Android Kotlin在其他活动中调用函数

Question

I call a function of MainActivity in other Activity, the app shut down and show the error as below, please help to solve this issue.

MainActivity function:

fun checkInternet():Boolean {
    val cm = baseContext.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
    val networkInfo = cm.activeNetworkInfo
    if (networkInfo != null && networkInfo.isConnected) {

        alertPopUp("internet ok", "")
        return true
    }
    else {
        val title = getString(R.string.No_Internet)
        val message = getString(R.string.need_internet_for_service)
        alertPopUp(title, message)

        return false
    }

}

Call this function in other activity:

var internetStatus:Boolean = MainActivity().checkInternet()

Error Message:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference at com.gph.qpon.MainActivity.checkInternet(MainActivity.kt:148) at com.gph.qpon.qponClickedActivity.onCreate(qponClickedActivity.kt:32)

Answer 1

I would rather extract all logic to a different helper or utility class. It's a big mistake to have it within an activity if you're gonna reuse it. A pretty neat solution could be to have a ConnectivityUtils utility class like the famous iosched project has , just passing the application context to it:

/**
 * Utility methods for dealing with connectivity
 */
object ConnectivityUtils {
  fun isConnected(context: Context): Boolean {
    val connectivityManager = context.applicationContext.getSystemService(
        Context.CONNECTIVITY_SERVICE) as ConnectivityManager
    val activeNetworkInfo = connectivityManager.activeNetworkInfo
    return activeNetworkInfo != null && activeNetworkInfo.isConnectedOrConnecting
  }
}

Then, you just need to invoke it within any activity like this:

ConnectivityUtils.isConnected(this)

Answer 2

Either you should have the context of the MainActivity rather than creating a new one or you could also pass a context in your checkInternet() method so that it could get context in other Activity. I have modified the code below.

companion object {
    fun checkInternet(context: Context):Boolean {
        val cm = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
        val networkInfo = cm.activeNetworkInfo
        if (networkInfo != null && networkInfo.isConnected) {

            // alertPopUp("internet ok", "")
            return true
        }
        else {
            val title = "no Internet"
            val message = "Need Internet Service"
            // alertPopUp(title, message)

            return false
        }

    }
}

In the above code, I have replaced the baseContext with the context that I have passed in the constructor of the method. Also from the other activity from which the checkInternet is called, I have send context of that activity. Code for it is below.

MainActivity.checkInternet(this)

Hope that helps.

Answer 3

First, dont create activities directly by constructor . Activity has own lifecycle, and it's OS that manage it. The only proper way to initialize Activity is by using Intent (but I guess it's not what you want to do here).

Also, dont try to keep your Activity in any field (it leads to memory leaks). If I was to propose a solution, I would suggest use extension function (because you can check connection in any Context class, not only that one):

fun Context.checkInternet() {
   /* your code here, to use context use this */
   val cm = this.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
   /* some code here */
}

When you declare this function, you can use it in any context class (like Activity, Fragment etc. - they are subtypes of context). Now, how to use it?

class MyActivity(): Activity() {
    fun foo() {
        this.checkInternet()
    }
}