如何声明我的LinearLayout元素？

Question

在我的IDE中，LinearLayout-tag'被标记为红色，错误消息显示为："Element LinearLayout must be declared"。我的目录设置如下：

https://imgur.com/a/deneVxN

activity_main.xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">


<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/nav_first_fragment"
            android:icon="@drawable/ic_1"
            android:title="First"
            tools:ignore="HardcodedText" />
        <item
            android:id="@+id/nav_second_fragment"
            android:icon="@drawable/ic_2"
            android:title="Second"
            tools:ignore="HardcodedText" />
        <item
            android:id="@+id/nav_third_fragment"
            android:icon="@drawable/ic_3"
            android:title="Third"
            tools:ignore="HardcodedText" />
    </group>
</menu>

</LinearLayout>

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout     xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">



<ImageButton
    android:id="@+id/twitchBtn"
    android:layout_width="250dp"
    android:layout_height="250dp"
    android:layout_marginStart="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginTop="124dp"
    android:layout_marginEnd="8dp"
    android:layout_marginRight="8dp"
    android:adjustViewBounds="true"
    android:cropToPadding="true"
    android:scaleType="centerInside"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.496"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:srcCompat="@drawable/twitch" />

<ImageButton
    android:id="@+id/youtubeBtn"
    android:layout_width="253dp"
    android:layout_height="249dp"
    android:layout_marginStart="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginTop="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginRight="8dp"
    android:layout_marginBottom="8dp"
    android:adjustViewBounds="true"
    android:cropToPadding="true"
    android:scaleType="centerInside"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/twitchBtn"
    app:layout_constraintVertical_bias="0.336"
    app:srcCompat="@drawable/youtube" />


<TextView
    android:id="@+id/textPlattform"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginStart="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginTop="36dp"
    android:layout_marginEnd="8dp"
    android:layout_marginRight="8dp"
    android:text="Choose platform"
    android:textAppearance="@style/TextAppearance.AppCompat"
    android:textSize="38sp"
    app:fontFamily="sans-serif"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<!-- This DrawerLayout has two children at the root  -->
<android.support.v4.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <!-- The navigation drawer that comes from the left -->
    <!-- Note that `android:layout_gravity` needs to be set to 'start' -->
    <android.support.design.widget.NavigationView
        android:id="@+id/nvView"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:layout_weight="1"
        android:background="@android:color/white"
        app:menu="@menu/drawer_view.xml"/>

    <!-- This LinearLayout represents the contents of the screen  -->
    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <!-- The ActionBar displayed at the top -->
        <include
            layout="@layout/toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />

        <!-- The main content view where fragments are loaded -->
        <FrameLayout
            android:id="@+id/flContent"
            app:layout_behavior="@string/appbar_scrolling_view_behavior"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />
    </FrameLayout>


   </android.support.v4.widget.DrawerLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

我想要实现一个汉堡包菜单，我正在遵循这个教程：https://github.com/codepath/android_guides/wiki/Fragment-Navigation-Drawer但是如果我现在运行该程序我得到一个NullPointerExeptionbut我认为这是因为drawer_view.xml无法正常工作。

Answer 1

是因为抽屉view.xml的定义是错误的

drawer_view.xml是

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/nav_first_fragment"
            android:icon="@drawable/ic_1"
            android:title="First"
            tools:ignore="HardcodedText" />
        <item
            android:id="@+id/nav_second_fragment"
            android:icon="@drawable/ic_2"
            android:title="Second"
            tools:ignore="HardcodedText" />
        <item
            android:id="@+id/nav_third_fragment"
            android:icon="@drawable/ic_3"
            android:title="Third"
            tools:ignore="HardcodedText" />
    </group>
</menu>

在activity_main.xml中，菜单不包含扩展名.xml