class的class-static-func使用Self作为返回值

Posted on In

一个问题

  • Xcode: Version 11.4.1 (11E503a)
  • swift: Swift 5
1
2
3
4
5
6
7
import UIKit
class MDButton: UIButton {
    // 为了更清楚展示问题,代码经过精简
    open class func make() -> Self {
        return MDButton()
    }
}

上面的代码无法编译:

1
2
3
4
ViewController.swift: error: cannot convert return expression of type 'MDButton' to return type 'Self'
        return MDButton()
               ^~~~~~~~~~
                          as! Self

查找关于Self的新特性

Proposal: SE-0068这里有说到关于这个Self的问题.

其目的是为了解决:

  • dynamicType remains an exception to Swift’s lowercased keywords rule. This change eliminates a special case that’s out of step with Swift’s new standards.
  • Self is shorter and clearer in its intent. It mirrors self, which refers to the current instance.
  • It provides an easier way to access static members. As type names grow large, readability suffers. MyExtremelyLargeTypeName.staticMember is unwieldy to type and read.
  • Code using hardwired type names is less portable than code that automatically knows its type.
  • Renaming a type means updating any TypeName references in code.
  • Using self.dynamicType fights against Swift’s goals of concision and clarity in that it is both noisy and esoteric.

所以说该问题在struct中并不存在,也就是下面的代码完全没有问题:

1
2
3
4
5
struct xxx {
    static func make() -> Self {
        return xxx()
    }
}

究竟如何修改

好吧,上面测试过这几个关键字之后,尝试修改函数内的返回对象吧.此时既然Self是dynamicType,那么可以使用self去调用初始化方法就行了吧, 于是:

1
2
3
4
5
6
7
class MDButton2 {
    required init() {}
     class func make() -> Self {
        // 或 return self.init()
        return Self()
    }
}

一点看法

首先, 整个过程(包括Xcode的提示)不使用as!的解决方案, 对,我就是那个坚决不使用as!的人(刷题除外).

然后, 可以翻一下这个Self的实现apple/swift#22863,内容比较多.