Question

I am writing a flashlight app that is a “screen light” for any phone without a torch. I want to have a setting that enables a Strobe Light feature. However, with this type of “flashlight” I thought the easiest way to do this, would be to change the background of the view. I do this in a separate thread because I use a SeekBar to get how fast the strobelight should flash. However, I keep getting the error that the Thread cannot access the View because it is not the Thread that created it. Any suggestions?

Here is what is contained within the onCreate():

        mSeekBar = (SeekBar) findViewById(R.id.seekBar);
        mSeekBar.setOnSeekBarChangeListener(new OnSeekBarChangeListener(){
          public void onProgressChanged(SeekBar seekBar, int progress, boolean fromTouch) {

                final Thread strober = new Thread(new Runnable(){
                    public void run(){
                        strobeLight();
                    }
                });             
                if (mSeekBar.getProgress()>0) {
                    runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        strober.start();
                    }
                    });
                }
                else {
                    runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        strober.interrupt();
                        relLayout.setBackgroundColor(color.all_white);
                    }
                    });
                }
          }
          public void onStartTrackingTouch(SeekBar seekBar) {}
          public void onStopTrackingTouch(SeekBar seekBar) {}          
        }); 

Here is the strobeLight() method:

public void strobeLight(){
    boolean lightIsOn=true;

    runOnUiThread(new Runnable() {
    @Override
    public void run() { 
            do {
                    if (lightIsOn){
                        relLayout.setBackgroundColor(color.all_black);
                        try { Thread.sleep(1000/(mSeekBar.getProgress()+2)); }
                            catch (InterruptedException e) {}
                        lightIsOn=false; 
                        strobeActivated=true;}
                    else{
                        relLayout.setBackgroundColor(color.all_white);  
                        try { Thread.sleep(1000/(mSeekBar.getProgress()+2)); }
                            catch (InterruptedException e) {}                       
                        lightIsOn=true; }
                    if (mSeekBar.getProgress()<1)
                        strobeActivated=false;
            } while (strobeActivated);
            relLayout.setBackgroundColor(color.all_white);
    }
});   
}   

Here is the LogCat for the code above:

11-28 17:17:58.383: E/ActivityManager(16877): ANR in com.marshall.meadows182 (com.marshall.meadows182/.LightNoTorch),  time=603431510
11-28 17:17:58.383: E/ActivityManager(16877): Reason: keyDispatchingTimedOut
11-28 17:17:58.383: E/ActivityManager(16877): Load: 1.53 / 1.62 / 1.62
11-28 17:17:58.383: E/ActivityManager(16877): CPU usage from 14662ms to 0ms ago:
11-28 17:17:58.383: E/ActivityManager(16877):   0.8% 16877/system_server: 0.4% user + 0.4% kernel / faults: 7 minor
11-28 17:17:58.383: E/ActivityManager(16877):   0% 14673/com.qikffc.android: 0% user + 0% kernel / faults: 151 minor
11-28 17:17:58.383: E/ActivityManager(16877):   0.2% 16957/com.android.systemui: 0.2% user + 0% kernel / faults: 1 minor
11-28 17:17:58.383: E/ActivityManager(16877):   0.1% 16999/com.android.phone: 0% user + 0% kernel
11-28 17:17:58.383: E/ActivityManager(16877):   0.1% 13/kondemand/0: 0% user + 0.1% kernel
11-28 17:17:58.383: E/ActivityManager(16877):   0% 68/rild: 0% user + 0% kernel / faults: 6 minor
11-28 17:17:58.383: E/ActivityManager(16877):   0% 14830/logcat: 0% user + 0% kernel
11-28 17:17:58.383: E/ActivityManager(16877):   0.1% 14868/com.marshall.meadows182: 0% user + 0% kernel / faults: 9 minor
11-28 17:17:58.383: E/ActivityManager(16877):   0% 24825/adbd: 0% user + 0% kernel
11-28 17:17:58.383: E/ActivityManager(16877):   0% 5446/com.google.android.apps.maps:NetworkLocationService: 0% user + 0% kernel
11-28 17:17:58.383: E/ActivityManager(16877):   0% 7839/wpa_supplicant: 0% user + 0% kernel / faults: 3 minor
11-28 17:17:58.383: E/ActivityManager(16877):   0% 17054/com.google.process.gapps: 0% user + 0% kernel / faults: 4 minor
11-28 17:17:58.383: E/ActivityManager(16877):   0% 25055/iqd: 0% user + 0% kernel
11-28 17:17:58.383: E/ActivityManager(16877): 2.7% TOTAL: 1.8% user + 0.8% kernel
11-28 17:17:58.383: E/ActivityManager(16877): CPU usage from 416ms to 935ms later with 99% awake:
11-28 17:17:58.383: E/ActivityManager(16877):   7.4% 16877/system_server: 0% user + 7.4% kernel / faults: 6 minor
11-28 17:17:58.383: E/ActivityManager(16877):     9.2% 16923/InputDispatcher: 1.8% user + 7.4% kernel
11-28 17:17:58.383: E/ActivityManager(16877): 3.8% TOTAL: 0% user + 3.8% kernel

Answer 1

The error message is self-explanatory. You are not allowed to call many methods of views outside of the Thread they were created on, known as the UI Thread.

The problem is the lines

relLayout.setBackgroundColor(...);

which are run in your thread. You should run them on the UI thread like this:

runOnUiThread(new Runnable() {
    @Override
    public void run() {
        relLayout.setBackgroundColor(...);
    }
});