一、ActivityLifecycleCallbacks接口介绍

官方地址：https://developer.android.com/reference/android/app/Application.ActivityLifecycleCallbacks

在 Android API 14之后的Application类中，为我们提供了一个应用生命周期回调的注册方法，用来对应用的生命周期进行集中管理，这个接口叫registerActivityLifecycleCallbacks，可以通过它注册自己的ActivityLifeCycleCallback，每一个Activity的生命周期都会回调到这里的对应方法。

有了ActivityLifeCycleCallback接口，我们就可以完成之前我们想做的类似限制制定Activity个数等相关需求，因为所有Activity的生命周期都会在这里回调，我们可以根据条件随心处理。

Activity 的生命周期图为：https://developer.android.com/images/activity_lifecycle.png；

ActivityLifecycleCallbacks 的方法列表文档：https://developer.android.com/reference/android/app/Application.ActivityLifecycleCallbacks

两者几乎是一一对应的，不管是做Activity的限制还是Activity的状态统计都是非常方便的，而且里面还有一个void onActivitySaveInstanceState(Activity activity, Bundle outState) 方法，非常方便我们来保存Activity状态数据。

使用ActivityLifecycleCallbacks我们可以完成类似如下的功能：

1. 限制指定的Activity的数量

2. 控制在特定情况下只会有一个Activity被打开

3. 判断App前后台状态

二、ActivityLifecycleCallbacks的应用

Application.ActivityLifecycleCallbacks是Application中的一个接口，使用起来也很简单，只需要调用registerActivityLifecycleCallbacks方法即可完成注册。Application.ActivityLifecycleCallbacks中对应的监听的生命周期方法会在Activity中的生命方法调用父类的方法之后被触发。

import android.app.Activity;
import android.app.Application;
import android.os.Bundle;
import android.util.Log;

public class LifecycleApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        init();
    }

    private void init() {
        registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
                Log.e("Lifecycle",activity.getLocalClassName()+" was Created"+"activity==null   "
                        +(activity==null)+"     activity.isFinishing()  "+(activity.isFinishing())+"    activity.isDestroyed()  "+activity.isDestroyed());
            }

            @Override
            public void onActivityStarted(Activity activity) {
                Log.e("Lifecycle",activity.getLocalClassName()+" was Started"+"activity==null   "
                            +(activity==null)+"     activity.isFinishing()   "+(activity.isFinishing())+"   activity.isDestroyed()  "+activity.isDestroyed());
            }

            @Override
            public void onActivityResumed(Activity activity) {
                 Log.e("Lifecycle",activity.getLocalClassName()+" was oResumed"+"activity==null   "
                        +(activity==null)+"activity.isFinishing()   "+(activity.isFinishing())+"activity.isDestroyed() "+activity.isDestroyed());
            }

            @Override
            public void onActivityPaused(Activity activity) {
                Log.e("Lifecycle",activity.getLocalClassName()+" was Pauseed"+"activity==null   "
                        +(activity==null)+"activity.isFinishing()   "+(activity.isFinishing())+"activity.isDestroyed()  "+activity.isDestroyed());
            }

            @Override
            public void onActivityStopped(Activity activity) {
                Log.e("Lifecycle",activity.getLocalClassName()+" was Stoped"+"activity==null    "
                        +(activity==null)+"activity.isFinishing()   "+(activity.isFinishing())+"activity.isDestroyed() "+activity.isDestroyed());
            }

            @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
                Log.e("Lifecycle",activity.getLocalClassName()+" was SaveInstanceState"+"activity==null "
                        +(activity==null)+"activity.isFinishing()   "+(activity.isFinishing())+"activity.isDestroyed()  "+activity.isDestroyed());
            }

            @Override
            public void onActivityDestroyed(Activity activity) {
                Log.e("Lifecycle",activity.getLocalClassName()+" was Destroyed"+"activity==null"
                        +(activity==null)+"  activity.isFinishing()  "+(activity.isFinishing())+"  activity.isDestroyed()"+activity.isDestroyed());
            }
        });
    }
}

在清单中声明Application，无需在Activity添加额外的代码就可以实现监控：

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;

public class MainActivity extends AppCompatActivity {

    public static final String LIFECYCLE = "MainActivity:Lifecycle";

    @Override
        protected void onCreate(Bundle savedInstanceState) {
        Log.e(LIFECYCLE, "onCreate() is Running__before super.onCreate called");
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Log.e(LIFECYCLE, "onCreate() is Running__after super.onCreate called");
    }

    @Override
        protected void onRestart() {
        Log.e(LIFECYCLE, "onRestart() is Running__before super's called");
        super.onRestart();
        Log.e(LIFECYCLE, "onRestart() is Running__after super's called");
    }

