計算屬性中的自調用

Question

爲什麼在早些時候，當我們在像這個例子那樣的計算屬性中調用self時，我們需要編寫lazy var，但現在我們不必這樣做。爲什麼？

let(lazy var in earlier times) pauseButton: UIButton = { 
    let button = UIButton(type: .system) 
    let image = UIImage(named: "pause") 
    button.setImage(image, for: .normal) 
    button.translatesAutoresizingMaskIntoConstraints = false 
    button.tintColor = .white 
    button.addTarget(self, action: #selector(handlePause), for: .touchUpInside) 

    return button 
    }() 

Answer 1

我認爲這是一個誤解，這是你的代碼段提到的是不一個計算的財產！它只是一個已存儲的財產已被封閉初始化; As mentioned in the Swift Initialization - Setting a Default Property Value with a Closure or Function：

If a stored property’s default value requires some customization or setup, you can use a closure or global function to provide a customized default value for that property. Whenever a new instance of the type that the property belongs to is initialized, the closure or function is called, and its return value is assigned as the property’s default value.

_{你可以檢查：Difference between computed property and property set with closure。是的pauseButton會被關閉，甚至沒有使用它，如果你嘗試檢查它（在它添加斷點）來執行，}

注意，你會發現這一點。我認爲這是而不是你期待什麼 - 而不是你瞄準什麼 - 所以你應該聲明它爲lazy var而不是let。

然而，

指同一Swift documentation：

If you use a closure to initialize a property, remember that the rest of the instance has not yet been initialized at the point that the closure is executed. This means that you cannot access any other property values from within your closure, even if those properties have default values. You also cannot use the implicitselfproperty, or call any of the instance’s methods.

言下之意是：

class MyViewController: UIViewController { 
    let btnTitle = "pause" 

    let pauseButton: UIButton = { 
     let button = UIButton(type: .system) 
     let image = UIImage(named: btnTitle) 
     button.setImage(image, for: .normal) 
     button.translatesAutoresizingMaskIntoConstraints = false 
     button.tintColor = .white 
     button.addTarget(self, action: #selector(handlePause), for: .touchUpInside) 

     return button 
    }() 

    func handlePause() { } 
} 

威爾給出的let image = UIImage(named: btnTitle)的錯誤：

這也應該適用於任何其他實例成員，例如，如果你想嘗試添加view.addSubview(button)到關閉，你會得到同樣的錯誤view實例成員。

但對於一個原因（我不知道爲什麼），與選擇的工作似乎是一個特例，因爲button.addTarget(self, action: #selector(handlePause), for: .touchUpInside)對我（的Xcode 9.0）工作得很好，但如果你想添加self它，因爲：

button.addTarget(self, action: #selector(self.handlePause), for: .touchUpInside) 

你會得到以下錯誤：