On 3 Sep 2015, at 8:55 AM, has <hengist.p...@virgin.net> wrote:
> Stuck and looking for ideas here. I need to define a base class whose methods
> vends instances of its subclasses (thus enabling chained method calls; your
> basic query builder).
I’m assuming Swift 1.2. The following works in an OS X 10.10 playground:
//////////////////////////////
class Factory: Printable {
required init() {}
class func makeNew() -> Self {
return self()
}
var description: String { return "This is a Factory." }
}
class Product: Factory, Printable {
override var description: String { return "This is a Product." }
}
let f = Factory.makeNew() // => __lldb_expr_580.Factory
f.description // => "This is a Factory."
let p = Product.makeNew() // => {__lldb_expr_580.Factory}
p.description // => "This is a Product."
//////////////////////////////
* Using an instance as a factory for its own class is contrary to general
usage, so is a smell at best. It’s a job for a class func. (Yes,
functional-programming operators do exactly that, but this is an example of the
Factory pattern, not an operator. You shouldn’t have to instantiate a Product
to make a Product.)
* Explicitly calling init should be legitimate, as the changes to map{} and the
like now allow you to use it in the closure. I could still imagine support for
that usage being withdrawn. I’m not comfortable as a matter of style with
making it Just Another Func — and the solution above makes it moot.
* Self is the new instancetype. It takes care of casting issues. (I haven’t
thought out all the corner cases, so don’t hit me. I wear glasses.)
* I was confused by the REPL’s calling Product.makeNew() an
{__lldb_expr_580.Factory}, but apparently the braces mean “some subclass of.”
If anyone can offer an authoritative explanation, I’d be glad to know.
* I assume the call to self() in makeNew() goes through to an initializer for
Product, but my day job calls.
— F
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