当我触摸屏幕时，我的应用程序崩溃-OnTouch方法失败

Question

我创建了一个应用程序，其中有一个相机预览。我正在尝试创建一个触摸事件，这样当我触摸预览时，就会发生一些事情（比如说，会出现祝酒词）。问题是，当我触摸预览时，2秒钟后应用程序崩溃。我有两个班：

相机活动：

public class CameraActivity extends Activity {
    private static final String TAG = "CameraDemo";
    Preview preview;
    public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.main);
         preview = new Preview(this);
        ((FrameLayout) findViewById(R.id.preview)).addView(preview);
        ((FrameLayout) findViewById(R.id.preview)).setOnTouchListener(preview);
        Log.d(TAG, "Camera Activity Created.");
    }
}

和预览：

class Preview extends SurfaceView implements SurfaceHolder.Callback, OnTouchListener {
private static final String TAG = "Preview";
SurfaceHolder mHolder;
public Camera camera;
Preview(Context context) {
    super(context);
    // Install a SurfaceHolder.Callback so we get notified when the
    // underlying surface is created and destroyed.
    mHolder = getHolder();
    mHolder.addCallback(this);
    mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
}
// Called once the holder is ready
public void surfaceCreated(SurfaceHolder holder) {
    camera = Camera.open();
    try {
        camera.setDisplayOrientation(90);
        camera.setPreviewDisplay(holder);
        camera.setPreviewCallback(new PreviewCallback() {
            // Called for each frame previewed
            public void onPreviewFrame(byte[] data, Camera camera) {
                Log.d(TAG, "onPreviewFrame called at: " +  System.currentTimeMillis());
                Preview.this.invalidate();
            }
        });
    } catch (IOException e) {
      e.printStackTrace();
    }

}

// Called when the holder is destroyed
public void surfaceDestroyed(SurfaceHolder holder) {
    camera.stopPreview();
    camera.release();
    camera = null;
}
// Called when holder has changed
public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
    camera.startPreview();
}
public boolean onTouch(View v, MotionEvent event) {
            Toast.makeText(findViewById(R.id.preview).getContext(), "BEFORE SHUTTER", Toast.LENGTH_LONG).show();
    return false;
}
 }

有人能帮忙告诉我我做错了什么吗？

以下是LogCat（崩溃出现的部分）：

03-08 12:11:48.335: D/AndroidRuntime(1167): Shutting down VM
03-08 12:11:48.335: W/dalvikvm(1167): threadid=1: thread exiting with uncaught    exception (group=0x40015578)
03-08 12:11:48.343: E/AndroidRuntime(1167): FATAL EXCEPTION: main
03-08 12:11:48.343: E/AndroidRuntime(1167): java.lang.NullPointerException
03-08 12:11:48.343: E/AndroidRuntime(1167):     at com.marakana.Preview.onTouch(Preview.java:80)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at     android.view.View.dispatchTouchEvent(View.java:3934)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:903)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at  android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:869)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:869)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:869)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at  android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:869)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1733)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1151)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at android.app.Activity.dispatchTouchEvent(Activity.java:2096)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1717)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at android.view.ViewRoot.deliverPointerEvent(ViewRoot.java:2215)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1886)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at android.os.Handler.dispatchMessage(Handler.java:99)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at android.os.Looper.loop(Looper.java:123)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at android.app.ActivityThread.main(ActivityThread.java:3687)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at java.lang.reflect.Method.invokeNative(Native Method)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at java.lang.reflect.Method.invoke(Method.java:507)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:842)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
03-08 12:11:48.343: E/AndroidRuntime(1167):     at dalvik.system.NativeStart.main(Native Method)
03-08 12:11:48.378: D/dalvikvm(1167): GC_FOR_MALLOC freed 943K, 59% free 2633K/6407K, external 1596K/2108K, paused 15ms
03-08 12:11:48.531: D/dalvikvm(1167): GC_CONCURRENT freed 21K, 52% free 3084K/6407K, external 1596K/2108K, paused 2ms+26ms
03-08 12:11:56.109: I/Process(1167): Sending signal. PID: 1167 SIG: 9

感谢

Answer 1

上下文有问题

Toast.makeText(findViewById(R.id.preview).getContext(), "BEFORE SHUTTER", Toast.LENGTH_LONG).show(); 

你不能用这种方式获取上下文。在构造函数中，您传入上下文：

Preview(Context context) {

我会创建一个上下文的私有实例

private Context ctx;

然后在构造函数中添加

this.ctx = context;

然后将toast消息更改为

Toast.makeText(ctx, "BEFORE SHUTTER", Toast.LENGTH_LONG).show();

这也会让你在课堂上的大多数其他地方都有背景，从而让生活变得更轻松。