[swift-evolution] [Proposal] Remove behavior on AnyObject that allows any obj-c method to be called on it

Kevin Lundberg kevin at klundberg.com
Tue Mar 22 20:59:50 CDT 2016

In "Using Swift with Cocoa and Objective-C", this behavior is described as part of how AnyObject works:

“You can also call any Objective-C method and access any property without casting to a more specific class type." … "However, because the specific type of an object typed as AnyObject is not known until runtime, it is possible to inadvertently write unsafe code. As in Objective-C, if you invoke a method or access a property that does not exist on an AnyObject typed object, it is a runtime error.”

I propose that we remove this behavior entirely to push swift further in the direction of type safety.

Even if you don’t mean to write code that relies on this behavior, it's easy to accidentally do so when interfacing with various Cocoa APIs due to type inference. A developer may not even realize that their code is unsafe since their code will compile just fine when calling obj-c visible methods. Removing this behavior would alleviate any confusion that a developer may have while writing this, especially as it is not a highly advertised feature of AnyObject. Furthermore, anyone who reads swift code using this will know with more certainty what types the author expects to be using here since an explicit cast will be required.

If this is done, the way I see AnyObject behaving is similar to Any, where you need to manually downcast in order to call methods on things. Code would change from this:

class Foo: NSObject { func bar() {} }
let things = NSOrderedSet(object: Foo())

for thing in things { // thing is AnyObject
	thing.bar() // happens to work but not verified by compiler, may crash in the future

to something like this:


for thing in things {
	if let foo = thing as? Foo { // needs an explicit cast
		foo.bar() // type checked, verified by compiler, won’t crash due to missing method

One ancillary benefit that I can see of doing this is that it could make AnyObject consistent across darwin and other platforms. As far as I can tell, this behavior only exists on platforms where swift integrates with the objective-c runtime, and doing this will help swift code be more portable as it doesn’t rely on this implicit behavior.

Any thoughts?

Kevin Lundberg
kevin at klundberg.com

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