Swift 覆盖默认初始化没有子类

Question

Is it possible to override a default init without subclassing?

I want every instance of DateFormatter created in my app to have the same locale attached to it. Right now, upon each instance creation, I have to:

let dateFormatter = DateFormatter()
dateFormatter.locale = Locale(identifier: "en_US_POSIX"

Is it possible to have this happen without the need to write out the locale every single time? I'd prefer to not create a DateFormatter subclass as I feel that's a bit of overkill

Thanks!

Answer 1

There is no way to override an existing method of a class without subclassing it or modifying the source code (which you obviously can't do in case of built-in types).

However, for your specific case, you could simply add a method/static variable that returns a DateFormatter specific to your needs.

extension DateFormatter {
    static func usPosix() -> DateFormatter {
        let dateFormatter = DateFormatter()
        dateFormatter.locale = Locale(identifier: "en_US_POSIX")
        return dateFormatter
    }
}

And then instead of calling let dateFormatter = DateFormatter() , you do let dateFormatter = DateFormatter.usPosix()

Answer 2

While I personally HIGHLY advise against swizzling, that is certainly an option here.

First, start by creating your own locale that you want to be used throughout your app:

extension DateFormatter {
    // Note the "@objc" here, this is necessary for altering the runtime logic
    @objc var swizzledLocale: Locale {
        return Locale(identifier: "en_US_POSIX")
    }
}

Next, you'll want to tell Swift that whenever a DateFormatter 's locale is accessed, you want your new swizzledLocale to be used instead:

// get reference to the two getter methods for the properties you want to "switch"
let orig = #selector(getter: DateFormatter.locale)
let new = #selector(getter: DateFormatter.swizzledLocale)
let origMethod = class_getInstanceMethod(DateFormatter.self, orig)!
let newMethod = class_getInstanceMethod(DateFormatter.self, new)!
// performs the actual switch so that whenever "locale" is accessed in code, the returned value will be the value of "swizzledLocale"
method_exchangeImplementations(origMethod, newMethod)

And that's all there is to it! Now whenever you access any DateFormatter 's locale , a Locale("en_US_POSIX") will be returned.

Some things to note: first, this will alter ALL instances of locale , not just the locale s that you personally use in your own code. That is, if there's any internal code that relies on a formatter's locale , en_US_POSIX will always be used. Second, the second block of code (the one that performs the actual swizzling) should be called exactly one time; look into wrapping it in a dispatch_once block to ensure it only runs once.