我一直在尝试在一个类中使用常量和变量,该类通过名称引用到类级别的同一个类中的其他常量和变量. AFAIK,截至
Xcode 6 beta 4,Swift仍然没有类变量支持.我想知道的是,当试图引用其他常量(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？
您可以参考一个方法或类之外的常量和变量,但您似乎无法在类级别中引用名称.以下课程显示了您将在Xcode 6 beta 4中显示的错误以及几个变体.
这可以在操场或常规.swift文件中进行测试.
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,其中使用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];
}
解决方法
您收到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,不存在,直到该类型被认为是完全初始化为止,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体的“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.通过这个定义和事实,你不能使用自己的默认属性值,很明显,这个错误信息是Swift寻找名为someConstant的类变量的结果. Beta 4中仍然不支持类变量.
结构体的类似代码,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“初始化程序”章节的回忆.
