如何在Kotlin中实现生成器模式?

嗨，我是Kotlin世界的新手。我喜欢我目前所看到的，并开始考虑将我们在应用程序中使用的一些库从Java转换为Kotlin。

这些库充满了带有setter、getter和Builder类的pojo。现在我已经在谷歌上找到了在Kotlin中实现Builders的最佳方法，但没有成功。

第二次更新:问题是如何写一个建设者设计模式的一个简单的pojo与一些参数在Kotlin?下面的代码是我尝试编写java代码，然后使用eclipse-kotlin-plugin转换为Kotlin。

class Car private constructor(builder:Car.Builder) {
    var model:String? = null
    var year:Int = 0
    init {
        this.model = builder.model
        this.year = builder.year
    }
    companion object Builder {
        var model:String? = null
        private set

        var year:Int = 0
        private set

        fun model(model:String):Builder {
            this.model = model
            return this
        }
        fun year(year:Int):Builder {
            this.year = year
            return this
        }
        fun build():Car {
            val car = Car(this)
            return car
        }
    }
}

当前回答

首先，在大多数情况下，您不需要在Kotlin中使用构建器，因为我们有默认参数和命名参数。这使你能够写作

class Car(val model: String? = null, val year: Int = 0)

像这样使用它:

val car = Car(model = "X")

如果你绝对想要使用构建器，你可以这样做:

将Builder作为伴生对象没有意义，因为对象是单例对象。相反，将其声明为一个嵌套类(在Kotlin中默认是静态的)。

将属性移动到构造函数中，这样对象也可以以常规方式实例化(如果不应该，则将构造函数设置为私有)，并使用接受构造函数并将其委托给主构造函数的辅助构造函数。代码如下所示:

class Car( //add private constructor if necessary
        val model: String?,
        val year: Int
) {

    private constructor(builder: Builder) : this(builder.model, builder.year)

    class Builder {
        var model: String? = null
            private set

        var year: Int = 0
            private set

        fun model(model: String) = apply { this.model = model }

        fun year(year: Int) = apply { this.year = year }

        fun build() = Car(this)
    }
}

用法:val car = car . builder ().model("X").build()

这段代码可以通过使用构建器DSL进一步缩短:

class Car (
        val model: String?,
        val year: Int
) {

    private constructor(builder: Builder) : this(builder.model, builder.year)

    companion object {
        inline fun build(block: Builder.() -> Unit) = Builder().apply(block).build()
    }

    class Builder {
        var model: String? = null
        var year: Int = 0

        fun build() = Car(this)
    }
}

用法:val car = car。建立{模型= "X"}

如果某些值是必需的，并且没有默认值，你需要将它们放在构建器的构造函数中，也需要放在我们刚刚定义的构建方法中:

class Car (
        val model: String?,
        val year: Int,
        val required: String
) {

    private constructor(builder: Builder) : this(builder.model, builder.year, builder.required)

    companion object {
        inline fun build(required: String, block: Builder.() -> Unit) = Builder(required).apply(block).build()
    }

    class Builder(
            val required: String
    ) {
        var model: String? = null
        var year: Int = 0

        fun build() = Car(this)
    }
}

用法:val car = car。build(required = "requiredValue") {model = "X"}

2016-03-22 09:48:07

其他回答

我看到过许多将额外乐趣作为构建者的例子。我个人喜欢这种方法。节省编写构建程序的精力。

package android.zeroarst.lab.koltinlab

import kotlin.properties.Delegates

class Lab {
    companion object {
        @JvmStatic fun main(args: Array<String>) {

            val roy = Person {
                name = "Roy"
                age = 33
                height = 173
                single = true
                car {
                    brand = "Tesla"
                    model = "Model X"
                    year = 2017
                }
                car {
                    brand = "Tesla"
                    model = "Model S"
                    year = 2018
                }
            }

            println(roy)
        }

        class Person() {
            constructor(init: Person.() -> Unit) : this() {
                this.init()
            }

            var name: String by Delegates.notNull()
            var age: Int by Delegates.notNull()
            var height: Int by Delegates.notNull()
            var single: Boolean by Delegates.notNull()
            val cars: MutableList<Car> by lazy { arrayListOf<Car>() }

            override fun toString(): String {
                return "name=$name, age=$age, " +
                        "height=$height, " +
                        "single=${when (single) {
                            true -> "looking for a girl friend T___T"
                            false -> "Happy!!"
                        }}\nCars: $cars"
            }
        }

        class Car() {

            var brand: String by Delegates.notNull()
            var model: String by Delegates.notNull()
            var year: Int by Delegates.notNull()

            override fun toString(): String {
                return "(brand=$brand, model=$model, year=$year)"
            }
        }

        fun Person.car(init: Car.() -> Unit): Unit {
            cars.add(Car().apply(init))
        }

