Kotlin函数返回null[重复]

Question

这个函数总是返回null. t是不被分配与令牌值.有人能帮助请？

fun getToken(): String? {
    var t: String? = ""
    FirebaseMessaging.getInstance().token.addOnSuccessListener { token: String? ->
        Log.i(TAG,"Token: ${token.toString()}")
        t = token
    }.addOnFailureListener { e: Exception? ->
        Log.e(TAG,"Couldn't get token", e)
    }
    return t
  }

Answer 1

getToken返回null，因为此函数在调用成功侦听器之前完成getToken不会等待请求完成。它所做的唯一一件事是设置成功和失败侦听器，当请求返回响应时将调用这些侦听器。

有几种方法可以解决这个问题：

• Mark getToken as suspend and use FirebaseMessaging.getInstance().token.await() to wait until the response is received.
• Instead of returning the token, you can pass a callback to getToken which will be invoked when the response comes.

  fun getToken(onTokenReceive: (String?) -> Unit) {
      FirebaseMessaging.getInstance().token.addOnSuccessListener { token: String? ->
          Log.i(TAG,"Token: ${token.toString()}")
          onTokenReceive(token)
      }.addOnFailureListener { e: Exception? ->
          Log.e(TAG,"Couldn't get token", e)
          onTokenReceive(null)
      }
  }
  

  val token = getToken()
  // using the `token` here
  

  getToken { token ->
      // use the `token` here
  }