在ViewPager中使用片段

Question

I'm new to Android, this is my first project, I've spent a while trying to solve this now and have hit a bit of a dead end. I'm wanting to open a fragment using ViewPager, and then access the fragment from the Activity it was opened from. I have tried creating the fragment in the Activity rather than the ViewPager but that throws up a different error.

public class TorchActivity extends SherlockFragmentActivity {

private static TorchFragment torchFrag;

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    ViewPager pager = (ViewPager) findViewById(R.id.pager);
    pager.setAdapter(new ViewPagerAdapter(getSupportFragmentManager()));
}   

public static class ViewPagerAdapter extends FragmentPagerAdapter {

    public ViewPagerAdapter(FragmentManager mgr) {
        super(mgr);
    }

    @Override
    public int getCount() {
        return (2);
    }

    @Override
    public Fragment getItem(int position) {
        if(position == 0){
            torchFrag = new TorchFragment();
            return torchFrag;
        }else{
            ColourPickerFragment colourPickerFrag = new ColourPickerFragment();
            return colourPickerFrag;
        }
    }
}
}

main.xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/pager"
android:layout_width="match_parent"
android:layout_height="match_parent" >
</android.support.v4.view.ViewPager>

When I come to access the fragment later on in the same Activity I get a NullPointerException.

@Override
public void lightSensorCheck(boolean isThereALightSensor) {
    torchFrag.setIsThereALightSensor(isThereALightSensor);
}

In the fragment:

public void setIsThereALightSensor(boolean isThereALightSensor) {
    this.isThereALightSensor = isThereALightSensor;
}

It works fine if I take the ViewPager code out and run it, any help is appreciated.

    02-17 16:01:30.132: E/AndroidRuntime(32006): FATAL EXCEPTION: main
02-17 16:01:30.132: E/AndroidRuntime(32006): java.lang.RuntimeException: Unable to start activity ComponentInfo{uk.co.bimrose.android.survivaltorch/uk.co.bimrose.android.survivaltorch.TorchActivity}: java.lang.NullPointerException
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2517)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2574)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.app.ActivityThread.access$600(ActivityThread.java:162)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1413)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.os.Handler.dispatchMessage(Handler.java:99)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.os.Looper.loop(Looper.java:158)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.app.ActivityThread.main(ActivityThread.java:5789)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at java.lang.reflect.Method.invokeNative(Native Method)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at java.lang.reflect.Method.invoke(Method.java:525)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1027)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:843)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at dalvik.system.NativeStart.main(Native Method)
02-17 16:01:30.132: E/AndroidRuntime(32006): Caused by: java.lang.NullPointerException
02-17 16:01:30.132: E/AndroidRuntime(32006):    at uk.co.bimrose.android.survivaltorch.TorchActivity.lightSensorCheck(TorchActivity.java:199)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at uk.co.bimrose.android.survivaltorch.LightFragment.updateLightSensorStatus(LightFragment.java:180)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at uk.co.bimrose.android.survivaltorch.LightFragment.sensorCheck(LightFragment.java:134)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at uk.co.bimrose.android.survivaltorch.LightFragment.onAttach(LightFragment.java:65)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:883)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1460)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:556)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at uk.co.bimrose.android.survivaltorch.TorchActivity.onStart(TorchActivity.java:109)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1239)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.app.Activity.performStart(Activity.java:5205)
02-17 16:01:30.132: E/AndroidRuntime(32006):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2490)
02-17 16:01:30.132: E/AndroidRuntime(32006):    ... 11 more

