1. 概述

判断App在前台还是后台有2种方式。

第一种：自己定义一个MyApplication继承Application，在onCreate()方法中：

/**
 * Email: 2185134304@qq.com
 * Created by Novate 2018/4/25 17:34
 * Version 1.0
 * Params:
 * Description:  
 */
public class MyApplication extends Application{
    public int count = 0;
    @Override
    public void onCreate() {
        super.onCreate();

        registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {

            @Override
            public void onActivityStopped(Activity activity) {
                Log.v("viclee", activity + "onActivityStopped");
                count--;
                if (count == 0) {
                    Log.v("viclee", ">>>>>>>>>>>>>>>>>>>切到后台  lifecycle");
                }
            }

            @Override
            public void onActivityStarted(Activity activity) {
                Log.v("viclee", activity + "onActivityStarted");
                if (count == 0) {
                    Log.v("viclee", ">>>>>>>>>>>>>>>>>>>切到前台  lifecycle");
                }
                count++;
            }

            @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
                Log.v("viclee", activity + "onActivitySaveInstanceState");
            }

            @Override
            public void onActivityResumed(Activity activity) {
                Log.v("viclee", activity + "onActivityResumed");
            }

            @Override
            public void onActivityPaused(Activity activity) {
                Log.v("viclee", activity + "onActivityPaused");
            }

            @Override
            public void onActivityDestroyed(Activity activity) {
                Log.v("viclee", activity + "onActivityDestroyed");
            }

            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
                Log.v("viclee", activity + "onActivityCreated");
            }
        });
    }
}

第二种：直接在具体的某个 Activity中写一个判断是否在前台还是后台的方法，然后在 该Activity的 onStart()、onStop()中判断：

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    public static final String TAG = "viclee";
    private boolean isCurrentRunningForeground = true;

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

        findViewById(R.id.btn).setOnClickListener(this);
    }

    @Override
    protected void onStart() {
        super.onStart();
        if (!isCurrentRunningForeground) {
            Log.e(TAG, ">>>>>>>>>>>>>>>>>>>切到前台 activity process");
            Toast.makeText(MainActivity.this , ">>>>>>>>>>>>>>>>>>>切到前台 activity process" , Toast.LENGTH_SHORT).show();
        }
    }

    @Override
    protected void onStop() {
        super.onStop();
        isCurrentRunningForeground = isRunningForeground();
        if (!isCurrentRunningForeground) {
            Log.e(TAG, ">>>>>>>>>>>>>>>>>>>切到后台 activity process");
            Toast.makeText(MainActivity.this , ">>>>>>>>>>>>>>>>>>>切到后台 activity process" , Toast.LENGTH_SHORT).show();
        }
    }

    public boolean isRunningForeground() {
        ActivityManager activityManager = (ActivityManager) this.getSystemService(Context.ACTIVITY_SERVICE);
        List<ActivityManager.RunningAppProcessInfo> appProcessInfos = activityManager.getRunningAppProcesses();
        // 枚举进程
        for (ActivityManager.RunningAppProcessInfo appProcessInfo : appProcessInfos) {
            if (appProcessInfo.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND) {
                if (appProcessInfo.processName.equals(this.getApplicationInfo().processName)) {
                    Log.e(TAG, "MainActivity isRunningForeGround");  // 前台进程
                    return true;
                }
            }
        }
        Log.e(TAG, "MainActivity isRunningBackGround");  // 后台进程
        return false;
    }

    @Override
    public void onClick(View v) {
        if (v.getId() == R.id.btn) {
            startActivity(new Intent(this,Main2Activity.class));
        }
    }
}

直接使用以上方式进行判断就ok。