
            selector: #selector(DisplayLinkTarget.prepareNextFrame))

        if let runloop = runloop {
            displayLink.add(to: runloop, forMode: .default)
        }
    }

    deinit {
        displayLink.invalidate()
    }
}

private class DisplayLinkTarget {
    let callback: (CADisplayLink) -> ()

    init(_ callback: @escaping (CADisplayLink) -> ()) {
        self.callback = callback
    }

    @objc func prepareNextFrame(displaylink: CADisplayLink) {
        callback(displaylink)
    }
}