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♻️ Extract delegate swizzling to single implementation
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| Original file line number | Diff line number | Diff line change |
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| // | ||
| // Swizzler.swift | ||
| // AppcuesKit | ||
| // | ||
| // Created by Matt on 2024-04-11. | ||
| // Copyright © 2024 Appcues. All rights reserved. | ||
| // | ||
|
|
||
| import Foundation | ||
|
|
||
| internal enum Swizzler { | ||
| /// Swizzling for delegate objects. | ||
| /// | ||
| /// This is unique because, | ||
| /// 1. We aren't certain of the class type that implements the delegate protocol at compile time. | ||
| /// This is the reason why this function takes an instance of the delegate instead of the delegate type. | ||
| /// 2. Delegate methods are frequently optional, so we can't rely on the implementation being there to swizzle. | ||
| /// If this is the case, we add an empty placeholder implementation and then swizzle that. | ||
| /// | ||
| /// - Parameters: | ||
| /// - targetInstance: Instance of the class to replace the method in. | ||
| /// - targetSelector: Selector of the method to replace. | ||
| /// - replacementOwner: Class containing the methods selected by `swizzleSelector` and `placeholderSelector`. | ||
| /// - placeholderSelector: Selector of the method to use the `targetSelector` method is not implemented. | ||
| /// This should be an empty function. | ||
| /// - swizzleSelector: Selector of the method to use as the replacement. | ||
| static func swizzle( | ||
| targetInstance: AnyObject, | ||
| targetSelector: Selector, | ||
| replacementOwner: AnyClass, | ||
| placeholderSelector: Selector, | ||
| swizzleSelector: Selector | ||
| ) { | ||
| // see if the currently assigned delegate has an implementation for the target selector already. | ||
| // these are optional methods in the protocol, and if they are not there already, we'll need to add | ||
| // a placeholder implementation so that we can consistently swap it with our override, which will attempt | ||
| // to call back into it, in case there was an implementation already - if we don't do this, we'll | ||
| // get invalid selector errors in these cases. | ||
| let targetClass: AnyClass = type(of: targetInstance) | ||
| let originalMethod = class_getInstanceMethod(targetClass, targetSelector) | ||
|
|
||
| if originalMethod == nil { | ||
| // this is the case where the existing delegate does not have an implementation for the target selector | ||
|
|
||
| guard let placeholderMethod = class_getInstanceMethod(replacementOwner, placeholderSelector) else { | ||
| // this should never be nil as it would be a developer error, but we must nil check this call | ||
| return | ||
| } | ||
|
|
||
| // add the placeholder, so it can be swizzled uniformly | ||
| class_addMethod( | ||
| targetClass, | ||
| targetSelector, | ||
| method_getImplementation(placeholderMethod), | ||
| method_getTypeEncoding(placeholderMethod) | ||
| ) | ||
| } | ||
|
|
||
| // swizzle the new implementation to inject our own custom logic | ||
|
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| // this should never be nil as it would be a developer error, but we must nil check this call | ||
| guard let swizzleMethod = class_getInstanceMethod(replacementOwner, swizzleSelector) else { return } | ||
|
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| // add the swizzled version - this will only succeed once for this instance, if its already there, we've already | ||
| // swizzled, and we can exit early in the next guard | ||
| let addMethodResult = class_addMethod( | ||
| targetClass, | ||
| swizzleSelector, | ||
| method_getImplementation(swizzleMethod), | ||
| method_getTypeEncoding(swizzleMethod) | ||
| ) | ||
|
|
||
| guard addMethodResult, | ||
| let originalMethod = originalMethod ?? class_getInstanceMethod(targetClass, targetSelector), | ||
| let swizzledMethod = class_getInstanceMethod(targetClass, swizzleSelector) else { | ||
| return | ||
| } | ||
|
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||
| // finally, here is where we swizzle in our custom implementation | ||
| method_exchangeImplementations(originalMethod, swizzledMethod) | ||
| } | ||
| } |