Swift 3 – dynamic vs @objc

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A function/variable declared as @objc is accessible from Objective-C, but Swift will continue to access it directly via static or virtual dispatch.
This means if the function/variable is swizzled via the Objective-C framework, like what happens when using Key-Value Observing or the various Objective-C APIs to modify classes, calling the method from Swift and Objective-C will produce different results.

Using dynamic tells Swift to always refer to Objective-C dynamic dispatch.
This is required for things like Key-Value Observing to work correctly. When the Swift function is called, it refers to the Objective-C runtime to dynamically dispatch the call.

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