Kotlin version 1.2.50
I have been following some examples on this tutorial on youtube https://www.youtube.com/watch?v=gPH9XnvpoXE. And there are a few things that I have understood, but there is still some confusion. I have left the comments in the code below where I am not sure what is happening.
fun main(args: Array<String>) {
val javaClient = createClient {
firstName = "joe"
lastName = "bloggs"
twitter {
handle = "@joebloggs"
}
}
println(javaClient.toConsole)
}
/* Are we passing in a lambda and receiver. What will the receiver be */
private fun JavaClientBuilder.twitter(suppler: JavaTwitterBuilder.() -> Unit) {
/* We call JavaTwitterBuilder().apply(..) Will apply return the newly created object? Not sure why we have to pass the suppler in the apply */
twitter = JavaTwitterBuilder().apply(suppler).build()
}
/* Are we passing in a lambda and receiver that return nothing */
private fun createClient(suppler: JavaClientBuilder.() -> Unit): JavaClient {
val javaClientBuilder = JavaClientBuilder()
/* confusion: Not sure about this, as we are calling suppler. Just wondering is the suppler the the JavaClientBuilder that was called in the above javaClient {} lambda */
javaClientBuilder.suppler()
return javaClientBuilder.build()
}
/* I understand this, an extension function that formats and returns the string from the JavaClient object it was called on */
private val JavaClient.toConsole: String
get() =
"Created client is: ${twitter.handle} ${company.name}"
/* Are we passing in a lambda and receiver. What will the receiver be */
private fun JavaClientBuilder.twitter(suppler: JavaTwitterBuilder.() -> Unit)
We've indeed got a receiver in this function, and it's the instance of JavaClientBuilder that this function will be invoked on.
/* We call JavaTwitterBuilder().apply(..) Will apply return the newly created object? Not sure why we have to pass the suppler in the apply */
twitter = JavaTwitterBuilder().apply(suppler).build()
To understand how apply() works, take a look at its source code (simplified version):
public inline fun <T> T.apply(block: T.() -> Unit): T {
block()
return this
}
It's an extension function declared on a receiver of type T and returning an instance of T, which takes in a block - another extension function with receiver of type T returning Unit. It's usually used to replace the Builder pattern - apply custom initialization logic to an object. In your case, suppler is a block that contains initialization logic for an instance of JavaTwitterBuilder. The function code creates the instance and uses apply() with the logic in suppler to initialize that instance.
/* Are we passing in a lambda and receiver that return nothing */
private fun createClient(suppler: JavaClientBuilder.() -> Unit): JavaClient
In this case, createClient() doesn't have a receiver, it's a top-level function.
/* confusion: Not sure about this, as we are calling suppler. Just wondering is the suppler the the JavaClientBuilder that was called in the above javaClient {} lambda */
javaClientBuilder.suppler()
suppler is a lambda with JavaClientBuilder being the receiver type, which allows us to call it on the newly-created instance of JavaClientBuilder.
/* I understand this, an extension function that formats and returns the string from the JavaClient object it was called on */
private val JavaClient.toConsole: String get() = "Created client is: ${twitter.handle} ${company.name}"
Right! Just a small correction, it's an extension property. Properties can have custom getters and setters. This property defines a custom getter, so indeed whenever this property is accessed it will produce a String with the format described by the getter code.