答案 0 :(得分:8)
Swift 1.2的更新
@objc
关键字的需求。实际上,以下简单示例现在无法使用@objc
关键字:
protocol Ap {
func hello()
}
class A: Ap {
func hello() {
println("hello, world")
}
}
var a = A()
if (a as AnyObject) is Ap {
a.hello()
} else {
println("nope")
}
// hello, world
此外,现在联系只是这样:
protocol-conformance-1-2:
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1213.0.0)
/usr/lib/libobjc.A.dylib (compatibility version 1.0.0, current version 228.0.0)
@rpath/libswiftCore.dylib (compatibility version 0.0.0, current version 0.0.0)
ORIGINAL:
让我们看一个例子。请注意,我还使用了额外的(varName as AnyObject)
调用,否则编译器会抱怨'is' test is always true
- 因为它确切地知道编译时类型是什么。
import Foundation
protocol Swifty {
func s()
// protocol-conformance.swift:5:2: error: 'optional' can only be applied to members of an @objc protocol
// optional var a: Int { get }
// ^
/*
optional var a: Int { get }
*/
}
protocol SwiftyClass: class {
func scl()
// protocol-conformance.swift:13:2: error: 'optional' can only be applied to members of an @objc protocol
// optional var a: Int { get }
// ^
/*
optional var a: Int { get }
*/
}
@objc protocol SwiftyConformance {
func scon()
optional var a: Int { get }
}
class SwiftyOnly: Swifty {
func s() {
println("s")
}
}
class SwiftyClassOnly: SwiftyClass {
func scl() {
println("scl")
}
}
class SwiftyConformanceOnly: SwiftyConformance {
func scon() {
println("scon")
}
}
class SwiftyConformanceWithOptional: SwiftyConformance {
func scon() {
println("sconwo")
}
var a: Int {
get { return 1; }
}
}
println("swifty")
var swifty = SwiftyOnly()
//protocol-conformance.swift:49:26: error: cannot downcast from 'AnyObject' to non-@objc protocol type 'Swifty'
//if (swifty as AnyObject) is Swifty {
// ~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~
/*
if (swifty as AnyObject) is Swifty {
println("swifty is Swifty")
}
*/
// protocol-conformance.swift:47:34: error: cannot downcast from 'AnyObject' to non-@objc protocol type 'Swifty'
// if let s = (swifty as AnyObject) as? Swifty {
// ~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~
/*
if let s = (swifty as AnyObject) as? Swifty {
s.s()
}
*/
println("")
println("swiftyClass")
var swiftyClass = SwiftyClassOnly()
//protocol-conformance.swift:61:31: error: cannot downcast from 'AnyObject' to non-@objc protocol type 'SwiftyClass'
/*
if (swiftyClass as AnyObject) is SwiftyClass {
println("swiftyClass is SwiftyClass")
}
*/
//protocol-conformance.swift:80:39: error: cannot downcast from 'AnyObject' to non-@objc protocol type 'SwiftyClass'
//if let s = (swiftyClass as AnyObject) as? SwiftyClass {
// ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~
/*
if let s = (swiftyClass as AnyObject) as? SwiftyClass {
s.scl()
}
*/
println("")
println("swiftyConformanceOnly")
var swiftyConformanceOnly = SwiftyConformanceOnly()
if (swiftyConformanceOnly as AnyObject) is SwiftyConformance {
println("swiftyConformanceOnly is SwiftyConformance")
}
if let s = (swiftyConformanceOnly as AnyObject) as? SwiftyConformance {
s.scon()
if let a = s.a? {
println("a: \(a)")
}
}
println("")
println("swiftyConformanceWithOptional")
var swiftyConformanceWithOptional = SwiftyConformanceWithOptional()
if (swiftyConformanceWithOptional as AnyObject) is SwiftyConformance {
println("swiftyConformanceWithOptional is SwiftyConformance")
}
if let s = (swiftyConformanceWithOptional as AnyObject) as? SwiftyConformance {
s.scon()
if let a = s.a? {
println("a: \(a)")
}
}
println("")
...和(在不取消破解代码测试用例的情况下),输出为:
swifty
swiftyClass
swiftyConformanceOnly
