Does Swift support reflection?

The documentation speaks about a dynamic type system, mainly about

Type and dynamicType

See Metatype Type (in Language Reference)

Example:

var clazz = TestObject.self
var instance: TestObject = clazz()

var type = instance.dynamicType

println("Type: \(type)") //Unfortunately this prints only "Type: Metatype"

Now assuming TestObject extends NSObject

var clazz: NSObject.Type = TestObject.self
var instance : NSObject = clazz()

if let testObject = instance as? TestObject {
    println("yes!") //prints "yes!"
}

Currently, there is no reflection implemented.

EDIT: I was apparently wrong, see stevex's answer. There is some simple readonly reflection for properties build in, probably to allow IDEs to inspect object contents.


Looks like there's the start of some reflection support:

class Fruit {
    var name="Apple"
}

reflect(Fruit()).count         // 1
reflect(Fruit())[0].0          // "name"
reflect(Fruit())[0].1.summary  // "Apple"

From mchambers gist, here: https://gist.github.com/mchambers/fb9da554898dae3e54f2


If a class extends NSObject, then all of Objective-C's introspection and dynamism works. This includes:

  • The ability to ask a class about its methods and properties, and to invoke methods or set properties.
  • The ability to exchange method implementations. (add functionality to all instances).
  • The ability to generate and assign a new sub-class on the fly. (add functionality to a given instance)

One shortcoming of this functionality is support for Swift optional value types. For example Int properties can be enumerated and modified but Int? properties cannot. Optional types can be enumerated partially using reflect/MirrorType, but still not modified.

If a class does not extend NSObject, then only the new, very limited (and in progress?) reflection works (see reflect/MirrorType), which adds limited ability to ask a instance about its class and properties, but none of the additional features above.

When not extending NSObject, or using the '@objc' directive, Swift defaults to static- and vtable-based dispatch. This is faster, however, in the absence of a virtual machine does not allow runtime method interception. This interception is a fundamental part of Cocoa and is required for the following types of features:

  • Cocoa's elegant property observers. (Property observers are baked right in to the Swift language).
  • Non-invasively applying cross-cutting concerns like logging, transaction management (i.e Aspect Oriented Programming).
  • Proxies, message forwarding, etc.

Therefore its recommended that clases in Cocoa/CocoaTouch applications implemented with Swift:

  • Extend from NSObject. The new class dialog in Xcode steers in this direction.
  • Where the overhead of of a dynamic dispatch leads to performance issues, then static dispatch can be used - in tight loops with calls to methods with very small bodies, for example.

Summary:

  • Swift can behave like C++, with fast static/vtable dispatch and limited reflection. This makes it suitable for lower level or performance intensive applications, but without the complexity, learning curve or risk of error associated with C++
  • While Swift is a compiled language, the messaging style of method invocation adds the introspection and dynamism found in modern languages like Ruby and Python, just like Objective-C, but without Objective-C's legacy syntax.

Reference data: Execution overhead for method invocations:

  • static : < 1.1ns
  • vtable : ~ 1.1ns
  • dynamic : ~4.9ns

(actual performance depends on hardware, but the ratios will remain similar).

Also, the dynamic attribute allows us to explicitly instruct Swift that a method should use dynamic dispatch, and will therefore support interception. 

public dynamic func foobar() -> AnyObject {
}

Tags:

Ios

Reflection

Swift

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