What if I want to assign a property to itself?
There are some good workarounds but there is little point in doing that. If a programmer (future maintainer of the code) sees code like this:
a = a
They will remove it.
Such a statement (or a workaround) should never appear in your code.
If your property looks like this:
var a: Int {
didSet {
// code
}
}
then it's a not a good idea to invoke the didSet
handler by assignment a = a
.
What if a future maintainer adds a performance improvement to the didSet
like this?
var a: Int {
didSet {
guard a != oldValue else {
return
}
// code
}
}
The real solution is to refactor:
var a: Int {
didSet {
self.updateA()
}
}
fileprivate func updateA() {
// the original code
}
And instead of a = a
directly call updateA()
.
If we are speaking about outlets, a suitable solution is to force the loading of views before assigning for the first time:
@IBOutlet weak var photographerLabel: UILabel?
var photographer: Photographer? {
didSet {
_ = self.view // or self.loadViewIfNeeded() on iOS >= 9
photographerLabel?.text = photographer.name // we can use ! here, it makes no difference
}
}
That will make the code in viewDidLoad
unnecessary.
Now you might be asking "why should I load the view if I don't need it yet? I want only to store my variables here for future use". If that's what you are asking, it means you are using a view controller as your model class, just to store data. That's an architecture problem by itself. If you don't want to use a controller, don't even instantiate it. Use a model class to store your data.
Prior to Swift 3.1 you could assign the property name
to itself with:
name = (name)
but this now gives the same error: "assigning a property to itself".
There are many other ways to work around this including introducing a temporary variable:
let temp = name
name = temp
This is just too fun not to be shared. I'm sure the community can come up with many more ways to do this, the crazier the better
class Test: NSObject {
var name: String? {
didSet {
print("It was set")
}
}
func testit() {
// name = (name) // No longer works with Swift 3.1 (bug SR-4464)
// (name) = name // No longer works with Swift 3.1
// (name) = (name) // No longer works with Swift 3.1
(name = name)
name = [name][0]
name = [name].last!
name = [name].first!
name = [1:name][1]!
name = name ?? nil
name = nil ?? name
name = name ?? name
name = {name}()
name = Optional(name)!
name = ImplicitlyUnwrappedOptional(name)
name = true ? name : name
name = false ? name : name
let temp = name; name = temp
name = name as Any as? String
name = (name,0).0
name = (0,name).1
setValue(name, forKey: "name") // requires class derive from NSObject
name = Unmanaged.passUnretained(self).takeUnretainedValue().name
name = unsafeBitCast(name, to: type(of: name))
name = unsafeDowncast(self, to: type(of: self)).name
perform(#selector(setter:name), with: name) // requires class derive from NSObject
name = (self as Test).name
unsafeBitCast(dlsym(dlopen("/usr/lib/libobjc.A.dylib",RTLD_NOW),"objc_msgSend"),to:(@convention(c)(Any?,Selector!,Any?)->Void).self)(self,#selector(setter:name),name) // requires class derive from NSObject
unsafeBitCast(class_getMethodImplementation(type(of: self), #selector(setter:name)), to:(@convention(c)(Any?,Selector!,Any?)->Void).self)(self,#selector(setter:name),name) // requires class derive from NSObject
unsafeBitCast(method(for: #selector(setter:name)),to:(@convention(c)(Any?,Selector,Any?)->Void).self)(self,#selector(setter:name),name) // requires class derive from NSObject
_ = UnsafeMutablePointer(&name)
_ = UnsafeMutableRawPointer(&name)
_ = UnsafeMutableBufferPointer(start: &name, count: 1)
withUnsafePointer(to: &name) { name = $0.pointee }
//Using NSInvocation, requires class derive from NSObject
let invocation : NSObject = unsafeBitCast(method_getImplementation(class_getClassMethod(NSClassFromString("NSInvocation"), NSSelectorFromString("invocationWithMethodSignature:"))),to:(@convention(c)(AnyClass?,Selector,Any?)->Any).self)(NSClassFromString("NSInvocation"),NSSelectorFromString("invocationWithMethodSignature:"),unsafeBitCast(method(for: NSSelectorFromString("methodSignatureForSelector:"))!,to:(@convention(c)(Any?,Selector,Selector)->Any).self)(self,NSSelectorFromString("methodSignatureForSelector:"),#selector(setter:name))) as! NSObject
unsafeBitCast(class_getMethodImplementation(NSClassFromString("NSInvocation"), NSSelectorFromString("setSelector:")),to:(@convention(c)(Any,Selector,Selector)->Void).self)(invocation,NSSelectorFromString("setSelector:"),#selector(setter:name))
var localVarName = name
withUnsafePointer(to: &localVarName) { unsafeBitCast(class_getMethodImplementation(NSClassFromString("NSInvocation"), NSSelectorFromString("setArgument:atIndex:")),to:(@convention(c)(Any,Selector,OpaquePointer,NSInteger)->Void).self)(invocation,NSSelectorFromString("setArgument:atIndex:"), OpaquePointer($0),2) }
invocation.perform(NSSelectorFromString("invokeWithTarget:"), with: self)
}
}
let test = Test()
test.testit()