獲取當前Activity最主要的作用當然是為了能隨時獲取Context，超多地方都需要用到它

什么是Context

源碼中的注釋是這么來解釋Context的：Context提供了關于應用環境全局信息的接口。Context到底可以實現哪些功能呢？彈出Toast、啟動Activity、啟動Service、發送廣播、操作數據庫等等都需要用到Context

Context作用域

如何獲取當前Activity

我的思路是創建一個Activity管理類，通過在Application監聽所有Activity的生命周期回調來獲取當前Activity。
首先創建Activity管理類（object聲明相當于單例）

object ActivityManager {
    // 弱引用
    private var sCurrentActivityWeakRef: WeakReference<Activity>? = null

    fun getCurrentActivity(): Activity? = sCurrentActivityWeakRef?.get()

    fun setCurrentActivity(activity: Activity) {
        sCurrentActivityWeakRef = WeakReference(activity)
    }
}

創建自己的Application，并監聽所有Activity的生命周期回調。
P:這里的Application也是用的單例模式，可以通過WeatherApplication.getInstance()來獲取Application

class WeatherApplication : Application() {
    // 單例
    companion object {
        fun getInstance(): WeatherApplication = Inner.instance
    }

    private object Inner {
        lateinit var instance: WeatherApplication
    }

    override fun onCreate() {
        super.onCreate()
        Inner.instance = this

        // 監聽所有Activity的生命周期回調
        registerActivityLifecycleCallbacks(object : ActivityLifecycleCallbacks {
            override fun onActivityCreated(activity: Activity, savedInstanceState: Bundle?) {
            }

            override fun onActivityStarted(activity: Activity) {
            }

            override fun onActivityResumed(activity: Activity) {
                // 在此處設置當前的Activity
                ActivityManager.setCurrentActivity(activity)
            }

            override fun onActivityPaused(activity: Activity) {
            }

            override fun onActivityStopped(activity: Activity) {
            }

            override fun onActivitySaveInstanceState(activity: Activity, outState: Bundle) {
            }

            override fun onActivityDestroyed(activity: Activity) {
            }
        })

    }
}

別忘了在AndroidManifest.xml中注冊Application

android:name=".WeatherApplication"

現在通過ActivityManager.getCurrentActivity()就能在代碼任意地方獲取當前的Activity了