行为模式-运算符重载和迭代器

方案1：实现Iterator接口

data class Book(val name: String)
class Bookcase(val books: List<Book>) : Iterator<Book> {
    private val iterator: Iterator<Book>

    init {
        this.iterator = books.iterator()
    }

    override fun hasNext(): Boolean = this.iterator.hasNext()
    override fun next(): Book = this.iterator.next()
}

fun main() {
    val bookcase = Bookcase(
        listOf(
            Book("Kotlin Divide into"),
            Book("Thinking in Java")
        )
    )
    while (bookcase.hasNext()){
        println("This book name is ${bookcase.next().name}")
    }
    for(book in bookcase){
        println("This book name is ${book.name}")
    }

}

Bookcase 对象拥有和List<Book>实例相同的迭代器，我们可以直接调用后者迭代器的所有方法，所以用for 更加简洁的方式也可以

方案2：重载Iterator方法

Kotlin 有更好的方案，用operator重载，内置了很多重载运算符。重载iterator

data class Book(val name: String)
class BookCase2(val books: List<Book>) {
    operator fun iterator(): Iterator<Book> = this.books.iterator()
}

瞬间感觉真美好

方案3：通过扩展函数

假设现在Book是引入的一个类，你不能修改它，那么如何用扩展支持迭代

data class Book(val name: String)
class BookCase3(val books: List<Book>){}
operator fun BookCase3.iterator():Iterator<Book> = books.iterator()

如果想要更多的迭代器逻辑控制权，那么可以通过object表达

operator fun BookCase3.iterator():Iterator<Book> = object :Iterator<Book>{
    val iterator = books.iterator()
    override fun hasNext()=iterator.hasNext()
    override fun next()= iterator.next()
}