'Class.Type'没有名为'variable'的成员错误只是缺少类变量支持？

Question

我一直在尝试在类中使用常量和变量,这些类通过名称引用类级别的同一类中的其他常量和变量.AFAIK,从Xcode 6 beta 4开始,Swift仍然没有类变量支持.我想知道的是,在尝试引用其他常量(let)或变量(var)时我看到的错误是否仅仅是由于缺少类变量支持？

您可以引用方法内或类外的常量和变量,您似乎无法在类级别按名称引用.以下课程显示了几种变体以及您将在Xcode 6 beta 4中看到的错误.

这可以在游乐场或常规.swift文件中测试.

class Simple {

    let someConstant = 0.50

    var someVariable = 1

    // uncomment let and var lines to see the errors

    // error: 'Simple.Type' does not have a member named 'someConstant'
    // let referringConstant = someConstant
    // error: Use of unresolved identifier 'self'
    // let referringConstant = self.someConstant

    // error: 'Simple.Type' does not have a member named 'someVariable'
    // var referringVar = someVariable
    // error: Use of unresolved identifier 'self'
    // var referringVar = self.someVariable

    // can't do class constants or variables yet either
    // let referringConstant = Simple.someConstant
    // var referringVar = Simple.someVariable

    func simpleMethod() {
        // both of these forms are valid as long as it is unambiguous
        let referToConstant = someConstant
        let referToVariable = someVariable

        var anotherConstant = self.someConstant
        var anotherVariable = self.someVariable
    }
}

作为参考,在Swift中导致此问题的原始Objective-C代码来自CS193P SuperCard应用程序,其中C#define用于常量,然后该常量用于设置变量.

@interface PlayingCardView()
@property (nonatomic) CGFloat faceCardScaleFactor;
@end

@implementation PlayingCardView

#pragma mark - Properties

@synthesize faceCardScaleFactor = _faceCardScaleFactor;

#define DEFAULT_FACE_CARD_SCALE_FACTOR 0.90

- (CGFloat)faceCardScaleFactor
{
    if (!_faceCardScaleFactor) _faceCardScaleFactor = DEFAULT_FACE_CARD_SCALE_FACTOR;
        return _faceCardScaleFactor;
}

- (void)setFaceCardScaleFactor:(CGFloat)faceCardScaleFactor
{
    _faceCardScaleFactor = faceCardScaleFactor;
    [self setNeedsDisplay];
}

Answer 1

您获得的两个错误消息实际上是两个不同但相关的原因的结果.

错误:使用未解析的标识符"self"

self 是指类型的实例,在该类型被视为完全初始化之前不存在,因此不能用于默认属性值.

错误:'Simple.Type'没有名为'someConstant'的成员

类变量和静态变量分别被认为是类和结构的"类型变量".这意味着它们被定义为类型本身的变量,而不是该类型的实例.通过这个定义以及不能self用于默认属性值的事实,很明显这个错误消息是Swift查找名为的类变量的结果someConstant.从Beta 4开始,仍然不支持类变量.

结构的类似代码,考虑了类型变量,编译得很好:

struct Simple {
    static let someConstant = 0.50
    static var someVariable = 1

    let referringConstant = someConstant
    var referringVar = someVariable

    let explicitTypeProperty = Simple.someConstant
}

参考:我对Swift编程语言的"属性"和"初始化器"章节的回忆.