以下在视图控制器中设置视图的方法有什么区别
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)
}
}
在声明中构造：
class VC: UIViewController() {
var customView = UIView(frame: .zero)
override func viewDidLoad() {
super.viewDidLoad()
addSubview(customView)
}
}
weak声明中构造
class VC: UIViewController() {
weak var customView = UIView(frame: .zero))
override func viewDidLoad() {
super.viewDidLoad()
addSubview(customView)
}
}
可能是您的问题与Xcode创建插座有关：
@IBoutlet weak var label: UILabel!
苹果(和其他专家(已经讨论过是否应该在这里使用弱(因为这些观点几乎总是被view本身保留 - 我相信如果你四处挖掘，你会发现无休止的讨论。您通常会删除上面的weak，没有副作用(尝试一下，看看 - 在视图控制器dealloc中添加日志消息。
但是，让我们进入您的具体问题。我在测试项目中做了一个小班(每个人都想探索这种主题时应该这样做！
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")
}
}
但是，目标C并不关心这些。下面是引用所有这些类的另一个类：
@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)
*/