Android TextView 在 Kotlin 中显示致命错误

Question

我正尝试在 Kotlin 中初始化 TextView，如下所示

val textSelectedFood = findViewById<TextView>(R.id.textSelectedFood) as TextView

我也试过这个

val textSelectedFood: TextView = findViewById<TextView>(R.id.textSelectedFood) as TextView

还有这个

val textSelectedFood: TextView = findViewById(R.id.textSelectedFood) as TextView

还有这个

val textSelectedFood: TextView = findViewById<TextView>(R.id.textSelectedFood)

但仍然出现以下错误。不知道为什么

2021-01-10 23:38:26.640 29636-29636/com.imran.android.dinnerdecider E/d.dinnerdecide: Unknown bits set in runtime_flags: 0x8000
2021-01-10 23:38:27.091 29636-29636/com.imran.android.dinnerdecider E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.imran.android.dinnerdecider, PID: 29636
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.imran.android.dinnerdecider/com.imran.android.dinnerdecider.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3194)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:163)
        at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:174)
        at android.content.Context.obtainStyledAttributes(Context.java:738)
        at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:839)
        at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:806)
        at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:630)
        at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:223)
        at com.imran.android.dinnerdecider.MainActivity.<init>(MainActivity.kt:12)
        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:1243)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3182)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

Answer 1

我从问题中得到提示。

但代码在一年中发生了很大变化。

这就是为什么我在下面编写解决我的问题的当前解决方案代码的原因。

private lateinit var textSelectedFood: TextView // withoud lateinit code will show error

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    textSelectedFood = findViewById(R.id.textSelectedFood)
}

Answer 2

如果你不必“findViewById”有很多更好更安全的选择：

1.synthetic（已弃用）

Call the view directly in your code.

2.view绑定

For more information check this link:https://developer.android.com/topic/libraries/view-binding

Answer 3

这种技术现在已经很老了，只需使用“View binding”。或者你可以只通过它的 id 调用视图，就是这样。你的情况：

textSelectedFood.text = "Example of doing something"

Android TextView 在 Kotlin 中显示致命错误

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