swiftyConformanceOnly is SwiftyConformance
scon
swiftyConformanceWithOptional
swiftyConformanceWithOptional is SwiftyConformance
sconwo
a: 1
所以,简单的答案就像文档所说的那样:protocol conformance testing(和optionals)需要@objc
。
在Swift中,objc只是一个声明属性,通常表示编译器的提示或修改代码的生成方式。
但是更长的答案就引出了一个问题:"但为什么语言或运行时以这种方式编写?",而且这很难解决;我的猜测是@objc
属性将生成真正的Objective-C对象/协议引用,并且一致性测试只是在运行时使用它来实现。
您可以在上面的示例中对/ *和* / one之间的代码进行注释,并查看编译器抱怨的时间和位置。
更新:编译器和链接器更新
如果我们编译以上内容:xcrun swiftc -sdk $(xcrun --show-sdk-path --sdk macosx) protocol-conformance.swift
并检查它与otool -L protocol-conformance
的链接,我们会看到
协议一致性:
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1213.0.0)
/usr/lib/libobjc.A.dylib (compatibility version 1.0.0, current version 228.0.0)
@rpath/libswiftCore.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libswiftCoreGraphics.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libswiftDarwin.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libswiftDispatch.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libswiftFoundation.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libswiftObjectiveC.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libswiftSecurity.dylib (compatibility version 0.0.0, current version 0.0.0)
...所以我认为做这些一致性测试更公平,你需要与Objective-C运行时进行交互,但我不一定会说你需要互动使用Objective-C(对我来说,这意味着你需要写一些objc代码)。
查看一个非常简单的协议使用程序:
protocol Ap {
func hello()
}
class A: Ap {
func hello() {
println("hello, world")
}
}
var a = A()
a.hello()
//$ otool -L hello-world
//hello-world:
// /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1213.0.0)
// /usr/lib/libobjc.A.dylib (compatibility version 1.0.0, current version 228.0.0)
// @rpath/libswiftCore.dylib (compatibility version 0.0.0, current version 0.0.0)
...但是如果只是尝试实现一个没有其他更改的@objc协议:
@objc protocol Ap {
func hello()
}
class A: Ap {
func hello() {
println("hello, world")
}
}
var a = A()
a.hello()
//$ xcrun swiftc -sdk $(xcrun --show-sdk-path --sdk macosx) hello-world.swift
//hello-world.swift:1:2: error: @objc attribute used without importing module 'Foundation'
//@objc protocol Ap {
// ^~~~
...然后如果我们导入基金会:
import Foundation
@objc protocol Ap {
func hello()
}
class A: Ap {
func hello() {
println("hello, world")
}
}
var a = A()
a.hello()
//$ otool -L hello-world
//hello-world:
// /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1213.0.0)
// /usr/lib/libobjc.A.dylib (compatibility version 1.0.0, current version 228.0.0)
// @rpath/libswiftCore.dylib (compatibility version 0.0.0, current version 0.0.0)
// @rpath/libswiftCoreGraphics.dylib (compatibility version 0.0.0, current version 0.0.0)
// @rpath/libswiftDarwin.dylib (compatibility version 0.0.0, current version 0.0.0)
// @rpath/libswiftDispatch.dylib (compatibility version 0.0.0, current version 0.0.0)
// @rpath/libswiftFoundation.dylib (compatibility version 0.0.0, current version 0.0.0)
// @rpath/libswiftObjectiveC.dylib (compatibility version 0.0.0, current version 0.0.0)
// @rpath/libswiftSecurity.dylib (compatibility version 0.0.0, current version 0.0.0)
我甚至会说Swift标准库和运行时绝对使用Objective-C运行时,并期望访问defacto核心Objective-C框架,例如Foundation for core features。
答案 1 :(得分:1)
不是。
来自The Swift Programming Language:
您可以使用类型转换中描述的is和as运算符来检查协议一致性,并转换为特定协议。检查和转换为协议的语法与检查和转换为类型的语法完全相同...
protocol PersonBasedView {
var person: Person? {get set}
}
class EmployeeView : UIView, PersonBasedView {
var person: Person?
}
// Elsewhere
var view = ...
if view is PersonBasedView { ... }
// or
if let personView = view as? PersonBasedView { ... }