How does the smart app lock application in android detects application launch ?

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Android

How to detect the application launch in android application ?
How does the Smart App Lock applications in android detect the launch of an application ?

import android.app.ActivityManager;
import android.content.pm.ApplicationInfo;
import android.content.pm.PackageManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;

import java.util.Iterator;
import java.util.List;

public class MainActivity extends AppCompatActivity
{
public  static  final String TAG ="MainActivity";

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

        final PackageManager packageManager = getPackageManager();
        //get a list of installed apps.
        List<ApplicationInfo> packages = packageManager.getInstalledApplications(PackageManager.GET_META_DATA);
        for (ApplicationInfo packageInfo : packages) {
            Log.d(TAG, "Installed package :" + packageInfo.packageName);
            Log.d(TAG, "Source dir : " + packageInfo.sourceDir);
            Log.d(TAG, "Launch Activity :" + packageManager.getLaunchIntentForPackage(packageInfo.packageName));
        }

        ActivityManager activityManager = (ActivityManager) this.getSystemService(ACTIVITY_SERVICE);
        List l = activityManager.getRecentTasks(1, ActivityManager.RECENT_WITH_EXCLUDED);
        Iterator i = l.iterator();
        
        while (i.hasNext()) {
            ActivityManager.RunningAppProcessInfo info = (ActivityManager.RunningAppProcessInfo) i.next();
            try
            {
                CharSequence c = packageManager.getApplicationLabel(packageManager.getApplicationInfo(
                        info.processName, PackageManager.GET_META_DATA));
                Log.w("LABEL", c.toString());
            } catch (Exception e)
            {}
        }
    }
}

What I have tried:

I tried this snippet to detect atleast, the applications currently running. But this throws me a Class Cast Exception :

android.app.ActivityManager$RecentTaskInfo cannot be cast to android.app.ActivityManager$RunningAppProcessInfo