如何判断APP在前台还是后台？

如何判断一个APP在前台还是后台？可以通过RunningTasks，RunningProcess 以及ActivityLifecycleCallback判定。78a28资讯网——每日最新资讯28at.com

RunningTasks方式

注意：getRunningTask方法在5.0以上已经被废弃，只能返回自己和系统的一些不敏感的task，不再返回其他应用的task，用此方法来判断自身App是否处于后台是有效的，但是无法判断其他应用是否处于前台。78a28资讯网——每日最新资讯28at.com

private fun getTopApplication() {    //首先获取到ActivityManager    val activityManager = getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager    if (activityManager.getRunningTasks(1) == null){        Log.e(TAG, "getForegroundActivity: ")        return    }    var runningTaskInfo = activityManager.getRunningTasks(1)[0]    if (runningTaskInfo == null) {        Log.e(TAG, "runningTaskInfo is null")        return    }    runningTaskInfo.topActivity?.let {        Log.e(TAG, "top application is ${it.packageName}")    }}

RunningProcess方式

注意：RunningProcess方法在5.0以上已经被废弃，例如，在聊天类型的App中，大部分时间需要常驻后台来不间断地获取服务器的消息，就必须把Service设置成START_STICKY，kill后会被重启（等待5s左右）来保证Service常驻后台。如果Service设置了这个属性，这个App的进程就可以判断为前台。78a28资讯网——每日最新资讯28at.com

appProcess.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND

private fun isAppForeground(): Boolean {    val activityManager = getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager    var runningAppProcesses = activityManager.runningAppProcesses    if (runningAppProcesses == null) {        Log.e(TAG, "runningAppProcesses is null")        return false    }    runningAppProcesses.forEach {        if (it.processName == packageName && (it.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND)) {            return true        }    }    return false}

ActivityLifecycleCallbacks方式

class MyActivityLifecycleCallbacks(    var onActivityCreatedAction: ((Activity, Bundle?) -> Unit)? = null,    var onActivityStartedAction: ((Activity) -> Unit)? = null,    var onActivityResumedAction: ((Activity) -> Unit)? = null,    var onActivityPausedAction: ((Activity) -> Unit)? = null,    var onActivityStoppedAction: ((Activity) -> Unit)? = null,    var onActivitySaveInstanceStateAction: ((Activity, Bundle) -> Unit)? = null,    var onActivityDestroyedAction: ((Activity) -> Unit)? = null) : Application.ActivityLifecycleCallbacks {    private var mCount=0    override fun onActivityCreated(activity: Activity, savedInstanceState: Bundle?) {        onActivityCreatedAction?.invoke(activity, savedInstanceState)    }    override fun onActivityStarted(activity: Activity) {        mCount++        onActivityStartedAction?.invoke(activity)    }    override fun onActivityResumed(activity: Activity) {        onActivityResumedAction?.invoke(activity)    }    override fun onActivityPaused(activity: Activity) {        onActivityPausedAction?.invoke(activity)    }    override fun onActivityStopped(activity: Activity) {        mCount--        onActivityStoppedAction?.invoke(activity)    }    override fun onActivitySaveInstanceState(activity: Activity, outState: Bundle) {        onActivitySaveInstanceStateAction?.invoke(activity, outState)    }    override fun onActivityDestroyed(activity: Activity) {        onActivityDestroyedAction?.invoke(activity)    }    /**     * 这里我们把mCount的实际数值返回回去     */    fun getCount():Int = mCount}

这里我们对ActivityLifecycleCallbacks的实现类做了一层封装，利用Kotlin的高阶函数，当我们需要去实现那个声明周期的回调的时候，就通过高阶函数来提供回调处理，否则默认不做任何处理。然后我们在Application的onCreate中进行注册:78a28资讯网——每日最新资讯28at.com

class LifeApplication : Application() {    private val TAG = "LifeApplication"    private val mActivityLifecycleCallbacks by lazy {        MyActivityLifecycleCallbacks(            onActivityCreatedAction = { activit, bundle ->            Log.e(TAG, "onCreate: ")        },        onActivityStoppedAction = { activity ->            Log.e(TAG, "onStop ")        },        onActivityDestroyedAction = { activity ->            Log.e(TAG, "onDestroy")        })    }    override fun onCreate() {        super.onCreate()        instance = this        //注册生命周期回调事件        registerActivityLifecycleCallbacks(mActivityLifecycleCallbacks)    }    /**     * 用于判断当前进程是否处于前台     */    fun isForegroundMethod(): Boolean = mActivityLifecycleCallbacks.getCount() > 0    companion object{        private var instance :LifeApplication?= null        fun getInstance () = instance!!    }

当我们不管是点击Back键还是Home键都会回调到onStop方法，我们在onStart和onStop中分别对mCount值做了加减，这样我们可以通过该数值来判断当前App是前台还是后台。78a28资讯网——每日最新资讯28at.com

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