Hot reload on Swift app using @_dynamicReplacement

SwiftHotReload is an experimental project. We investigate a real world application of the @_dynamicReplacement feature of Swift 5.1+. Many portions are subject to change, including the library name (it's simple & naive name. we don't plan to publish to CocoaPods/Specs before resolving them.)

• Xcode 16.x
• Host macOS 14.x, 15.x

We can use either Standalone Reloader or Proxy Reloader. Standalone Reloader runs all required tasks on the runtime target process. Proxy Reloader runs on the runtime target process and receives dylibs from BuildHelper via network. BuildHelper runs on the host Mac and monitors file changes to build the file and send dylibs to Proxy on the target.

• Monitor a swift file for trigger a build (standalone, run on the app runtime process)
• Build a swift file and emit dylib (standalone, run on the app runtime process)
  • Estimate build environmentd and intermediate interfaces
• Load a dylib while the app on runtime
• Supports apps on macOS and simulators for iOS, iPadOS, and visionOS
  • SPM project structures
  • CocoaPods project structures
• Update trigger for SwiftUI views
• Helper app on host & Reload on devices
• Compatible for App Store submission, as long as caller side suppress any calls in Release build

• Less invasive: be easy to adopt & compatible for App Store submission
  • Build settings (-Xfrontend ...)
  • Sandbox restrictions for macOS app
• Load history
• In-place editing

• Open SwiftHotReload.xcworkspace
• Run SwiftHotReloadExample on Mac or any simulators
• Edit ReplaceView.swift and save

SPM

https://github.com/banjun/SwiftHotReload.git

CocoaPods

pod 'SwiftHotReload', :git => "https://github.com/banjun/SwiftHotReload.git", :branch => "main"

or manual copy

Set up app as described below and build & run on a supported platform.

extension App {
    static let reloader = StandaloneReloader(monitoredSwiftFile: URL(fileURLWithPath: #filePath).deletingLastPathComponent()
        // file path to be monitored
        .appendingPathComponent("RuntimeOverrides.swift")
    :        
    _ = App.reloader // use to load the lazy static property above and start a file monitor
}

If the app is for iOS Device, use ProxyReloader instead of StandaloneReloader. Run BuildHelper separately on the host Mac:

git clone https://github.com/banjun/SwiftHotReload.git
cd SwiftHotReload

swift run BuildHelper -c debug

Alternatively to swift run, we can run BuildHelper as an app (not CLI) using BuildHelper target on SwiftHotReload.xcworkspace.

Modify the app entitlements file:

App Sandbox = NO

• Add to OTHER_SWIFT_FLAGS of the app target
  • -Xfrontend -enable-implicit-dynamic
    • use the flag instead of explicitly marking dynamic before funcs or vars
  • -Xfrontend -enable-private-imports
    • use the flag instead of making related funcs or vars visible by removing private

or copy-and-paste on the Xcode build settings GUI to overwrite in 1 step.

OTHER_SWIFT_FLAGS[config=Debug] = -Xfrontend -enable-implicit-dynamic -Xfrontend -enable-private-imports

@ObservedObject private var reloader = App.reloader

Any funcs/vars can be replaced (not only for SwiftUI).

import AppModuleName

extension ContentView { // <- typically use extension for a type containing func/var to be replaced
    @_dynamicReplacement(for: body) // <- func/var name to be replaced
    var body2: some View { // <- use different name than the original
    :
    }
}