也即判断当前App应用是否被用户按home键切换至后台（不可见）/前台（可见）。

package zhangphil.test;

import android.app.ActivityManager;;
import android.content.Context;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;

import java.util.List;
import java.util.concurrent.TimeUnit;

public class JavaActivity extends AppCompatActivity {
    private ActivityManager activityManager;
    private String packageName;
    private boolean stop = false;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.drawable_activity);
        activityManager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
        packageName = this.getPackageName();

        new Thread(new AppStatus()).start();
    }

    private class AppStatus implements Runnable {
        @Override
        public void run() {
            stop = false;

            while (!stop) {
                try {
                    if (appOnForeground()) {
                        System.out.println("当前App处于前台");
                    } else {
                        System.out.println("当前App处于后台");
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }

                try {
                    TimeUnit.SECONDS.sleep(1);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }

    private boolean appOnForeground() {
        List<ActivityManager.RunningAppProcessInfo> appProcesses = activityManager.getRunningAppProcesses();

        if (appProcesses == null)
            return false;

        for (ActivityManager.RunningAppProcessInfo appProcess : appProcesses) {
            if (appProcess.processName.equals(packageName) && appProcess.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND) {
                return true;
            }
        }

        return false;
    }
}

以上这个方案是全局性的判断当前App是否处于前台可见，抑或后台不可见。

需要明白的一点儿是，在大型Android项目中，存在很多Activity时候，仅靠让全部Activity继承自一个类似Base的Activity，然后在Base的Activity生命周期中做相应逻辑处理，进而判断当前App是否处于前台后台的方法是不可取行不通的。