Instructions for using the swift programming language in Chrome for iOS.

Build rules

When adding or removing swift source code files, the build rules must be updated. The BUILD.gn file in the same folder or in a parent folder of the source file typically contains a build rule that includes a given source code file.

Swift-specific instructions

• Swift source files must be referenced by a swift_source_set build rule which defines a build target that contains only swift code.
• The build rule file must have import("//build/config/apple/swift_source_set.gni") to be able to use the swift_source_set rule.

C++ interop

Use swift's direct C++ interop, if possible, whenever C++ code needs to be invoked from swift. When needed, use the helpers in the C++ include file base/apple/swift_interop_util.h to solve special cases.

Instructions for calling C++ directly from swift

• Ensure that the C++ symbols are exported to swift using a modulemap.
• The swift modulemap should be defined in the same folder as the C++ build rule that builds the exported C++ symbol.
• In the module map, use nested modules when appropriate. The nesting hierarchy should reflect the nesting of the C++ namespaces that contain the exported symbols.
• In the swift_source_set rule that has a dependency on C++ code, use the cxx_modulemap argument to specify a module map that imports the required C++ symbols.
• Add the necessary import statements to the swift source files to import the modules defined in the modulemap
• For an example of how to do this correctly, look at file ios/chrome/test/swift_interop/BUILD.gn and the files referenced by its build rules.
• Refer to the documentation for C++ interop

UI Programming instructions

• Prefer using SwiftUI for defining views. Only use UIKit when the required functionality is not exposed in SwiftUI.
• Use colors from the Chromium UI theme via the shims defined in file ios/chrome/common/ui/colors/Color+Chrome.swift.
• When image resources are needed in a view, import them using a variation of this syntax:

if let uiImage = UIImage(named: "<image_resource_name>") {
  Image(uiImage: uiImage)
}

• All UI strings must be imported using NSLocalizedString.
• The string resources for Chrome for iOS are defined in files ios/chrome/app/strings/ios_strings.grd and ios/chrome/app/resources/chrome_localize_strings_config.plist

Delegate to the user

When you encounter non-trivial build errors related to swift/C++ interoperability, do not try to fix the problem yourself. Instead, annotate the the code with comments that explain what you understand about the problem, send a message to the user instructing them to fix the issue themselves, then quit.