android - opendrawer() 在 drawerlayout 中抛出空指针错误

Question

我是 Android 新手。
通过这个简单的程序，我想使用按钮 onclick() 打开一个抽屉。但是这给出了 null 异常错误。我通过谷歌搜索尝试了解决方案，但不幸的是没有解决它。
这是onCreate()方法

         protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            navListener();
        }

navListener()

           public void navListener(){


                final DrawerLayout mDrawerLayout = (DrawerLayout) findViewById(R.id.drawerlayout);
                ImageView nav = (ImageView) findViewById(R.id.navbarimagebutton);
                nav.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {

                        mDrawerLayout.openDrawer(GravityCompat.START);
                    }
                });



            }

activity_main.xml

       <?xml version="1.0" encoding="utf-8"?>
      <RelativeLayout   
        xmlns:android="http://schemas.android.com/apk/res/android"    
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:paddingBottom="@dimen/activity_vertical_margin"
        android:paddingLeft="@dimen/activity_horizontal_margin"
        android:paddingRight="@dimen/activity_horizontal_margin"
        android:paddingTop="@dimen/activity_vertical_margin"
        tools:context=".MainActivity">

        <include layout="@layout/app_bar" />


    </RelativeLayout>

DrawerLayout

    <?xml version="1.0" encoding="utf-8"?>
    <android.support.v4.widget.DrawerLayout
        android:id="@+id/drawerlayout"
        android:layout_height="match_parent"
        android:layout_width="120dp"
        xmlns:android="http://schemas.android.com/apk/res/android">

        <FrameLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">

            <ListView
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_gravity="start"></ListView>



        </FrameLayout>
    </android.support.v4.widget.DrawerLayout>

app_bar

        <?xml version="1.0" encoding="utf-8"?>
       <android.support.v7.widget.Toolbar
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/app_bar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="@color/red">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="horizontal">

            <ImageView
                android:id="@+id/navbarimagebutton"
                android:layout_width="0dp"
                android:layout_weight=".25"
                android:layout_height="fill_parent"
                android:src="@drawable/menu"
                />


            <View
                android:id="@+id/homepagefirstview"

                android:layout_width="10dp"
                android:layout_height="fill_parent"></View>

            <ImageView
                android:layout_weight="1"
                android:layout_width="0dp"
                android:src="@drawable/hometitle"
                android:layout_height="fill_parent" />
            <View
                android:layout_width="10dp"
                android:layout_height="fill_parent"/>
            <ImageView
                android:layout_weight=".25"
                android:layout_width="0dp"
                android:src="@drawable/search"
                android:layout_height="fill_parent" />

        </LinearLayout>


    </android.support.v7.widget.Toolbar>

Answer 1

你的问题是你要求你的 Activity setContentView(R.layout.activity_main);

并且这个文件不包含你的DrawerLayout

你需要做的:

 <?xml version="1.0" encoding="utf-8"?>
  <RelativeLayout   
    xmlns:android="http://schemas.android.com/apk/res/android"    
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity">
    <!-- add an id here since your using RelativeLayout -->
    <include layout="@layout/app_bar" android:id="@+id/app_bar" />

    <!-- include the drawer this way, probably need to add layout_below="@+id/app_bar" or whatever the id is for your app bar -->
    <!-- Uncomment this <include layout="@layout/drawer_layout"/> -->

    <!-- Or simply copy the drawer layout xml and add layout_below="@+id/app_bar" -->
    <android.support.v4.widget.DrawerLayout
        android:id="@+id/drawerlayout"
        android:layout_below="@+id/app_bar"
        android:layout_height="match_parent"
        android:layout_width="match_parent">

        <!-- main content area -->
        <FrameLayout 
            android:id="@+id/content_area"
            android:layout_height="match_parent"
            android:layout_width="match_parent"/>

        <!-- Slide out drawer section -->
        <FrameLayout
            android:layout_gravity="start"
            android:layout_width="320dp"
            android:layout_height="match_parent">

            <ListView
                android:layout_width="match_parent"
                android:layout_height="match_parent"/>

        </FrameLayout>
    </android.support.v4.widget.DrawerLayout>
</RelativeLayout>

您遇到了 NullPointerException，因为您的膨胀 View 不包含您正在引用的这个 DrawerLayout