Swift type conversion
Swift language type conversion determines the type of instance. It can also be used to detect whether an instance type belongs to its parent or child class.
Type conversion in Swift is implemented using the is and as operators, and is used to detect the type of value, and as is used to convert the type.
Type conversion can also be used to check whether a class implements a protocol.
Define a class hierarchy
Type conversion is used to detect whether an instance type belongs to a specific instance type.
You can use it on the hierarchy of classes and sub-classes, check the type of a particular class instance, and transform the type of that class instance into another type in that hierarchy.
Here's an example:
class Subjects { var physics: String init(physics: String) { self.physics = physics } } class Chemistry: Subjects { var equations: String init(physics: String, equations: String) { self.equations = equations super.init(physics: physics) } } class Maths: Subjects { var formulae: String init(physics: String, formulae: String) { self.formulae = formulae super.init(physics: physics) } } let sa = [ Chemistry(physics: "固体物理", equations: "赫兹"), Maths(physics: "流体动力学", formulae: "千兆赫")] let samplechem = Chemistry(physics: "固体物理", equations: "赫兹") print("实例物理学是: \(samplechem.physics)") print("实例方程式: \(samplechem.equations)") let samplemaths = Maths(physics: "流体动力学", formulae: "千兆赫") print("实例物理学是: \(samplemaths.physics)") print("实例公式是: \(samplemaths.formulae)")
The output of the above program execution is:
实例物理学是: 固体物理 实例方程式: 赫兹 实例物理学是: 流体动力学 实例公式是: 千兆赫
Check the type
Type checking uses the is keyword.
Operator is to check whether an instance belongs to a specific subsype. If the instance belongs to that subsype, the type check operator returns true, otherwise false is returned.
class Subjects { var physics: String init(physics: String) { self.physics = physics } } class Chemistry: Subjects { var equations: String init(physics: String, equations: String) { self.equations = equations super.init(physics: physics) } } class Maths: Subjects { var formulae: String init(physics: String, formulae: String) { self.formulae = formulae super.init(physics: physics) } } let sa = [ Chemistry(physics: "固体物理", equations: "赫兹"), Maths(physics: "流体动力学", formulae: "千兆赫"), Chemistry(physics: "热物理学", equations: "分贝"), Maths(physics: "天体物理学", formulae: "兆赫"), Maths(physics: "微分方程", formulae: "余弦级数")] let samplechem = Chemistry(physics: "固体物理", equations: "赫兹") print("实例物理学是: \(samplechem.physics)") print("实例方程式: \(samplechem.equations)") let samplemaths = Maths(physics: "流体动力学", formulae: "千兆赫") print("实例物理学是: \(samplemaths.physics)") print("实例公式是: \(samplemaths.formulae)") var chemCount = 0 var mathsCount = 0 for item in sa { // 如果是一个 Chemistry 类型的实例,返回 true,相反返回 false。 if item is Chemistry { ++chemCount } else if item is Maths { ++mathsCount } } print("化学科目包含 \(chemCount) 个主题,数学包含 \(mathsCount) 个主题")
The output of the above program execution is:
实例物理学是: 固体物理 实例方程式: 赫兹 实例物理学是: 流体动力学 实例公式是: 千兆赫 化学科目包含 2 个主题,数学包含 3 个主题
Transform down
Transform down and convert operators by type (as? or as!)
When you are not sure that the downward transition will succeed, use the conditional form of type conversion (as?). A type conversion in the form of a condition always returns an optional value, and if it is not possible to down, the optional value will be nil.
Use the mandatory form (as!) only if you are sure that the downward transition will succeed. When you try to transition down to an incorrect type, a forced type conversion triggers a runtime error.
class Subjects { var physics: String init(physics: String) { self.physics = physics } } class Chemistry: Subjects { var equations: String init(physics: String, equations: String) { self.equations = equations super.init(physics: physics) } } class Maths: Subjects { var formulae: String init(physics: String, formulae: String) { self.formulae = formulae super.init(physics: physics) } } let sa = [ Chemistry(physics: "固体物理", equations: "赫兹"), Maths(physics: "流体动力学", formulae: "千兆赫"), Chemistry(physics: "热物理学", equations: "分贝"), Maths(physics: "天体物理学", formulae: "兆赫"), Maths(physics: "微分方程", formulae: "余弦级数")] let samplechem = Chemistry(physics: "固体物理", equations: "赫兹") print("实例物理学是: \(samplechem.physics)") print("实例方程式: \(samplechem.equations)") let samplemaths = Maths(physics: "流体动力学", formulae: "千兆赫") print("实例物理学是: \(samplemaths.physics)") print("实例公式是: \(samplemaths.formulae)") var chemCount = 0 var mathsCount = 0 for item in sa { // 类型转换的条件形式 if let show = item as? Chemistry { print("化学主题是: '\(show.physics)', \(show.equations)") // 强制形式 } else if let example = item as? Maths { print("数学主题是: '\(example.physics)', \(example.formulae)") } }
The output of the above program execution is:
实例物理学是: 固体物理 实例方程式: 赫兹 实例物理学是: 流体动力学 实例公式是: 千兆赫 化学主题是: '固体物理', 赫兹 数学主题是: '流体动力学', 千兆赫 化学主题是: '热物理学', 分贝 数学主题是: '天体物理学', 兆赫 数学主题是: '微分方程', 余弦级数
Type conversion for Any and AnyObject
Swift offers two special types of alias for indeterminate types:
-
AnyObject
represent instances of any class type. -
Any
can represent any type, including method types.