    }
}

我还没有找到一种方法，可以强制在DSL中初始化一些字段，比如显示错误而不是抛出异常。如果有人知道就告诉我。

2017-06-06 02:03:02

class Foo private constructor(@DrawableRes requiredImageRes: Int, optionalTitle: String?) {

    @DrawableRes
    @get:DrawableRes
    val requiredImageRes: Int

    val optionalTitle: String?

    init {
        this.requiredImageRes = requiredImageRes
        this.requiredImageRes = optionalTitle
    }

    class Builder {

        @DrawableRes
        private var requiredImageRes: Int = -1

        private var optionalTitle: String? = null

        fun requiredImageRes(@DrawableRes imageRes: Int): Builder {
            this.intent = intent
            return this
        } 

        fun optionalTitle(title: String): Builder {
            this.optionalTitle = title
            return this
        }

        fun build(): Foo {
            if(requiredImageRes == -1) {
                throw IllegalStateException("No image res provided")
            }
            return Foo(this.requiredImageRes, this.optionalTitle)
        }

    }

}

2018-05-25 21:59:16

因为我使用Jackson库从JSON解析对象，我需要一个空构造函数，我不能有可选字段。而且所有字段都必须是可变的。然后我可以使用这个漂亮的语法，它做的事情与生成器模式相同:

val car = Car().apply{ model = "Ford"; year = 2000 }

2016-06-29 11:41:37

首先，在大多数情况下，您不需要在Kotlin中使用构建器，因为我们有默认参数和命名参数。这使你能够写作

class Car(val model: String? = null, val year: Int = 0)

像这样使用它:

val car = Car(model = "X")

如果你绝对想要使用构建器，你可以这样做:

将Builder作为伴生对象没有意义，因为对象是单例对象。相反，将其声明为一个嵌套类(在Kotlin中默认是静态的)。

class Car( //add private constructor if necessary
        val model: String?,
        val year: Int
) {

    private constructor(builder: Builder) : this(builder.model, builder.year)

    class Builder {
        var model: String? = null
            private set

        var year: Int = 0
            private set

        fun model(model: String) = apply { this.model = model }

        fun year(year: Int) = apply { this.year = year }

        fun build() = Car(this)
    }
}

用法:val car = car . builder ().model("X").build()

这段代码可以通过使用构建器DSL进一步缩短:

class Car (
        val model: String?,
        val year: Int
) {

    private constructor(builder: Builder) : this(builder.model, builder.year)

    companion object {
        inline fun build(block: Builder.() -> Unit) = Builder().apply(block).build()
    }

    class Builder {
        var model: String? = null
        var year: Int = 0

        fun build() = Car(this)
    }
}

用法:val car = car。建立{模型= "X"}

如果某些值是必需的，并且没有默认值，你需要将它们放在构建器的构造函数中，也需要放在我们刚刚定义的构建方法中:

class Car (
        val model: String?,
        val year: Int,
        val required: String
) {

    private constructor(builder: Builder) : this(builder.model, builder.year, builder.required)

    companion object {
        inline fun build(required: String, block: Builder.() -> Unit) = Builder(required).apply(block).build()
    }

    class Builder(
            val required: String
    ) {
        var model: String? = null
        var year: Int = 0

        fun build() = Car(this)
    }
}

用法:val car = car。build(required = "requiredValue") {model = "X"}

2016-03-22 09:48:07

一种方法是做以下事情:

class Car(
  val model: String?,
  val color: String?,
  val type: String?) {

    data class Builder(
      var model: String? = null,
      var color: String? = null,
      var type: String? = null) {

        fun model(model: String) = apply { this.model = model }
        fun color(color: String) = apply { this.color = color }
        fun type(type: String) = apply { this.type = type }
        fun build() = Car(model, color, type)
    }
}

使用示例:

val car = Car.Builder()
  .model("Ford Focus")
  .color("Black")
  .type("Type")
  .build()

2019-04-05 11:09:11

如何在Kotlin中实现生成器模式?

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