为 android 应用程序制作动画背景不起作用

Question

尝试在 null object 参考上调用虚拟方法 'android.graphics.drawable.Drawable android.widget.RelativeLayout.getBackground()'

我正在创建一个 android 应用程序并希望为其设置动画背景。 不幸的是，我在这里收到了这个错误，我不知道为什么会这样。

这是我拥有的 XML：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    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:id="@+id/myLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/my_bg_animate"
    tools:context=".AccessControlActivity">

    <LinearLayout
         ..../>

    <TextView
        .... />


    <ImageView
         .... />

</RelativeLayout>

这是 java 文件/类：

    public class AccessControlActivity extends AppCompatActivity {

        ConstraintLayout myLayout;


        NfcAdapter nfcAdapter;
        Button btnPositive, btnNegative;
        Dialog epicDialog;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            RelativeLayout relativeLayout = findViewById(R.id.myLayout);
            AnimationDrawable animationDrawable = (AnimationDrawable) relativeLayout.getBackground();
            animationDrawable.setEnterFadeDuration(4500);
            animationDrawable.setExitFadeDuration(4500);
            animationDrawable.start();




            }
}

这就是我尝试运行程序时看到的错误：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.nfc.netvision/com.nfc.netvision.AccessControlActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.graphics.drawable.Drawable android.widget.RelativeLayout.getBackground()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        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.graphics.drawable.Drawable android.widget.RelativeLayout.getBackground()' on a null object reference
        at com.nfc.netvision.AccessControlActivity.onCreate(AccessControlActivity.java:37)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        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) 

先感谢您！ 期待一些答案！

Answer 1

您尚未在 onCreate 方法上定义要在其中膨胀视图的布局，请在 super.. 之后编写下一行：

setcontentview(R.layout.yourLayoutname);

Answer 2

super.onCreate(savedInstanceState);
RelativeLayout relativeLayout = findViewById(R.id.myLayout);

在 onCreate 你没有setContentView并直接执行findViewById因为你的RelativeLayout是 null 并且它的运行方法会导致异常。