Swift - Custom setter on property

问题

I am converting a project in to Swift code and have come across an issue in a setter. My Objective-C code looked like this:

- (void)setDocument:(MyDocument *)document
{
    if (![_document isEqual:document]) {
        _document = document;

        [self useDocument];
    }
}

and allowed my View Controller to run this each time the document was set (typically in the prepareForSegue: method of the presenting View Controller).

I have found the property observers willSet and didSet but they only work when the property is being updated, not when it’s initialised and updated.

Any ideas? Thanks

UPDATE

after trying get{} and set{} I get the EXC_BAD_ACCESS error

var document: UIDocument? {
    get {
        return self.document!
    }
    set {
        self.document = newValue

        useDocument()
    }
}

回答1:

You can't use set like that because when you call self.document = newValue you're just calling the setter again; you've created an infinite loop.

What you have to do instead is create a separate property to actually store the value in:

private var _document: UIDocument? = nil
var document: UIDocument? {
    get {
        return self._document
    }
    set {
        self._document = newValue
        useDocument()
    }
}

回答2:

Here's a Swift 3 version

var document : UIDocument? {
    didSet {
        useDocument()
    }
}

来源：https://stackoverflow.com/questions/25828632/swift-custom-setter-on-property