如何編寫一個採用泛型類型參數的函數，但基於泛型對象的類型返回不同的類型？

Question

我想在swift3中編寫一個基本插值函數。不過，我收到了很多錯誤。這顯然不是使用泛型的正確方法，但也許我對它們的應用有一個根本性的誤解。

class func interpolate<T>(from: T, to: T, progress: CGFloat) -> T 
{ 
    // Safety 
    assert(progress >= 0 && progress <= 1, "Invalid progress value: \(progress)") 

    if let from = from as? CGFloat, let to = to as? CGFloat 
    { 
     return from + (to - from) * progress // No + candidates produce the expected contextual result type 'T' 
    } 
    if let from = from as? CGPoint, let to = to as? CGPoint 
    { 
     var returnPoint = CGPoint() 
     returnPoint.x  = from.x + (to.x-from.x) * progress 
     returnPoint.y  = from.y + (to.y-from.y) * progress 
     return returnPoint // Cannot convert return expression of type 'CGPoint' to return type 'T' 
    } 
    if let from = from as? CGRect, let to = to as? CGRect 
    { 
     var returnRect = CGRect() 
     returnRect.origin.x  = from.origin.x + (to.origin.x-from.origin.x) * progress 
     returnRect.origin.y  = from.origin.y + (to.origin.y-from.origin.y) * progress 
     returnRect.size.width = from.size.width + (to.size.width-from.size.width) * progress 
     returnRect.size.height = from.size.height + (to.size.height-from.size.height) * progress 
     return returnRect // Cannot convert return expression of type 'CGRect' to return type 'T' 
    } 

    return nil // Nil is incompatible with return type 'T' 
} 

Answer 1

當你有幾個不同的類型執行相同的操作時，通用函數很有用。這基本上就是你在這裏所擁有的。問題是您沒有爲您關心的兩種類型定義操作，即CGPoint和CGRect。

如果您創建單獨的函數來添加，減少和乘以這些類型，則可以使該通用函數有效。這將簡化爲

class func interpolate<T>(from: T, to: T, progress: CGFloat) -> T 
{ 
    // Safety 
    assert(0.0...1.0 ~= progress, "Invalid progress value: \(progress)") 

    return from + (to - from) * progress 
}