How Do "App Lock" app work internally?

I need to develop app just like android App lock that opens password activity when app assigned application get launch. I have read below question but didn't find proper solution. I just want to ask that what service or method they are using? how to know about any other app launch event?

In my current implementation, I have added a service and in MyService, I have added a timer to check currentApp and then I am comparing current app to assigned app.

I don't like this approach because timer takes lot of battery as well as device get slow.

public void isForeground(String myPackage){
        String currentApp="";

        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.LOLLIPOP) {
            UsageStatsManager usm = (UsageStatsManager) getSystemService("usagestats");
            long time = System.currentTimeMillis();
            List<UsageStats> appList = usm.queryUsageStats(UsageStatsManager.INTERVAL_DAILY,
                    time - 1000 * 1000, time);
            if (appList != null && appList.size() > 0) {
                SortedMap<Long, UsageStats> mySortedMap = new TreeMap<Long, UsageStats>();
                for (UsageStats usageStats : appList) {
                    mySortedMap.put(usageStats.getLastTimeUsed(),
                            usageStats);
                }
                if (mySortedMap != null && !mySortedMap.isEmpty()) {
                    currentApp = mySortedMap.get(
                            mySortedMap.lastKey()).getPackageName();
                }
            }
        } else {
            ActivityManager am = (ActivityManager) getBaseContext().getSystemService(ACTIVITY_SERVICE);
            currentApp = am.getRunningTasks(1).get(0).topActivity .getPackageName();
        }
        processCurrentApp(currentApp,myPackage);

        }

Any one can help me in this? Thanks in advance.