ObjCRuntimeTools

ObjCRuntimeTools is a Swift library that provides powerful tools for working with Objective-C runtime features, including property association and method swizzling. It leverages modern Swift macros to simplify and enhance the use of these runtime capabilities.

Features

• Property Association: easily associate properties with Objective-C objects using the @Associated macro.
• Swizzling: perform type-safe and boilerplate-free runtime swizzling with the #swizzle macro.

Installation

Add ObjCRuntimeTools to your Swift Package Manager dependencies:

.package(url: "https://github.com/davdroman/objc-runtime-tools", from: "0.1.0"),

Then, add the dependency to your desired target:

.product(name: "ObjCRuntimeTools", package: "objc-runtime-tools"),

Usage

Property Association

Use the @Associated macro to associate properties with Objective-C objects:

class MyClass: NSObject {}

extension MyClass {
    @Associated(.retain(.nonatomic))
    var associatedProperty: String = "Default Value"
}

Swizzling

Use the #swizzle macro to perform function, getter, or setter swizzling.

Function Swizzling

try #swizzle(UIViewController.viewDidLoad) { $self in
    print("Before")
    self.viewDidLoad()
    print("After")
}

try #swizzle(UIViewController.viewDidAppear, param: Bool.self) { $self, animated in
    print("Before")
    self.viewDidAppear(animated)
    print("After")
}

try #swizzle(UITextField.resignFirstResponder, returning: Bool.self) { $self in
    print("Before")
    let result = self.resignFirstResponder()
    print("After")
    return result
}

try #swizzle(
    UIScrollView.touchesShouldBegin,
    params: Set<UITouch>.self, UIEvent?.self, UIView.self,
    returning: Bool.self
) { $self, touches, event, view in
    print("Before")
    let result = self.touchesShouldBegin(touches, with: event, in: view)
    print("After")
    return result
}

Getter and Setter Swizzling

try #swizzle(getter: \UIView.isHidden, returning: Bool.self) { $self in
    print("Before")
    let isHidden = self.isHidden
    print("After")
    return isHidden
}

try #swizzle(setter: \UIView.isHidden, param: Bool.self) { $self, isHidden in
    print("Before")
    self.isHidden = isHidden
    print("After")
}

⚠️ Important: how to swizzle only once ⚠️

It's up to you to ensure that the swizzling is done only once. You can use a static variable to track whether the swizzling has already been performed:

class MyClass: NSObject {
    static var isSwizzled = false

    static func swizzleOnce() throws {
        guard !MyClass.isSwizzled else { return }
        MyClass.isSwizzled = true

        try #swizzle(MyClass.doSomething) { $self in
            print("Before")
            self.doSomething()
            print("After")
        }
    }
}

However this is error prone, noisy and not very maintainable, especially if you have multiple swizzling points. Not to mention Swift 6's strict concurrency rules, which will make this approach even more difficult because static variables aren't thread-safe by default.

A better approach is to use a macro such as #once which ensures any action is executed only once, regardless of how many times it is called over the lifetime of the app.

class MyClass: NSObject {
    static func swizzleOnce() throws {
        try #once {
            try #swizzle(MyClass.doSomething) { $self in
                print("Before")
                self.doSomething()
                print("After")
            }
        }
    }
}

Credits

This library is inspired by and builds upon the following projects:

• AssociatedObject
• InterposeKit