Changing activities prevents ImageButton setImageR

I have a main activity that changes the image on the ImageButton on every click.

public class MainActivity extends Activity {
    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

        static ImageButton mGetClickTime;
            mGetClickTime.setOnClickListener(new View.OnClickListener() {
            mUpdateBackground();
            }
        }
    }

public static void mUpdateBackground() {
        int[] imageIds = { 
                R.drawable.bg1,
                R.drawable.bg2,
                R.drawable.bg3,
                R.drawable.bg4,
                };
        Random generator = new Random();
        randomImageId = imageIds[generator.nextInt(imageIds.length)];
                mGetClickTime.setImageResource(randomImageId);
        }

This works fine, until the menu button is clicked and another activity is accessed

public class settings extends MainActivity{


@Override
   public void onCreate(Bundle savedInstanceState) {                
   super.onCreate(savedInstanceState);    
   setContentView(R.layout.settings);
   }
}

When the menu or back button is pressed, we return to the main activity. But once this has happened, the ImageButton no longer has the image updated with every click. Everything else still works, including text boxes updating on each click.

What am I doing wrong?

EDIT

Thanks for all your help. At the moment, it seems to continue working after I return from the sub-activity. (Hooray!)

But now I can't get the mUpdateBackground method to work when called from onResume(), or from the other activity. Here's where I'm at:

1 public class MainActivity extends Activity {
2   public static ImageButton mGetClickTime;
3 @Override
4   protected void onResume() {
5        super.onResume();
6       //Get shared preferences
7               mSharedPreferences = getSharedPreferences(PREFERENCES_NAME, MODE_PRIVATE);
8           dp = mSharedPreferences.getInt("DecimalPlaces", 0);
9               length_setting = mSharedPreferences.getInt("MSSelector", 1);
10              backgroundPic = mSharedPreferences.getBoolean("BackgroundPic", true);
11              //mUpdateBackground();  
12          }
12  @Override
13  protected void onCreate(Bundle savedInstanceState) {
14      super.onCreate(savedInstanceState);
15      setContentView(R.layout.activity_main);
16
17      //Get shared preferences
18      mSharedPreferences = getSharedPreferences(PREFERENCES_NAME, MODE_PRIVATE);
19      dp = mSharedPreferences.getInt("DecimalPlaces", 0);
20      length_setting = mSharedPreferences.getInt("MSSelector", 5);
21      mUpdateBackground();
22          mGetClickTime.setOnClickListener(new View.OnClickListener() {
23      mUpdateBackground();
24          }
25     }
26 }
27  public void mUpdateBackground() {
28      if (backgroundPic) {
29          int[] imageIds = { 
30                  R.drawable.bg1,
31                  R.drawable.bg2,
32                  R.drawable.bg3,
33                  R.drawable.bg4,
34
35          };
36          Random generator = new Random();
37          randomImageId = imageIds[generator.nextInt(imageIds.length)];
38          Log.d("1", "backgroundPic: "+randomImageId);
39      }
40      else {
41          randomImageId = R.drawable.bg0;
42          Log.d("1", "backgroundPicCALLED: "+randomImageId);
43      }
44      mGetClickTime = (ImageButton) findViewById(R.id.clicker);
45      mGetClickTime.setImageResource(randomImageId);
46  }

The problem I have with this is that if I uncomment line 11 I get a NullPointerException. Could that be because the mUpdateBackground() method at line 27 isn't static? If I make it static then at line 44 I get an error on findViewById, Cannot make a static reference to the non-static method findViewById(int) from the type Activity. I'm really stumped. I'm obviously not following the logic correctly but I've gone over it a few times and I can't see what it could be.

回答1:

Ha, now I see what is bothering you. Your problem is either onResume or onCreate happens second time and this resets your image to your default value you set in your XML. You can simply log and see which of these occurs. Once you see where your image is returned to default, simply create field in your activity class which will remember your current image id or what you want. Something like this

private int currentImage = R.drawable.firstImage;

public class MainActivity extends Activity{

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

    static ImageButton mGetClickTime;
    mGetClickTime.setImageResource(currentImage);
        mGetClickTime.setOnClickListener(new View.OnClickListener() {
        mUpdateBackground();
        }
    }
   }

public static void mUpdateBackground() {
    int[] imageIds = { 
            R.drawable.bg1,
            R.drawable.bg2,
            R.drawable.bg3,
            R.drawable.bg4,
            };
    Random generator = new Random();
    randomImageId = imageIds[generator.nextInt(imageIds.length)];
            mGetClickTime.setImageResource(randomImageId);
            currentImage = randomImageId;
    }

Hope this helps and enjoy your work.

回答2:

I got the answer from this thread: Android: What's the best hierarchy for this app?

The key was learning the concept of checking if an object is null before calling it.

The code I used was this:

    if (mGetClickTime != null) {
        Log.d("mGetClickTime: ", "is not null");
        mGetClickTime.setImageResource(randomImageId);
    }
    else {
        mGetClickTime = (ImageButton) findViewById(R.id.clicker);
        mGetClickTime.setImageResource(randomImageId);
        Log.d("mUpdateBackground", " was null");
    }