如何判斷一個APP在前臺還是后臺？可以通過RunningTasks，RunningProcess 以及ActivityLifecycleCallback判定。VNA28資訊網——每日最新資訊28at.com

RunningTasks方式

注意：getRunningTask方法在5.0以上已經被廢棄，只能返回自己和系統的一些不敏感的task，不再返回其他應用的task，用此方法來判斷自身App是否處于后臺是有效的，但是無法判斷其他應用是否處于前臺。VNA28資訊網——每日最新資訊28at.com

private fun getTopApplication() {    //首先獲取到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以上已經被廢棄，例如，在聊天類型的App中，大部分時間需要常駐后臺來不間斷地獲取服務器的消息，就必須把Service設置成START_STICKY，kill后會被重啟（等待5s左右）來保證Service常駐后臺。如果Service設置了這個屬性，這個App的進程就可以判斷為前臺。VNA28資訊網——每日最新資訊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)    }    /**     * 這里我們把mCount的實際數值返回回去     */    fun getCount():Int = mCount}

這里我們對ActivityLifecycleCallbacks的實現類做了一層封裝，利用Kotlin的高階函數，當我們需要去實現那個聲明周期的回調的時候，就通過高階函數來提供回調處理，否則默認不做任何處理。然后我們在Application的onCreate中進行注冊:VNA28資訊網——每日最新資訊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        //注冊生命周期回調事件        registerActivityLifecycleCallbacks(mActivityLifecycleCallbacks)    }    /**     * 用于判斷當前進程是否處于前臺     */    fun isForegroundMethod(): Boolean = mActivityLifecycleCallbacks.getCount() > 0    companion object{        private var instance :LifeApplication?= null        fun getInstance () = instance!!    }

當我們不管是點擊Back鍵還是Home鍵都會回調到onStop方法，我們在onStart和onStop中分別對mCount值做了加減，這樣我們可以通過該數值來判斷當前App是前臺還是后臺。VNA28資訊網——每日最新資訊28at.com

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