Attention:
Use Any andAnyObject
only if you clearlyAny
its behavior and functionality. It's always better to use the explicit types you expect in your code.
Any instance
class Subjects { var physics: String init(physics: String) { self.physics = physics } } class Chemistry: Subjects { var equations: String init(physics: String, equations: String) { self.equations = equations super.init(physics: physics) } } class Maths: Subjects { var formulae: String init(physics: String, formulae: String) { self.formulae = formulae super.init(physics: physics) } } let sa = [ Chemistry(physics: "固体物理", equations: "赫兹"), Maths(physics: "流体动力学", formulae: "千兆赫"), Chemistry(physics: "热物理学", equations: "分贝"), Maths(physics: "天体物理学", formulae: "兆赫"), Maths(physics: "微分方程", formulae: "余弦级数")] let samplechem = Chemistry(physics: "固体物理", equations: "赫兹") print("实例物理学是: \(samplechem.physics)") print("实例方程式: \(samplechem.equations)") let samplemaths = Maths(physics: "流体动力学", formulae: "千兆赫") print("实例物理学是: \(samplemaths.physics)") print("实例公式是: \(samplemaths.formulae)") var chemCount = 0 var mathsCount = 0 for item in sa { // 类型转换的条件形式 if let show = item as? Chemistry { print("化学主题是: '\(show.physics)', \(show.equations)") // 强制形式 } else if let example = item as? Maths { print("数学主题是: '\(example.physics)', \(example.formulae)") } } // 可以存储Any类型的数组 exampleany var exampleany = [Any]() exampleany.append(12) exampleany.append(3.14159) exampleany.append("Any 实例") exampleany.append(Chemistry(physics: "固体物理", equations: "兆赫")) for item2 in exampleany { switch item2 { case let someInt as Int: print("整型值为 \(someInt)") case let someDouble as Double where someDouble > 0: print("Pi 值为 \(someDouble)") case let someString as String: print("\(someString)") case let phy as Chemistry: print("主题 '\(phy.physics)', \(phy.equations)") default: print("None") } }
The output of the above program execution is:
实例物理学是: 固体物理 实例方程式: 赫兹 实例物理学是: 流体动力学 实例公式是: 千兆赫 化学主题是: '固体物理', 赫兹 数学主题是: '流体动力学', 千兆赫 化学主题是: '热物理学', 分贝 数学主题是: '天体物理学', 兆赫 数学主题是: '微分方程', 余弦级数 整型值为 12 Pi 值为 3.14159 Any 实例 主题 '固体物理', 兆赫
AnyObject instance
class Subjects { var physics: String init(physics: String) { self.physics = physics } } class Chemistry: Subjects { var equations: String init(physics: String, equations: String) { self.equations = equations super.init(physics: physics) } } class Maths: Subjects { var formulae: String init(physics: String, formulae: String) { self.formulae = formulae super.init(physics: physics) } } // [AnyObject] 类型的数组 let saprint: [AnyObject] = [ Chemistry(physics: "固体物理", equations: "赫兹"), Maths(physics: "流体动力学", formulae: "千兆赫"), Chemistry(physics: "热物理学", equations: "分贝"), Maths(physics: "天体物理学", formulae: "兆赫"), Maths(physics: "微分方程", formulae: "余弦级数")] let samplechem = Chemistry(physics: "固体物理", equations: "赫兹") print("实例物理学是: \(samplechem.physics)") print("实例方程式: \(samplechem.equations)") let samplemaths = Maths(physics: "流体动力学", formulae: "千兆赫") print("实例物理学是: \(samplemaths.physics)") print("实例公式是: \(samplemaths.formulae)") var chemCount = 0 var mathsCount = 0 for item in saprint { // 类型转换的条件形式 if let show = item as? Chemistry { print("化学主题是: '\(show.physics)', \(show.equations)") // 强制形式 } else if let example = item as? Maths { print("数学主题是: '\(example.physics)', \(example.formulae)") } } var exampleany = [Any]() exampleany.append(12) exampleany.append(3.14159) exampleany.append("Any 实例") exampleany.append(Chemistry(physics: "固体物理", equations: "兆赫")) for item2 in exampleany { switch item2 { case let someInt as Int: print("整型值为 \(someInt)") case let someDouble as Double where someDouble > 0: print("Pi 值为 \(someDouble)") case let someString as String: print("\(someString)") case let phy as Chemistry: print("主题 '\(phy.physics)', \(phy.equations)") default: print("None") } }
The output of the above program execution is:
实例物理学是: 固体物理 实例方程式: 赫兹 实例物理学是: 流体动力学 实例公式是: 千兆赫 化学主题是: '固体物理', 赫兹 数学主题是: '流体动力学', 千兆赫 化学主题是: '热物理学', 分贝 数学主题是: '天体物理学', 兆赫 数学主题是: '微分方程', 余弦级数 整型值为 12 Pi 值为 3.14159 Any 实例 主题 '固体物理', 兆赫
Use the mandatory type conversion operator (as, not as?) in the case of a switch statement to examine and convert to an explicit type.