Question

我有此函数尝试使用以下firebase addchildevent侦听器调用android活动中的方法，但出现“ NullPointer”错误。我需要实现此代码的帮助，以便可以在其他kotlin类中访问该方法，并在适当的上下文中调用它。

// ==UserScript==
// @name     USTVGO, sort "Recent Posts"
// @match    *://ustvgo.tv/*
// @noframes
// @grant    none
// ==/UserScript==
/* global jQuery, $ */
/* eslint-disable no-multi-spaces, curly */
'use strict';

var $ = jQuery;  //  The page loads jQuery (and we are in grant none mode), but doesn't set `$`.
var recentPostsLst  = $("#recent-posts-3 > ul");
var itemsToSort     = recentPostsLst.find ("li");

itemsToSort.sort (sortByLinkTextAscending).appendTo (recentPostsLst);

function sortByLinkTextAscending (nodeA, nodeB) {
     var valA_Text  = $(nodeA).find ("a").text ().trim ();
     var valB_Text  = $(nodeB).find ("a").text ().trim ();

     return valA_Text.localeCompare (valB_Text, 'en', {sensitivity: 'base'} );
}

这是我尝试从kotlin类调用的android活动方法：

companion object {
     private fun checkAdmin(userId: String) {

            isAdmin = false

            val ref: DatabaseReference = firebaseDatabase!!.reference
                .child("administrators")
                .child(userId)

            childEventListener = ref.addChildEventListener(object : ChildEventListener {
                override fun onCancelled(p0: DatabaseError) {
                    Log.d("Cancelled", "FirebaseUtil cancel")
                }

                override fun onChildMoved(p0: DataSnapshot, p1: String?) {
                    TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
                }

                override fun onChildChanged(p0: DataSnapshot, p1: String?) {
                    TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
                }

                override fun onChildAdded(p0: DataSnapshot, p1: String?) {
                    isAdmin = true
                    val caller = ListActivity()
                    caller.showMenu()

                }

                override fun onChildRemoved(p0: DataSnapshot) {
                    TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
                }
            })
        }

        fun detachListener(callerActivity: Activity?) {
            firebaseAuth!!.removeAuthStateListener(object : AuthStateListener {
                override fun onAuthStateChanged(p0: FirebaseAuth) {
                    TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
                }
            })
        }
 }

错误：

fun showMenu() {
        this@ListActivity.invalidateOptionsMenu()
    }

Answer 1

我能够使用以下方法解决该问题：

步骤：

修改checkAdmin函数以接受两个参数，一个像以前一样为string，第二个为ListActivity类型的活动：

private fun checkAdmin(userId: String, activity: ListActivity) {

    isAdmin = false

    val ref: DatabaseReference = firebaseDatabase!!.reference
        .child("administrators")
        .child(userId)

    childEventListener = ref.addChildEventListener(object : ChildEventListener {
        override fun onCancelled(p0: DatabaseError) {
            Log.d("Cancelled", "FirebaseUtil cancel")
        }

        override fun onChildMoved(p0: DataSnapshot, p1: String?) {
            TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
        }

        override fun onChildChanged(p0: DataSnapshot, p1: String?) {
            TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
        }

        override fun onChildAdded(p0: DataSnapshot, p1: String?) {
            isAdmin = true
            activity.showMenu(activity)

        }

        override fun onChildRemoved(p0: DataSnapshot) {
            TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
        }
    })
}

调用此函数以传入两个参数，一个为字符串，另一个为活动，并将第二个强制转换为ListActivity类型：
```
checkAdmin(userId, callerActivity as ListActivity)
```

在ListActivity类中，如下设置showMenu函数：

fun showMenu(activity: ListActivity) {

   activity.invalidateOptionsMenu()
}

通过上述设置，我可以访问ListActivity的showManu函数。

NullPointerException：从另一个类调用方法时

1 个答案: