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如何判斷一個(gè)APP在前臺(tái)還是后臺(tái)？可以通過RunningTasks，RunningProcess 以及ActivityLifecycleCallback判定。

讓客戶滿意是我們工作的目標(biāo)，不斷超越客戶的期望值來自于我們對(duì)這個(gè)行業(yè)的熱愛。我們立志把好的技術(shù)通過有效、簡(jiǎn)單的方式提供給客戶，將通過不懈努力成為客戶在信息化領(lǐng)域值得信任、有價(jià)值的長(zhǎng)期合作伙伴，公司提供的服務(wù)項(xiàng)目有：空間域名、網(wǎng)頁空間、營(yíng)銷軟件、網(wǎng)站建設(shè)、禹王臺(tái)網(wǎng)站維護(hù)、網(wǎng)站推廣。

RunningTasks方式

注意：getRunningTask方法在5.0以上已經(jīng)被廢棄，只能返回自己和系統(tǒng)的一些不敏感的task，不再返回其他應(yīng)用的task，用此方法來判斷自身App是否處于后臺(tái)是有效的，但是無法判斷其他應(yīng)用是否處于前臺(tái)。

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(jīng)被廢棄，例如，在聊天類型的App中，大部分時(shí)間需要常駐后臺(tái)來不間斷地獲取服務(wù)器的消息，就必須把Service設(shè)置成START_STICKY，kill后會(huì)被重啟（等待5s左右）來保證Service常駐后臺(tái)。如果Service設(shè)置了這個(gè)屬性，這個(gè)App的進(jìn)程就可以判斷為前臺(tái)。

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(shí)際數(shù)值返回回去
     */
    fun getCount():Int = mCount
}

這里我們對(duì)ActivityLifecycleCallbacks的實(shí)現(xiàn)類做了一層封裝，利用Kotlin的高階函數(shù)，當(dāng)我們需要去實(shí)現(xiàn)那個(gè)聲明周期的回調(diào)的時(shí)候，就通過高階函數(shù)來提供回調(diào)處理，否則默認(rèn)不做任何處理。然后我們?cè)贏pplication的onCreate中進(jìn)行注冊(cè):

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(cè)生命周期回調(diào)事件
        registerActivityLifecycleCallbacks(mActivityLifecycleCallbacks)
    }

    /**
     * 用于判斷當(dāng)前進(jìn)程是否處于前臺(tái)
     */
    fun isForegroundMethod(): Boolean = mActivityLifecycleCallbacks.getCount() > 0

    companion object{
        private var instance :LifeApplication?= null
        fun getInstance () = instance!!
    }

當(dāng)我們不管是點(diǎn)擊Back鍵還是Home鍵都會(huì)回調(diào)到onStop方法，我們?cè)趏nStart和onStop中分別對(duì)mCount值做了加減，這樣我們可以通過該數(shù)值來判斷當(dāng)前App是前臺(tái)還是后臺(tái)。

當(dāng)前名稱：如何判斷APP在前臺(tái)還是后臺(tái)？
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