Here is the code for the fragment:

public class TorchFragment extends SherlockFragment implements OnClickListener {

private Button buttonOn;
private Button buttonSos;
private Button buttonOff;

private TextView luxStatic;
private TextView luxMessage;
private TextView timerStatic;
private TextView timerMessage;
private TextView batteryStatic;
private TextView batteryMessage;
private TextView shutdownMessage;
private TextView shutdownMessageStatic;
private TextView timesUsed;

private ImageView img;
private TextView heading;

private SharedPreferences prefs;
private int lightSensitivity;
private boolean timerOn;
private int batteryPct;

private boolean isThereALightSensor;

public static int serviceCount;

private AlertResetListener alertResetListener;
private ServiceListener sListener;
private SetDataListener sDListener;
private ClearShutdownMessageListener cSMListener;
private ColourSchemeListener cSListener;

private int[] colourScheme = new int[4];
private LinearLayout dataCompassLayout;

int x = 0;

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);

    // Makes sure that the container activity has implemented the callback interface
    try {
        alertResetListener = (AlertResetListener) activity;
    } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString() + " must implement AlertReset");
    }

    // start / stop service
    try {
        sListener = (ServiceListener) activity;
    } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString() + " must implement ServiceListener");
    }

    // Clear the dataFragment
    try {
        sDListener = (SetDataListener) activity;
    } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString() + " must implement SetDataListener");
    }

    // Clear the dataFragment shutdown message
    try {
        cSMListener = (ClearShutdownMessageListener) activity;
    } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString() + " must implement ClearShutdownMessageListener");
    }

    // Get the colourscheme
    try {
        cSListener = (ColourSchemeListener) activity;
    } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString() + " must implement ColourSchemeListener");
    }

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState) {
    View result = inflater.inflate(R.layout.torchfrag, parent, false);

    buttonOn = (Button) result.findViewById(R.id.button_on);
    buttonSos = (Button) result.findViewById(R.id.button_sos);
    buttonOff = (Button) result.findViewById(R.id.button_off);

    buttonOn.setText("ON");
    buttonOff.setText("OFF");
    buttonSos.setText("SOS");

    luxStatic = (TextView) result.findViewById(R.id.luxstatic);
    luxMessage = (TextView) result.findViewById(R.id.luxmessage);
    timerStatic = (TextView) result.findViewById(R.id.timerstatic);
    timerMessage = (TextView) result.findViewById(R.id.timermessage);
    batteryStatic = (TextView) result.findViewById(R.id.batterystatic);
    batteryMessage = (TextView) result.findViewById(R.id.batterymessage);
    shutdownMessage = (TextView) result.findViewById(R.id.shutdownmessage);
    shutdownMessageStatic = (TextView) result.findViewById(R.id.shutdownmessagestatic);
    timesUsed = (TextView) result.findViewById(R.id.timesused);

    timesUsed.setText(Integer.toString(TorchActivity.timesUsed));

    //Compass
    img = (ImageView) result.findViewById(R.id.compass);
    heading = (TextView) result.findViewById(R.id.heading);
    dataCompassLayout = (LinearLayout) result.findViewById(R.id.datacompasslayout);

    setColourScheme();

    buttonOn.setOnClickListener(this);
    buttonSos.setOnClickListener(this);
    buttonOff.setOnClickListener(this);

    return (result);
}

public void onResume() {
    setColourScheme();
    clearShutdownMessage();
    getPreferences();
    clearText();
    setStaticMessage();
    super.onResume();
}

@Override
public void onClick(View view) {
    alertResetListener.alertReset();
    TorchActivity.keepRunning = false;
    cSMListener.clearShutdownMessage();
    switch (view.getId()) {
    case R.id.button_on:
        serviceCount++;
        sListener.startService("on");
        break;
    case R.id.button_sos:
        serviceCount++;
        sListener.startService("sosPreset");
        break;
    case R.id.button_off:
        sListener.reset();
        sDListener.setData();
        break;
    default:
        throw new RuntimeException("Unknown button ID");
    }
}

private void setColourScheme(){
    //Calls the method from the activity which gets the colour scheme from the ColourSchemeFragment
    colourScheme = cSListener.getColourScheme();
    buttonOn.setBackgroundColor(getResources().getColor(colourScheme[0]));
    buttonOff.setBackgroundColor(getResources().getColor(colourScheme[1]));
    buttonSos.setBackgroundColor(getResources().getColor(colourScheme[2]));
    dataCompassLayout.setBackgroundColor(getResources().getColor(colourScheme[3]));
}

public void setSosTextColour(String s){
    int sosColour = 0;
    if(s.equals("white")){
        sosColour = R.color.button_text_sos;
    }else{
        sosColour = R.color.button_text_onoff;
    }
    try {
        XmlResourceParser parser = getResources().getXml(sosColour);
        ColorStateList colors = ColorStateList.createFromXml(getResources(), parser);
        buttonSos.setTextColor(colors);
    } catch (Exception e) {
        // handle exceptions
    }
}

