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我试图能够移动“长颈鹿”图像视图。我从 strings.xml 调用资源的方式似乎有问题，或者它在 giraffe.setOnTouchListener(this); 上不断返回 NullPointerException。这是代码：

public class MainGameActivity extends Activity implements OnTouchListener {

float x,y,dx,dy;
ImageView giraffe;


@Override
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.requestWindowFeature(Window.FEATURE_NO_TITLE);
        ImageView giraffe = (ImageView)findViewById(R.id.giraffe);
        setContentView(R.layout.activity_main_game);
        giraffe.setOnTouchListener(this);
}


public boolean onTouch(View v, MotionEvent me){
    switch(me.getAction()){
        case MotionEvent.ACTION_DOWN:
        {
            x = me.getX();
            y = me.getY();
            dx = x - giraffe.getX();
            dy = y - giraffe.getY();
        }
        break;
        case MotionEvent.ACTION_MOVE :
        {
            giraffe.setX(me.getX()-dx);
            giraffe.setY(me.getY()-dy);
        }
        break;
    }
    return true;
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.activity_main_game, menu);
    return true;
}

}

<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"
tools:context=".MainGameActivity" >

<ImageView
    android:id="@+id/giraffe"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true"
    android:layout_marginBottom="58dp"
    android:contentDescription="@string/giraffe"
    android:src="@drawable/giraffeicon" />


<?xml version="1.0" encoding="utf-8"?>


<string name="app_name">Falling Objects</string>
<string name="menu_settings">Settings</string>
<string name="splash_text">Start dodging in:</string>
<string name="splash_number_countdown" > </string>
<string name="title_activity_main_game" > </string>
<string name="version">Version 1.0</string>
<string name="giraffe">Giraffe</string>
<string name="s_and_b_productions">Created by: S&amp;B Productions</string>
<drawable name="giraffe">#F00</drawable>


    07-05 19:41:52.006: E/AndroidRuntime(1826): FATAL EXCEPTION: main
07-05 19:41:52.006: E/AndroidRuntime(1826): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.SandBProductions.fallingobjects/com.SandBProductions.fallingobjects.MainGameActivity}: java.lang.NullPointerException
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.os.Handler.dispatchMessage(Handler.java:99)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.os.Looper.loop(Looper.java:137)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.app.ActivityThread.main(ActivityThread.java:5039)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at java.lang.reflect.Method.invokeNative(Native Method)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at java.lang.reflect.Method.invoke(Method.java:511)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at dalvik.system.NativeStart.main(Native Method)
07-05 19:41:52.006: E/AndroidRuntime(1826): Caused by: java.lang.NullPointerException
07-05 19:41:52.006: E/AndroidRuntime(1826):     at com.SandBProductions.fallingobjects.MainGameActivity.onCreate(MainGameActivity.java:24)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.app.Activity.performCreate(Activity.java:5104)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
07-05 19:41:52.006: E/AndroidRuntime(1826):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
4

3 回答 3

6

需要在findViewById之前setContentView，否则activity不知道去哪里找。

于 2013-07-05T19:52:19.953 回答
1

这段代码是错误的：

ImageView giraffe = (ImageView)findViewById(R.id.giraffe);
setContentView(R.layout.activity_main_game);

您必须先调用SetContentView()搜索任何布局元素。这部分应该是

setContentView(R.layout.activity_main_game);
ImageView giraffe = (ImageView)findViewById(R.id.giraffe);
于 2013-07-05T19:52:49.447 回答
0

尝试：

setContentView(R.layout.activity_main_game);
ImageView giraffe = (ImageView)findViewById(R.id.giraffe);
        giraffe.setOnTouchListener(new OnTouchListener() {
    public boolean onTouch(View v, MotionEvent me){
    switch(me.getAction()){
        case MotionEvent.ACTION_DOWN:
        {
            x = me.getX();
            y = me.getY();
            dx = x - giraffe.getX();
            dy = y - giraffe.getY();
        }
        break;
        case MotionEvent.ACTION_MOVE :
        {
            giraffe.setX(me.getX()-dx);
            giraffe.setY(me.getY()-dy);
        }
        break;
    }
    return true;
}
});

您需要在调用 Imageview 之前设置内容视图

于 2013-07-05T19:55:43.933 回答