[Solved] “Assignments are not expressions” error in Kotlin/Android

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postDelayed() takes a Runnable as its first parameter. blank_fields_error.visibility = View.INVISIBLE is not a Runnable. It is an assignment statement.

Since Runnable is an interface defined in Java, and it has a single method, you can pass a Kotlin lambda expression as the first parameter, and the Kotlin compiler will convert that into a Runnable for you (see “SAM Conversions” in the Kotlin documentation).

So, while blank_fields_error.visibility = View.INVISIBLE is an assignment, {blank_fields_error.visibility = View.INVISIBLE} is a lambda expression that happens to perform an assignment. You can pass the lambda expression to postDelayed().

For places where in Java you might use anonymous inner classes, where the interface or class being extended has more than one method, in Kotlin you can create an anonymous object:

someField.addTextChangedListener(object : TextWatcher {
  fun afterTextChanged(s: Editable) {
    TODO()
  }

  fun beforeTextChanged(s: CharSequence, start: Int, count: Int, after: Int) {
    TODO()
  }

  fun onTextChanged(s: CharSequence, start: Int, count: Int, after: Int) {
    TODO()
  }
})

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solved “Assignments are not expressions” error in Kotlin/Android