android - 从 arraylist 中的 firebase 读取数据并显示在 kotlin android 的 Recycler 视图中
问题描述
数据库 点击这里查看数据库图片
我的数据已上传到 firebase 我想读取我的数据并添加到 arraylist数据集,然后在回收器视图中显示,但我的应用程序崩溃了,我认为这是由于传递了空长度数组,我是 kotlin 的新手,但我认为下面方法在 java 帮助中是可行的,以找出错误或建议我的方法。
主要活动
abstract class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
var dataSet = ArrayList<User>()
var database = FirebaseDatabase.getInstance()
var usersAdapter= updateAdapter(dataSet)
var recyclerView = findViewById<RecyclerView>(R.id.recyclerViewId)
recyclerView.adapter=usersAdapter
database.getReference().child("Users").addValueEventListener(object: ValueEventListener {
override fun onDataChange(snapshot: DataSnapshot) {
Log.d("MAIN_ACTIVITY","onDataChange called")
dataSet.clear()
for(snapshot1: DataSnapshot in snapshot.children){
var user: User? = snapshot1.getValue<User>()
dataSet.add(user!!)
}
usersAdapter.notifyDataSetChanged()
}
override fun onCancelled(error: DatabaseError) {
Log.d("MAIN_ACTIVITY","onCancelled called")
}
})
}
更新适配器
类 updateAdapter(var dataset: ArrayList): RecyclerView.Adapter<updateAdapter.userViewHolder>() {
class userViewHolder(view: View): RecyclerView.ViewHolder(view) {
var username = view.findViewById<TextView>(R.id.username)
var lastMsg = view.findViewById<TextView>(R.id.lastMsg)
var msgTime = view.findViewById<TextView>(R.id.msgTime)
var profile = view.findViewById<ImageView>(R.id.profile)
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): userViewHolder {
Log.d("MAIN_ACTIVITY","onCreate View HOlder")
var view = LayoutInflater.from(parent.context).inflate(R.layout.row_conversation,parent,false)
return userViewHolder(view)
}
override fun onBindViewHolder(holder: userViewHolder, position: Int) {
Log.d("MAIN_ACTIVITY","onBLind ViewHolder")
var user = dataset[position]
holder.username.text = user.Uname
}
override fun getItemCount(): Int {
return dataset.size
}
}
用户
data class User(
var Uid :String?="",
var Uname:String?="",
var Unumber :String?= "",
var Uimage :String?="")
错误
E/Perf: Fail to get file list com.example.ranka
E/Perf: getFolderSize() : Exception_1 = java.lang.NullPointerException: Attempt to get length of null array
Fail to get file list com.example.ranka
getFolderSize() : Exception_1 = java.lang.NullPointerException: Attempt to get length of null array
错误
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.ranka, PID: 27503
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.ranka/com.example.ranka.Activities.MainActivity}: java.lang.InstantiationException: java.lang.Class<com.example.ranka.Activities.MainActivity> cannot be instantiated
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3404)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3636)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:140)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:100)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2222)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:228)
at android.app.ActivityThread.main(ActivityThread.java:7782)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:981)
Caused by: java.lang.InstantiationException: java.lang.Class<com.example.ranka.Activities.MainActivity> cannot be instantiated
at java.lang.Class.newInstance(Native Method)
at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:45)
at android.app.Instrumentation.newActivity(Instrumentation.java:1251)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3392)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3636)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:140)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:100)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2222)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:228)
at android.app.ActivityThread.main(ActivityThread.java:7782)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:981)
I/Process:发送信号。PID:27503 SIG:9
解决方案
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