Deprecate @UIApplicationMain and @NSApplicationMain

• Proposal: SE-NNNN
• Authors: Robert Widmann
• Review Manager: TBD
• Status: Awaiting implementation

Introduction

@UIApplicationMain and @NSApplicationMain used to be the standard way for iOS and macOS apps respectively to declare a synthesized platform-specific entrypoint for an app. These functions have since been obsoleted by SE-0281 and the @main attribute, and now represent a confusing bit of duplication in the language. This proposal seeks to deprecate these alternative entrypoint attributes in favor of @main in Swift 5.8, and makes their use in Swift 6 a hard error.

Swift-evolution thread: Discussion thread topic for that proposal

Motivation

UIKit and AppKit have fully embraced the @main attribute and have made application adoption as simple as conforming to the UIApplicationDelegate and NSApplicationDelegate protocols. This now means that an author of an application is presented with two different, but ultimately needless, choices for an entrypoint:

• Use hard coded framework-specific attributes
• Use the language’s general-purpose syntax for declaring an entrypoint

The right choice is clearly the latter. Moreover, having two functionally identical ways to express the concept of an app-specific entrypoint is clutter at best and confusing at worst. This proposal seeks to complete the migration work implied by SE-0281 by having the compiler push Swift authors towards the more general, unified solution.

Proposed solution

The use of both @UIApplicationMain and @NSApplicationMain under Swift 5 language modes will unconditionally warn and offer to replace these attributes with the appropriate conformances. In Swift 6 language modes, usage of these attributes will result in a hard error.

Detailed design

Because @UIApplicationMain and @NSApplicationMain have remarkably similar usages in abstract, this portion of the document will only use examples of @UIApplicationMain and assume that the case for @NSApplicationMain follows similarly.

Issue a diagnostic asking the user to remove @UIApplicationMain. The existing conformance to UIApplicationDelegate will kick in when the code is next built and the general-purpose entrypoint will be added instead.

@UIApplicationMain // warning: '@UIApplicationMain' is deprecated in Swift 5
                   // fixit: Change `@UIApplicationMain` to `@main`
final class MyApplication: UIResponder, UIApplicationDelegate {
  /**/
}

We can get away with this simple remedy because

• A Swift module can have at most one of: @UIApplicationMain, @main, and @NSApplicationMain
• A class annotated with @UIApplicationMain or @NSApplicationMain must conform directly to the corresponding delegate protocol anyways. They therefore inherit the main entrypoint provided by these protocols, they just aren't using it!

Source compatibility

This proposal is intentionally source-breaking, but that breakage is relegated to a new language mode and the diagnostics involved provide a clear and simple migration path for existing usages of these entrypoint attributes.

Effect on ABI stability

This proposal has no impact on ABI.

Effect on API resilience

None.

Alternatives considered

We could choose not to do this.