Android，使用按钮在不同图像的 2 个 View 之间交叉淡入淡出

Question

我在使用按钮在 2 个图像之间交叉淡入淡出时遇到问题。该按钮未实现，但问题是我什至无法加载 ImageView 。 Eclipse 不会在 Java 代码中给出错误，但是当我尝试运行该应用程序时，它崩溃了。我在下面包含了 MainActivity.java、LogCat 和 XML 文件。

public class MainActivity extends ActionBarActivity {

    private View imageView1;
    private View imageView2;
    private int mShortAnimationDuration;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
        }

        imageView1 = findViewById(R.id.imageView1);
        imageView2 = findViewById(R.id.imageView2);

        // Initially hide the content view.
        imageView2.setVisibility(View.GONE);

        // Retrieve and cache the system's default "short" animation time.
        mShortAnimationDuration = getResources().getInteger(
        android.R.integer.config_shortAnimTime);
    }

    private void crossfade() {

        // Set the content view to 0% opacity but visible, so that it is visible
        // (but fully transparent) during the animation.
        imageView1.setAlpha(0f);
        imageView1.setVisibility(View.VISIBLE);

        // Animate the content view to 100% opacity, and clear any animation
        // listener set on the view.
        imageView1.animate()
                .alpha(1f)
                .setDuration(mShortAnimationDuration)
                .setListener(null);

        // Animate the loading view to 0% opacity. After the animation ends,
        // set its visibility to GONE as an optimization step (it won't
        // participate in layout passes, etc.)
        imageView2.animate()
                .alpha(0f)
                .setDuration(mShortAnimationDuration)
                .setListener(new AnimatorListenerAdapter() {
                    @Override
                    public void onAnimationEnd(Animator animation) {
                        imageView2.setVisibility(View.GONE);
                    }
                });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            return rootView;
        }
    }

}

我的 xml:

<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="ca.mohawk.you.lab6a.MainActivity$PlaceholderFragment" >

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:src="@drawable/desert" />

    <ImageView
        android:id="@+id/imageView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:src="@drawable/desert" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="42dp"
        android:text="@string/button_click" />

</RelativeLayout>

日志:

04-07 21:51:13.463: D/AndroidRuntime(1099): Shutting down VM
04-07 21:51:13.473: W/dalvikvm(1099): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 21:51:13.483: E/AndroidRuntime(1099): FATAL EXCEPTION: main
04-07 21:51:13.483: E/AndroidRuntime(1099): Process: ca.mohawk.you.lab6a, PID: 1099
04-07 21:51:13.483: E/AndroidRuntime(1099): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.os.Looper.loop(Looper.java:136)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at dalvik.system.NativeStart.main(Native Method)
04-07 21:51:13.483: E/AndroidRuntime(1099): Caused by: java.lang.NullPointerException
04-07 21:51:13.483: E/AndroidRuntime(1099):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 21:51:13.483: E/AndroidRuntime(1099):     ... 11 more
04-07 21:54:14.453: D/AndroidRuntime(1164): Shutting down VM
04-07 21:54:14.453: W/dalvikvm(1164): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 21:54:14.463: E/AndroidRuntime(1164): FATAL EXCEPTION: main
04-07 21:54:14.463: E/AndroidRuntime(1164): Process: ca.mohawk.you.lab6a, PID: 1164
04-07 21:54:14.463: E/AndroidRuntime(1164): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.os.Looper.loop(Looper.java:136)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at dalvik.system.NativeStart.main(Native Method)
04-07 21:54:14.463: E/AndroidRuntime(1164): Caused by: java.lang.NullPointerException
04-07 21:54:14.463: E/AndroidRuntime(1164):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 21:54:14.463: E/AndroidRuntime(1164):     ... 11 more
04-07 21:54:17.973: I/Process(1164): Sending signal. PID: 1164 SIG: 9
04-07 22:01:16.333: D/AndroidRuntime(1184): Shutting down VM
04-07 22:01:16.333: W/dalvikvm(1184): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 22:01:16.353: E/AndroidRuntime(1184): FATAL EXCEPTION: main
04-07 22:01:16.353: E/AndroidRuntime(1184): Process: ca.mohawk.you.lab6a, PID: 1184
04-07 22:01:16.353: E/AndroidRuntime(1184): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.os.Looper.loop(Looper.java:136)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at dalvik.system.NativeStart.main(Native Method)
04-07 22:01:16.353: E/AndroidRuntime(1184): Caused by: java.lang.NullPointerException
04-07 22:01:16.353: E/AndroidRuntime(1184):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 22:01:16.353: E/AndroidRuntime(1184):     ... 11 more
04-07 22:01:59.343: I/Process(1184): Sending signal. PID: 1184 SIG: 9
04-07 22:02:01.843: D/AndroidRuntime(1205): Shutting down VM
04-07 22:02:01.843: W/dalvikvm(1205): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 22:02:01.853: E/AndroidRuntime(1205): FATAL EXCEPTION: main
04-07 22:02:01.853: E/AndroidRuntime(1205): Process: ca.mohawk.you.lab6a, PID: 1205
04-07 22:02:01.853: E/AndroidRuntime(1205): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.os.Looper.loop(Looper.java:136)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at dalvik.system.NativeStart.main(Native Method)
04-07 22:02:01.853: E/AndroidRuntime(1205): Caused by: java.lang.NullPointerException
04-07 22:02:01.853: E/AndroidRuntime(1205):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 22:02:01.853: E/AndroidRuntime(1205):     ... 11 more
04-07 22:02:51.663: D/AndroidRuntime(1251): Shutting down VM
04-07 22:02:51.663: W/dalvikvm(1251): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 22:02:51.673: E/AndroidRuntime(1251): FATAL EXCEPTION: main
04-07 22:02:51.673: E/AndroidRuntime(1251): Process: ca.mohawk.you.lab6a, PID: 1251
04-07 22:02:51.673: E/AndroidRuntime(1251): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.os.Looper.loop(Looper.java:136)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at dalvik.system.NativeStart.main(Native Method)
04-07 22:02:51.673: E/AndroidRuntime(1251): Caused by: java.lang.NullPointerException
04-07 22:02:51.673: E/AndroidRuntime(1251):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 22:02:51.673: E/AndroidRuntime(1251):     ... 11 more

Answer 1

看来您发布的 xml 来自文件 fragment_main.xml 文件。如果是这种情况，那么您遇到的问题是 findViewById() 正在 Activity 的 View 层次结构中搜索您的 View ，之前 fragment 的 ViewGroup 已附加到 Activity 。如果您不想/不需要使用 fragment ，则只需将发布的 xml 移动到 activity_main.xml，并删除相关的 fragment 代码块。如果您想使用 fragment ，请将您的 View 初始化和功能移动到 PlaceholderFragment。