    @Override
        protected void onStart() {
        Log.e(LIFECYCLE, "onStart() is Running__before super.onStart called");
        super.onStart();
        Log.e(LIFECYCLE, "onStart() is Running__after super.onStart called");
    }

    @Override
        protected void onResume() {
        Log.e(LIFECYCLE, "onResume() is Running__before super.onResume called");
        super.onResume();
        Log.e(LIFECYCLE, "onResume() is Running__after super.onResume called");
    }

    @Override
        protected void onPause() {
        Log.e(LIFECYCLE, "onPause() is Running__before super's called");
        super.onPause();
        Log.e(LIFECYCLE, "onPause() is Running__after super's called");
    }

    @Override
        protected void onStop() {
        Log.e(LIFECYCLE, "onStop() is Running__before super's called");
        super.onStop();
        Log.e(LIFECYCLE, "onStop() is Running__after super's called");
    }

    @Override
        protected void onDestroy() {
        Log.e(LIFECYCLE, "onDestroy() is Running__before super's called");
        super.onDestroy();
        Log.e(LIFECYCLE, "onDestroy() is Running__after super's called");
    }

    public void toTask(View view) {
        startActivity(new Intent(this, TaskActivity.class));
        //finish();
    }
}

三、使用 ActivityLifecycleCallbacks 判断App前后台状态

App 前后台的切换一般情况下都是按Home来进行，当然也有别的方式，但是此时Activity的生命周期是一样的：

HOME键前后台切换Activity的执行顺序：onPause->onStop->onRestart->onStart->onResume

BACK键前后台切换Activity键的顺序： onPause->onStop->onDestroy->onCreate->onStart->onResume

其实按BACK按键就是退出app了，不算是前台后切换。

现在我们知道App的由前台切换到后台所有打开的Activity会走：onPause->onStop

后台切换到前台所有打开的Activity会走：->onRestart->onStart->onResume

前后台切换App所有打开的Activity的生命周期都是一样的，这样我就可以在ActivityLifecycleCallbacks回调接口中记录生命周期：

public class App extends Application {
    //记录Activity的总个数
    public int count = 0;
    private static App mApp;
    public static Stack<ActivityDetail> store;
    //商品详情页最多个数,这里为了测试只写了2,大家根据自己的情况设值
    private static final int MAX_ACTIVITY_DETAIL_NUM = 2;

    @Override
        public void onCreate() {
        super.onCreate();
        mApp = this;
        store = new Stack<>();
        registerActivityLifecycleCallbacks(new SwitchBackgroundCallbacks());
    }

    public static App getAppContext() {
        return mApp;
    }

    public static boolean toGoodsDetail(String id){

        if(store == null || store.empty()){
            return false;
        }
        for(ActivityDetail activityDetail : store){
            if(id.equalsIgnoreCase(activityDetail.getID())){ //当前商品的详情页已经打开
                activityDetail.finish();
                // 这是你需要在AndroidManifest.xml中添加"Android.permission.STOP_APP_SWITCHES"用户权限，前提是必须是系统应用才可以。
                // ActivityManager am = (ActivityManager) getAppContext().getSystemService(Activity.ACTIVITY_SERVICE);
                // am.moveTaskToFront(activityDetail.getTaskId(), 0);
                return true;
            }
        }
        return false;
    }

    private class SwitchBackgroundCallbacks implements ActivityLifecycleCallbacks {

        @Override
            public void onActivityCreated(Activity activity, Bundle bundle) {
            if(activity instanceof ActivityDetail) {
                if(store.size() >= MAX_ACTIVITY_DETAIL_NUM){
                    store.peek().finish(); //移除栈底的详情页并finish,保证商品详情页个数最大为10
                }
                store.add((ActivityDetail) activity);
            }
        }

        @Override
            public void onActivityStarted(Activity activity) {
            if (count == 0) { //后台切换到前台
                Log.v("danxx", ">>>>>>>>>>>>>>>>>>>App切到前台");
            }
            count++;
        }

        @Override
            public void onActivityResumed(Activity activity) {

        }

        @Override
            public void onActivityPaused(Activity activity) {

        }

        @Override
            public void onActivityStopped(Activity activity) {
            count--;
            if (count == 0) { //前台切换到后台
                Log.v("danxx", ">>>>>>>>>>>>>>>>>>>App切到后台");
            }
        }

        @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle bundle) {

        }

        @Override
            public void onActivityDestroyed(Activity activity) {
            store.remove(activity);
        }
    }
    // 获取当前的Activity
    public Activity getCurActivity() {
        return store.lastElement();
    }
}