赋值不是表达式，在这种情况下只允许使用表达式 - Kotlin

Question

bindingSub.btnCoinup.setOnClickListener {
    fbFirestore?.collection("users")?.document(fbAuth?.uid.toString())?.get()?.addOnSuccessListener { document ->
        if (document != null) {
            var autoCoin = document.data?.values.toString().replace("[", "").replace("]", "").toLong()
            bindingSub.tvAutoCoin.setText("Coin: ${autoCoin += 1}")
        } else {
            bindingSub.tvAutoCoin.setText("Coin: 0")
        }
    } ?.addOnFailureListener { exception ->
        ...

bindingSub.tvAutoCoin.setText("Coin: ${autoCoin += 1}")

I tried really hard to fix this error, but I couldn't fix it.

Even if the code is dirty, I would appreciate it if you understand...

Answer 1

You are assigning a new value in this line to the autoCoin variable:

bindingSub.tvAutoCoin.setText("Coin: ${autoCoin += 1}")

String templates only allow expressions. An expression is something that results in a value of some type, even if that type is Unit and can be on the right hand side of an assignment statement (eg val x = 3 assigns the value of the expression 3 to x ). Some languages (like Java or C++) assignments are expressions, but in Kotlin they are not.

The x += 1 operator is a special shorthand syntax for x = x + 1 .