在Kotlin中传递和使用函数作为构造函数参数

Question

How to create a class that takes function as a constructor argument. Then, use this function at some later point in the class.

Answer 1

You can have a property with a function type just like you would with any other type:

class A(val f: () -> Unit) {

    fun foo() {
        f()
    }

}

From here, you can pass that function to the constructor as a method reference:

fun bar() {
    println("this is bar")
}

val a = A(::bar)
a.foo()             // this is bar

Or as a lambda:

val a = A({ println("this is the lambda") })

And you can even do the usual syntactic sugar for lambdas that are the last parameter of a function (although this is getting a little wild):

val a = A { println("this is the lambda") }

Answer 2

A real world example can be observed in SynchronizedLazyImpl , the class backing lazy delegates.

public fun <T> lazy(lock: Any?, initializer: () -> T): Lazy<T> = 
    SynchronizedLazyImpl(initializer, lock)

When we use val x by lazy {...} , the initializer , passed as a lambda, is actually stored as a property in an instance of SynchronizedLazyImpl and called later when the corresponding val x is being accessed for the first time.

Answer 3

If you have more than one constructor declarations you can use this

...

private var listener : (() -> Unit)? = null

constructor(context: Context, listener: (() -> Unit)?) : this(context){
        this.listener = listener
}

constructor(context: Context) : super(context, attrs = null)

...