无法在OnCreate中将视图添加到LinearLayout

问题描述

我正在创建一个启动器应用程序，我希望当用户启动该应用程序时，加载他先前添加的小部件。问题是，当我尝试在onCreate函数中加载Widget时，它崩溃，提示它找不到我的视图。

这是我的mainActivity.kt：

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

        AppDatabase.init(this,AppDatabase::class.java,DATABASE_NAME) // Just DB stuff using room 

        widgetSelector = WidgetSelector(this)

        // Fragment initialisation 
        homeScreen = HomeFragment()
        appListScreen = AppListFragment(this,widgetSelector)
        fragmentMgr = supportFragmentManager
        pagerAdapter =
            HomeAdapter(fragmentMgr)
        pagerAdapter.addFragments(homeScreen)
        pagerAdapter.addFragments(appListScreen)
        homeViewPager.adapter = pagerAdapter
        // End of fragment initialisation 

        widgetSelector.loadWidget() // /!\ This is the problematic line 
    }

这是loadWidget函数。每个小部件都已经存储在数据库中，我只是获取它们并尝试显示它们：

fun createWidget(id: Int) {
        val appWidgetInfo: AppWidgetProviderInfo = appWidgetManager.getAppWidgetInfo(id)
        val hostView: AppWidgetHostView =
            appWidgetHost.createView(mainActivity,id,appWidgetInfo)
        hostView.setAppWidget(id,appWidgetInfo)
        mainActivity.findViewById<LinearLayout>(R.id.list_widget).addView(hostView) // /!\ This line is the problem again
}

fun loadWidget() {
        AppDatabase.INSTANCE?.itemDAO()?.getAll()?.forEach {
            createWidget(it.appWidgetId)
        }
}

这是我尝试添加视图的基本片段

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/list_widget"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="@color/colorLMTBlueBlack">

</LinearLayout>

启动我的应用程序时出现此错误：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.letmethink.lmtapp,PID: 28499
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.letmethink.lmtapp/com.letmethink.lmtapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.addView(android.view.View)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3114)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3257)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1948)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7050)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.addView(android.view.View)' on a null object reference
        at com.letmethink.lmtapp.widget.WidgetSelector.createWidget(WidgetSelector.kt:59)
        at com.letmethink.lmtapp.widget.WidgetSelector.loadWidget(WidgetSelector.kt:71)
        at com.letmethink.lmtapp.MainActivity.onCreate(MainActivity.kt:49)
        at android.app.Activity.performCreate(Activity.java:7327)
        at android.app.Activity.performCreate(Activity.java:7318)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3094)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3257) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 

我已经尝试了许多操作，例如将addView移至mainActivity或尝试执行loadWidget函数onPostCreated（）。我非常确定问题出在我正在使用片段的事实上，这就是为什么我尝试在片段的onCreateView中调用该函数，但是它没有用。我稍后会在代码中调用此函数（例如onResult），它可以正常工作。

override fun onActivityResult(
        requestCode: Int,resultCode: Int,data: Intent?
    ) {
        super.onActivityResult(requestCode,resultCode,data)
        widgetSelector.reloadWidget()
        val widgetSelector = WidgetSelector(this)
        if (resultCode == Activity.RESULT_OK) {
            val id = data!!.getIntExtra(AppWidgetManager.EXTRA_APPWIDGET_ID,-1)
            if (requestCode == REQUEST_PICK_APPWIDGET) {
                widgetSelector.configureWidget(id)
            } else if (requestCode == REQUEST_CREATE_APPWIDGET) {
                widgetSelector.createWidget(id)
            }
            AppDatabase.INSTANCE?.itemDAO()?.insert(Widget(id))
            widgetSelector.reloadWidget()
        } else if (resultCode == Activity.RESULT_CANCELED && data != null) {
            val appWidgetId = data.getIntExtra(AppWidgetManager.EXTRA_APPWIDGET_ID,-1)
            if (appWidgetId != -1) {
                widgetSelector.appWidgetHost.deleteAppWidgetId(appWidgetId)
            }
        }
    }

解决方法

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