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.
Rationale:
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.
Considerations:
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
[email protected]
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