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143 lines
3.7 KiB
Markdown
143 lines
3.7 KiB
Markdown
# Swift 模式匹配
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## 模式匹配的底层实现
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为了去除其他语句对汇编研究造成的干扰，所以 case 和 default 里面都很简单，聚焦于研究 case 的模式匹配实现。测试代码如下：
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```swift
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var age = 28
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switch age {
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case 0...30:
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break
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default:
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break
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}
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```
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在 `case 0...30:` 处加断点可以看到汇编，然后看到可疑方法 `Swift.RangeExpression.~= infix(τ_0_0, τ_0_0.Bound) -> Swift.Bool`
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/SwiftPatternMatchExplore.png" style="zoom:25%">
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LLDB 输入 `si` 进去窥探下
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/SwiftPatternMatchExplore2.png" style="zoom:25%">
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看样子，函数还没到底，看到地址 `0x00007ff81a86f530` 很大很大，猜测应该是一个系统动态库方法地址，继续跟进去研究 `si`
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可以看到内部还有函数调用
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/SwiftPatternMatchExplore3.png" style="zoom:25%">
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继续跟进 `si`
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/SwiftPatternMatchExplore4.png" style="zoom:25%">
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看上去是在做继续 `clasedRange` 区间符合的判断，继续跟进 `si`，里面确实是在判断是否命中区间的判断。不一一研究了，这次目的是判断 switch...case pattern 的实现。
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/SwiftPatternMatchExplore5.png" style="zoom:25%">
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结论：switch case pattern 模式匹配，系统底层实现是依赖
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## 模式匹配的应用
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自定义模式匹配。根据上面通过汇编进行分析，我们知道
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- `static func ~=(pattern: case 后面的值, value: switch 后面的值)` pattern 代表的是 case 后面的值，value 代表 switch 后面的值。
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- 当 case 有不同类型的时候，如果编译报错，则需要重写 `static func ~=(pattern: ,value: )` 方法，调整 pattern 的数据类型，数据类型应该和 case 后的数据类型一致（可以是函数等）
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```swift
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struct Student {
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var score = 0
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var name = ""
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/// Student 和 Int 模式匹配的方法
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/// - Parameters:
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/// - pattern: case 后面的内容
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/// - value: switch 后面的内容
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/// - Returns: 是否命中
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static func ~= (pattern: Int, value: Student) -> Bool {
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value.score >= pattern
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}
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static func ~= (pattern: Range<Int>, value: Student) -> Bool {
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pattern.contains(value.score)
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}
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static func ~= (pattern: ClosedRange<Int>, value: Student) -> Bool {
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pattern.contains(value.score)
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}
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static func ~= (pattern: String, value: Student) -> Bool {
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Int(pattern) == value.score
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}
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}
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var student: Student = Student(score: 55, name: "杭城小刘")
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switch student {
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case 100:
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print(">=100")
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case 90:
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print(">=90")
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case 80..<90:
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print("[80, 90)")
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case 60...79:
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print("[60, 79]")
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case "55":
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print("斯国一")
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default:
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print("just so so")
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}
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// console
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斯国一
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```
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修改 `var student: Student = Student(score: 78, name: "杭城小刘")` 则输出 `[60, 79]`
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```swift
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extension String {
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static func ~=(pattern: (String) -> Bool ,value: String) -> Bool {
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pattern(value)
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}
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}
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func hasPrefix(_ prefix: String) -> (String) -> Bool {
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{$0.hasPrefix(prefix)}
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}
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func hasSuffix(_ prefix: String) -> (String) -> Bool {
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{$0.hasSuffix(prefix)}
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}
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var name = "城小刘"
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switch name {
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case hasPrefix("杭"):
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print("有杭哦")
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case hasSuffix("刘"):
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print("有刘哦")
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default:
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print("just a name")
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}
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// console
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有刘哦
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```
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##
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