Difference between with and run in Kotlin

问题:

Do with and run have the same functionality with only different syntax, or is there any major differences between with and run?

Which is the proper way?

adapter.run {
    notifyDataSetChanged()
    if (activityDetails.isEmpty())
        emptyText.visibility = View.VISIBLE
    else 
       emptyText.visibility = View.GONE
}


with(adapter){
   notifyDataSetChanged()
   if (activityDetails.isEmpty())
       emptyText.visibility = View.VISIBLE
   else 
       emptyText.visibility = View.GONE
}

回答1:

They have only syntactic difference, run is an extension function while with is not. Here are the definitions (in kotlin-sdlib:1.0.3):

public inline fun <T, R> T.run(block: T.() -> R): R = block() // equivalent to this.block()

public inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()

Since run is an extension function, it has one more implicit argument of type T, so the argument types are the same. The bodies of the functions are also effectively the same.

Their performance should also be equivalent since both are inline functions: the resulting bytecode should only contain the inlined block body.

---

The differences in the functions usage are all caused by the fact that run is an extension.

First, run is suitable for calls chaining:

foo.run { bar }.run { baz }

Second, and more important, if a declared variable type has run function with the same signature, it will be called instead of the extension. And run can be shadowed by another extension. This is how extensions are resolved. Example:

class MyClass {
     fun <R> run(blockIgnored: MyClass.() -> R): Nothing = throw RuntimeException()
}

"abcdefg".run { println("x") } // prints "x"
MyClass().run { println("x") } // throws RuntimeException
(MyClass() as Any).run { println("x") } // prints "x"

回答2:

In normal case, they are pretty similar, but for a nullable variable, it's better to use T.run()

E.g. if webview.settings is nullable, below would show the distinct

with(webview.settings) {
    this?.javaScriptEnabled = true
    this?.databaseEnabled = true
}

// similarly
webview.settings?.run {
    javaScriptEnabled = true
    databaseEnabled = true
}