[swift-evolution] [Proposal] Factory Initializers

Riley Testut rileytestut at gmail.com
Thu Dec 17 15:41:13 CST 2015

Recently, I proposed the idea of adding the ability to implement the "class cluster" pattern from Cocoa (Touch) in Swift. However, as we discussed it and came up with different approaches, it evolved into a functionality that I believe is far more beneficial to Swift, and subsequently should be the focus of its own proposal. So here is the improved (pre-)proposal:

# Factory Initializers

The "factory" pattern is common in many languages, including Objective-C. Essentially, instead of initializing a type directly, a method is called that returns an instance of the appropriate type determined by the input parameters. Functionally this works well, but ultimately it forces the client of the API to remember to call the factory method instead, rather than the type's initializer. This might seem like a minor gripe, but given that we want Swift to be as approachable as possible to new developers, I think we can do better in this regard.

Rather than have a separate factory method, I propose we build the factory pattern right into Swift, by way of specialized “factory initializers”. The exact syntax was proposed by Philippe Hausler from the previous thread, and I think it is an excellent solution:

class AbstractBase {
    public factory init(type: InformationToSwitchOn) {
        return ConcreteImplementation(type)

class ConcreteImplementation : AbstractBase {


Why exactly would this be useful in practice? In my own development, I’ve come across a few places where this would especially be relevant:

## Class Cluster/Abstract Classes
This was the reasoning behind the original proposal, and I still think it would be a very valid use case. The public superclass would declare all the public methods, and could delegate off the specific implementations to the private subclasses. Alternatively, this method could be used as an easy way to handle backwards-compatibility: rather than litter the code with branches depending on the OS version, simply return the OS-appropriate subclass from the factory initializer. Very useful.

## Protocol Initializers
Proposed by Brent Royal-Gordon, we could use factory initializers with protocol extensions to return the appropriate instance conforming to a protocol for the given needs. Similar to the class cluster/abstract class method, but can work with structs too. This would be closer to the factory method pattern, since you don’t need to know exactly what type is returned, just the protocol it conforms to.

## Initializing Storyboard-backed View Controller
This is more specific to Apple Frameworks, but having factory initializers could definitely help here. Currently, view controllers associated with a storyboard must be initialized from the client through a factory method on the storyboard instance (storyboard. instantiateViewControllerWithIdentifier()). This works when the entire flow of the app is storyboard based, but when a single storyboard is used to configure a one-off view controller, having to initialize through the storyboard is essentially use of private implementation details; it shouldn’t matter whether the VC was designed in code or storyboards, ultimately a single initializer should “do the right thing” (just as it does when using XIBs directly). A factory initializer for a View Controller subclass could handle the loading of the storyboard and returning the appropriate view controller.

Here are some comments from the previous thread that I believe are still relevant:

> On Dec 9, 2015, at 1:06 PM, Philippe Hausler <phausler at apple.com> wrote:
> I can definitely attest that in implementing Foundation we could have much more idiomatic swift and much more similar behavior to the way Foundation on Darwin actually works if we had factory initializers. 

> On Dec 7, 2015, at 5:24 PM, Brent Royal-Gordon <brent at architechies.com> wrote:
> A `protocol init` in a protocol extension creates an initializer which is *not* applied to types conforming to the protocol. Instead, it is actually an initializer on the protocol itself. `self` is the protocol metatype, not an instance of anything. The provided implementation should `return` an instance conforming to (and implicitly casted to) the protocol. Just like any other initializer, a `protocol init` can be failable or throwing.
> Unlike other initializers, Swift usually won’t be able to tell at compile time which concrete type will be returned by a protocol init(), reducing opportunities to statically bind methods and perform other optimization tricks. Frankly, though, that’s just the cost of doing business. If you want to select a type dynamically, you’re going to lose the ability to aggressively optimize calls to the resulting instance.

I’d love to hear everyone’s thoughts on this!

Riley Testut

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