为什么在 iOS 的 ViewController 中声明视图时在 Swift 中使用“weak”关键字

Question

What is the difference between the following methods of setting up views in a ViewController

force-unwrapped :

class VC: UIViewController() {
    var customView: UIView!

    override func viewDidLoad() {
        super.viewDidLoad()
        customView = UIView(frame: .zero)
        addSubview(customView)
    }
}

weak force-unwrapped :

class VC: UIViewController() {
    weak var customView: UIView!

    override func viewDidLoad() {
        super.viewDidLoad()
        customView = UIView(frame: .zero)
        addSubview(customView)
    }
}

weak :

class VC: UIViewController() {
    weak var customView: UIView

    override func viewDidLoad() {
        super.viewDidLoad()
        customView = UIView(frame: .zero)
        addSubview(customView)
    }
}

constructed in declaration:

class VC: UIViewController() {
    var customView = UIView(frame: .zero)

    override func viewDidLoad() {
        super.viewDidLoad()
        addSubview(customView)
    }
}

weak constructed in declaration

class VC: UIViewController() {
    weak var customView = UIView(frame: .zero))

    override func viewDidLoad() {
        super.viewDidLoad()
        addSubview(customView)
    }
}

Answer 1

It could be that your question was related to Xcode creating outlets as:

@IBoutlet weak var label: UILabel!

Apple (and other experts) have had discussions about whether weak should be used here (since these views are almost always retained by the view itself - I'm sure if you dig around you can find endless discussions on it. You an usually remove weak above with no side effects (try it and see - add a log message in the view controller dealloc .

But, lets get to your specific questions. I made a small class in a test project (something everyone should do when they want to explore this kind of topic!):

final class MemoryTest: NSObject {

    lazy var vc1 = VC1()
    lazy var vc2 = VC2()
    lazy var vc4 = VC4()
    lazy var vc5 = VC5()

    func test() {
        let _ = vc1
        let _ = vc2
//        let _ = vc3
        let _ = vc4
        let _ = vc5
    }
}

// What is the difference between the following methods of setting up views in a ViewController

// `force-unwrapped`:

/*
    Interoperability with Objective C required this.
    Any code accessing vc1 before its set will crash, forced unwrapped means that Swift will always try to unwrap what is
    otherwise an optional, and if nil boom down youg go.

    Since Objective C can deal with nil objects, you can access it and read properties, all of which will be nil
 */
    @objcMembers
    class VC1: UIViewController {
        var customView: UIView!

        init() {
            super.init(nibName: nil, bundle: nil)

            //print("BG Color 1", customView.backgroundColor)   // Compiler warning then a crash
            print("BG Color 2", customView?.backgroundColor ?? "none")   // crash
            customView = UIView(frame: .zero)
            view.addSubview(customView)
            customView = nil // the view is still there - its retained by your view, but your reference to it is gone
            //print("BG Color 3", customView.backgroundColor ?? "none")   // crash
        }
        required init?(coder: NSCoder) { // compiler forces this
            fatalError("init(coder:) has not been implemented")
        }
    }

//`weak force-unwrapped`:

    @objcMembers
    class VC2: UIViewController {
        weak var customView: UIView!

        init() {
            super.init(nibName: nil, bundle: nil)

            // customView = UIView(frame: .zero)   // Compiler error - cannot even build - compiler knows this will be released immediately
            view.addSubview(UIView(frame: .zero))   // view retains this newly added view
            customView = view.subviews.first        // since its retained already, we can reference it "weakly" (meaning its not further retained)

            customView.backgroundColor = .yellow
            print("Frame", customView.frame)
            customView.removeFromSuperview()        // since customView is weak, it got nilled with the view was released

            // immediate refernces still work - there is some delay until that customView actually gets dealloed
            print("CustomerView", String(format: "Pointer %p", customView)) // Still there ...

            DispatchQueue.main.async {
                if self.customView == .none {
                    print("Its gone now!")
                }
                //print("CustomerView", String(format: "Pointer %p", self.customView)) // Crash!
            }
        }
        required init?(coder: NSCoder) { // compiler forces this
            fatalError("init(coder:) has not been implemented")
        }
    }

//`weak`: this is an illegal syntax and you cannot compile it

//    class VC3: UIViewController {
//        weak var customView: UIView     // Compiler hard error, requires either a "!" or "?" suffix
//
//        init() {
//            super.init(nibName: nil, bundle: nil)
//
//            customView = UIView(frame: .zero)
//            view.addSubview(customView)
//        }
//        required init?(coder: NSCoder) { // compiler forces this
//            fatalError("init(coder:) has not been implemented")
//        }
//    }

//constructed in declaration:

    @objcMembers
    class VC4: UIViewController {
        var customView = UIView(frame: .zero)

        init() {
            super.init(nibName: nil, bundle: nil)

            print("Frame", customView.frame)    // fine - the view is constructed immediately, and since its "strong" the view persists until
                                                // dealloc.
            super.viewDidLoad()
            view.addSubview(customView)
        }
        required init?(coder: NSCoder) { // compiler forces this
            fatalError("init(coder:) has not been implemented")
        }
    }

//`weak` constructed in declaration

    @objcMembers
    class VC5: UIViewController {
        weak var customView = UIView(frame: .zero)  // Compiler warning, the view is instantiated, then immediately released.
        { didSet { if customView == nil { print("NIL !!!") } } }  // does not work - the change isn't ob

        init() {
            print("CUSTOMVIEW", customView?.frame ?? "already gone") // its gone

            super.init(nibName: nil, bundle: nil)

            print("CUSTOMVIEW", customView?.frame ?? "already gone") // its gone

            // At this point, customView was allocated, then immediately released as its not retained elsewhere
            super.viewDidLoad()
            //view.addSubview(customView!)    // Crash
        }
        required init?(coder: NSCoder) { // compiler forces this
            fatalError("init(coder:) has not been implemented")
        }
    }

But, Objective C doesn't care about any of this. Here is another class that references all these classes:

@implementation ObjcTest

- (void)test {
    VC1 *vc1 = [VC1 new];
    NSLog(@"vc1 %@", vc1.customView);

    VC2 *vc2 = [VC2 new];
    NSLog(@"vc2 %@", vc2.customView);

    VC4 *vc4 = [VC4 new];
    NSLog(@"vc4 %@", vc4.customView);

    VC5 *vc5 = [VC5 new];
    NSLog(@"vc5 %@", vc5.customView);
}

@end

/* Console:

 2019-12-05 16:37:52.534537-0500 TableSelections[51189:30629412] vc1 (null)
 2019-12-05 16:37:52.537241-0500 TableSelections[51189:30629412] vc2 <UIView: 0x7fcc73c34230; frame = (0 0; 0 0);
    layer = <CALayer: 0x600000001140>>
 2019-12-05 16:37:52.537535-0500 TableSelections[51189:30629412] vc4 <UIView: 0x7fcc760019e0; frame = (0 0; 0 0);
    layer = <CALayer: 0x6000000244a0>>
 2019-12-05 16:37:52.537734-0500 TableSelections[51189:30629412] vc5 (null)
*/