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 = os
    return 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 html

fun 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 content
                    a(href = "http://jetbrains.com/kotlin") { +"Kotlin" }

                    // mixed content
                    p {
                        +"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 arguments
                    p {
                        +"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: String
        get() = 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/