第十一章 Kotlin实现DSL

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DSL

DSL 即 domain-specific languages，领域特定语言。和一般的编程语言不同，领域特定语言只能用于特定的领域中并且表现形式有限。领域特定语言最大的功能就是可以让语言本身更容易阅读，方便开发者和领域专家进行交流。

实现 DSL

Java 中 DSL 的最简单实现方式就是构造器模式，而在 Kotlin 过去的版本中可以省略 .，所以可以写成更易读的代码，但是现在的版本已经不支持了。

构造器模式

Machine machine = new Machine.Builder()
.setCore(8)
.setArch("64 bits")
.setOs("Linux")
.build();
DSL 方式

定义必要的类和方法

data class Cpu(val core: Int, val arch: String)

class Machine {
var cpu: Cpu? = null
var os: String? = null

fun having(cores: Int, arch: String): Machine {cpu = Cpu(cores, arch)return this
}fun os(os: String): Machine {this.os = osreturn this
}override fun toString(): String {return "Machine{cpu=$cpu, os='$os'"
}

}
构建对象

val m1 = Machine().having(8, "64 bits").os("linux")
val m2 = Machine().having(4, "32 bits").os("Windows")
可以看到使用 DSL 后代码更加易读。

使用闭包构建 DSL

Kotlin 像 Groovy 一样也能通过闭包构建 DSL，语法看起来很像 Groovy。

例

定义必要的类和方法

class EmailSpec {
fun from(from: String) = println("From: $from")
fun to(to: String) = println("To: $to")
fun subject(subject: String) = println("Subject: $subject")
fun body(init: BodySpec.() -> Unit): BodySpec {
val body = BodySpec()
body.init()
return body
}
}

class BodySpec {
fun p(p: String) = println("P: $p")
}

fun email(init: EmailSpec.() -> Unit): EmailSpec {
val email = EmailSpec()
email.init()
return email
}
调用 DSL 语句

email {
from ("dsl-guru@mycompany.com")
to ("john.doe@waitaminute.com")
subject ("The pope has resigned!")
body {
p ("Really, the pope has resigned!")
}
}

val data = mapOf(1 to "one", 2 to "two")createHTML().table {//遍历数据for ((num, string) in data) {//创建 HTML 标签的函数tr {td { +"$num" } td { +string }}}
}

/*** This is an example of a Type-Safe Groovy-style Builder** Builders are good for declaratively describing data in your code.* In this example we show how to describe an HTML page in Kotlin.** See this page for details:* http://kotlinlang.org/docs/reference/type-safe-builders.html*/
package htmlfun main(args: Array<String>) {val result =html {head {title { +"XML encoding with Kotlin" }}body {h1 { +"XML encoding with Kotlin" }p { +"this format can be used as an alternative markup to XML" }// an element with attributes and text contenta(href = "http://jetbrains.com/kotlin") { +"Kotlin" }// mixed contentp {+"This is some"b { +"mixed" }+"text. For more see the"a(href = "http://jetbrains.com/kotlin") { +"Kotlin" }+"project"}p { +"some text" }// content generated from command-line argumentsp {+"Command line arguments were:"ul {for (arg in args)li { +arg }}}}}println(result)
}interface Element {fun render(builder: StringBuilder, indent: String)
}class TextElement(val text: String) : Element {override fun render(builder: StringBuilder, indent: String) {builder.append("$indent$text\n")}
}abstract class Tag(val name: String) : Element {val children = arrayListOf<Element>()val attributes = hashMapOf<String, String>()protected fun <T : Element> initTag(tag: T, init: T.() -> Unit): T {tag.init()children.add(tag)return tag}override fun render(builder: StringBuilder, indent: String) {builder.append("$indent<$name${renderAttributes()}>\n")for (c in children) {c.render(builder, indent + "  ")}builder.append("$indent</$name>\n")}private fun renderAttributes(): String? {val builder = StringBuilder()for (a in attributes.keys) {builder.append(" $a=\"${attributes[a]}\"")}return builder.toString()}override fun toString(): String {val builder = StringBuilder()render(builder, "")return builder.toString()}
}abstract class TagWithText(name: String) : Tag(name) {operator fun String.unaryPlus() {children.add(TextElement(this))}
}class HTML() : TagWithText("html") {fun head(init: Head.() -> Unit) = initTag(Head(), init)fun body(init: Body.() -> Unit) = initTag(Body(), init)
}class Head() : TagWithText("head") {fun title(init: Title.() -> Unit) = initTag(Title(), init)
}class Title() : TagWithText("title")abstract class BodyTag(name: String) : TagWithText(name) {fun b(init: B.() -> Unit) = initTag(B(), init)fun p(init: P.() -> Unit) = initTag(P(), init)fun h1(init: H1.() -> Unit) = initTag(H1(), init)fun ul(init: UL.() -> Unit) = initTag(UL(), init)fun a(href: String, init: A.() -> Unit) {val a = initTag(A(), init)a.href = href}
}class Body() : BodyTag("body")
class UL() : BodyTag("ul") {fun li(init: LI.() -> Unit) = initTag(LI(), init)
}class B() : BodyTag("b")
class LI() : BodyTag("li")
class P() : BodyTag("p")
class H1() : BodyTag("h1")class A() : BodyTag("a") {public var href: Stringget() = attributes["href"]!!set(value) {attributes["href"] = value}
}fun html(init: HTML.() -> Unit): HTML {val html = HTML()html.init()return html
}

https://www.kotliner.cn/2017/05/15/2017-5-11-KotlinDSL2/