Android中判断App是处于前台还是后台

简介

有时候我们需要判断App是处于前台还是后台，这样有利于我们处理一些业务
这里就说下如何判断App是处于前台还是后台
分别通过RunningTasks，RunningProcess 以及ActivityLifecycleCallbacks

实现

RunningTasks

 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}")
        }
    }

getRunningTask方法在5.0以上已经被废弃，只会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是否处于后台仍然有效，但是无法判断其他应用是否位于前台，因为不能再获取信息。

RunningProcess

/**
     * 判断当前应用是否处于前台
     */
    private fun isAppForeground(): Boolean {
        val activityManager = getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
        /**
         * 存活的App进程
         */
        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
    }

 /**
     * 用于判断那个应用是处于前台的
     */
    private fun getForegroundApp(): String? {

        val activityManager = getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
        var runningAppProcesses = activityManager.runningAppProcesses
        if (runningAppProcesses.isNullOrEmpty()) {
            return null
        }

        runningAppProcesses.forEach {

            if (it.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND || it.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_VISIBLE) {
                return it.processName
            }
        }

        return null
    }

Android5.0之后已经被废弃。

例如，在聊天类型的App中，常常需要常驻后台来不间断地获取服务器的消息，就需要把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的进程就会被判断为前台。代码表现为

appProcess.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND

上述code永远成立，这样就永远无法判断到底那个是前台了。

ActivityLifecycleCallbacks

这里我们对ActivityLifecycleCallbacks的实现类做了一层封装，利用高阶函数，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

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
}

然后我们在Application的onCreate中进行注册

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是否处于前台还是后台