// Called from TorchActivity whenever some new data is sent from one of the sensors
public void setMessage(int msg, String from) {
    if (from.equals("light")) {
        luxMessage.setText("Current LUX: " + Integer.toString(msg) + "\n");
    } else if (from.equals("timer")) {
        timerMessage.setText("Turns off in " + Integer.toString(msg) + " minutes\n");
    } else if (from.equals("battery")) {
        // Stops it updating battery % when the torch isn't on
        if (TorchActivity.keepRunning) {
            batteryMessage.setText("Current battery level: " + Integer.toString(msg) + "\n");
            //toast("********");
        }
    }
}

public void setStaticMessage() {
    if (lightSensitivity < 1000) {
        luxStatic.setText("Turns off at " + Integer.toString(lightSensitivity) + " LUX");
    }
    if (timerOn) {
        timerStatic.setText("Timer: ON");
    } else {
        timerStatic.setText("Timer: OFF");
    }
    if (batteryPct > 0) {
        batteryStatic.setText("Turns off at " + Integer.toString(batteryPct) + "% battery");
    }
    shutdownMessageStatic.setText("Torch stopped by: ");
}

public void clearText() {
    luxMessage.setText("\n");
    luxStatic.setText("Light Monitor: OFF");
    // only lets it clear the timer if the torch is off, && it's not the first time through. On the first time
    // through it was getting cleared when it shouldn't have been.
    if (!TorchActivity.keepRunning && !TorchActivity.autoOn) {
        timerMessage.setText("\n");
    }
    timerStatic.setText("\nTimer: OFF");
    batteryMessage.setText("\n");
    batteryStatic.setText("Battery monitor: OFF");
}

public void updateCompass(String s, RotateAnimation rA){
    img.startAnimation(rA);
    heading.setText(s);
}

public void setImageWhite(){
    img.setImageResource(R.drawable.compass_white);
}

public void setImageBlack(){
    img.setImageResource(R.drawable.compass);
}

private void getPreferences() {
    prefs = PreferenceManager.getDefaultSharedPreferences(getActivity());
    lightSensitivity = Integer.valueOf(prefs.getString("lightsensitivity", "1000000"));
    timerOn = prefs.getBoolean("timeron", false);
    batteryPct = Integer.valueOf(prefs.getString("batterypct", "50"));
}

// From TorchActivity
public void setShutdownMessage(String s) {
    shutdownMessage.setText("The " + s);
}

public void clearShutdownMessage() {
    shutdownMessage.setText("");
}

public void setIsThereALightSensor(boolean isThereALightSensor) {
    this.isThereALightSensor = isThereALightSensor;
}

public boolean getIsThereALightSensor() {
    return isThereALightSensor;
}

// Container Activity must implement this interface
public interface AlertResetListener {
    public void alertReset();
}

// Container Activity must implement this interface
public interface ServiceListener {
    public void startService(String s);

    // Resets the service and the timer
    public void reset();
}

// Container Activity must implement this interface
public interface SetDataListener {
    public void setData();
}

public interface ClearShutdownMessageListener {
    public void clearShutdownMessage();
}

public interface ColourSchemeListener {
    public int[] getColourScheme();
}

}

Answer 1

Looks like you are calling setIsThereALightSensor before the fragment is attached.

This is confirmed from your comment

Quoting

I solved the issue by starting the fragments in the onAttach in the TorchFragment (via the hosting activity) so it was up and running when it's methods were called