Builder模式在一些复杂实体类构建，或者封装的统一多样化控件中使用方便。

示例一：实体对象的构建User(Java版本)

    public class User {
    
        public String phone;
        public String area;
    
        public User() {
            this(new Builder());
        }
    
        public User(Builder builder) {
            this.phone = builder.phone;
            this.area = builder.area;
        }
    
        public static final class Builder{
            public String phone;
            public String area;
    
            //函数式编程，不用每个都setXX属性
            public Builder setPhone(String phone) {
                this.phone = phone;
                return this;
            }
    
            public Builder setArea(String area) {
                this.area = area;
                return this;
            }
    
            public Builder() {
                //默认初始化方法
                phone = "";
                area = "";
            }
    
            Builder(User user) {
                //回写builder属性
                phone = user.phone;
                area = user.area;
            }
    
            public User build() {
                //创建User对象
                return new User(this);
            }
    }
}

示例一：实体对象的构建User(Kotlin版本)

    class User @JvmOverloads constructor(builder: Builder = Builder()) {
        var phone: String
        var area: String
    
        class Builder {
            var phone: String
            var area: String
    
            //函数式编程，不用每个都setXX属性
            fun setPhone(phone: String): Builder {
                this.phone = phone
                return this
            }
    
            fun setArea(area: String): Builder {
                this.area = area
                return this
            }
    
            constructor() {
                //默认初始化方法
                phone = ""
                area = ""
            }
    
            internal constructor(user: User) {
                //回写builder属性
                phone = user.phone
                area = user.area
            }
    
            fun build(): User {
                //创建User对象
                return User(this)
            }
        }
    
        init {
            phone = builder.phone
            area = builder.area
        }
}

示例一：实体对象的构建

    /**
     * 类型安全的构造器
     */
    class Car(
        val model: String? ,
        val year: Int
    ){
        companion object {
            /**
             * 带接收者的函数类型,这意味着我们需要向函数传递一个Builder类型的实例
             */
            inline fun build(block: Builder.() -> Unit) = Builder().apply(block).build()
        }
    
        class Builder{
            var model: String? = null
            var year: Int = -1
    
            fun build() = Car(model ,year)
        }
}

使用

val  car = Car.build {
        model = "名字"
        year = 2017
    }

示例二：通用多样化控件的构建

    class Dialog private constructor(
        val title: String,
        val text: String?,
        val onAccept: (() -> Unit)?
    ) {
        class Builder(val title: String) {
            var text: String? = null
            var onAccept: (() -> Unit)? = null
            fun setText(text: String?): Builder {
                this.text = text
                return this
            }
            fun setOnAccept(onAccept: (() -> Unit)?): Builder {
                this.onAccept = onAccept
                return this
            }
            fun build() = Dialog(title, text, onAccept)
        }
    }

使用

    val dialog1 =Dialog.Builder("Some title")
        .setText("Great dialog")
        .setOnAccept { Log.i("tag" ,"I was clicked") }
        .build()
    val dialog2 = Dialog.Builder("Another dialog")
        .setText("I have no buttons")
        .build()
    val dialog3 = Dialog.Builder("Dialog with just a title").build()

感谢参考文章 Kotlin设计模式-Builder模式