sample.nim

jclass org.wiish.wiishexample.WiishActivity of JVMObject:
  proc evalJavaScript(js: string)

type
  WebviewWindow = ref object of BaseWindow
    wiishActivity: WiishActivity

proc nimwin() : WebviewWindow {.exportc.} = app.window

proc executeSomeJS(js:string) =
  app.window.wiishActivity.evalJavaScript("somefunc();")

{.emit: """
JNIEXPORT void JNICALL Java_org_wiish_exampleapp_WiishActivity_wiish_1init
  (JNIEnv * env, jobject obj) {
      NimMain();
      nimwin()->wiishActivity = obj;
    }
""" .}