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Tutkin diplomityössäni älypuhelinten hyödyntämistä maatalouskäytössä. Tutkimukseen liittyy GPS träkkäysohjelma, jota olen ollut kehittämässä jo lähes vuoden. Ohjelman tarkoitus on automatisoida maatalouteen liittyvän tyokirjanpidon muodostamista

Ohjelman on pystyttävä seuraavaan maanviljelijän liikkeitä koko vuorokauden. Ohjelman virrankulutus on pidettävä sellaisena, ettei se kuluta koko akkua loppuun ainakaan kymmeneen tuntiin. GPS palvelu on siis pystyttävä katkaisemaan väliajoin eikä muu puhelimen käyttö saa katkaista träkkäystä.

Android-implementaatiossani GPS kuuntelusta vastaa oma Androidin Service. Servicet sinänsä eivät ole erityisen soveltuvia tähän käyttöön, koska ne ajetaan samassa säikeessä muun ohjelman (käyttöliittymä) kanssa ja myös Servicen säie lopetetaan, jos laite menee näppäinlukkoon tai on käyttämättä jonkin aikaa. Ongelma ei poistu, vaikka Servicessä luotaisiin uusia säikeitä.

Ratkaisuna käytin WakeLockia, joka mahdollistaa näytön sammumisen ja näppäinlukon, mutta ei pysäytä prosessoria ja säikeet pysyvät päällä. Periaatteessa tämä on aika huono ominaisuus kännykkäohjelmalla, mutta paras löytämäni tapa pitää träkkäys hengissä. Jos GPS pollausintervalli pidetään yli minuutin mittaisena eli GPS on vähintään minuutin pois päältä ja taas minuutin päällä, virrankulutus pysyy melko inhimillisenä. Ohjelma lisää virrankulutusta noin 15mA.
private void startListening() {
  new Thread(new Runnable() {
   @Override
   public void run() {
    PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
    PowerManager.WakeLock wl = pm.newWakeLock(
      PowerManager.PARTIAL_WAKE_LOCK, "AlkioGPS");
    wl.acquire();
    Looper.prepare();
    while (!die) {
     mloc.requestLocationUpdates(LocationManager.GPS_PROVIDER,
       1000, 0, listener);
     try {
      Thread.sleep((gpsinterval / 2));
     } catch (InterruptedException e) {
      e.printStackTrace();
     }
     mloc.removeUpdates(listener);
     try {
      Thread.sleep(gpsinterval / 2);
     } catch (InterruptedException e) {
      e.printStackTrace();
     }
    }
    gpstimer.cancel();
    try {
     unregisterReceiver(rec);
    } catch (Exception e) {

    }
    wl.release();
   }
  }).start();
 }

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