MainActivity.kt

    override fun onNavigationItemSelected(item: MenuItem): Boolean {
        when (item.itemId) {
            R.id.main_drawer_login -> {
                if (service.identityManager.isUserSignedIn) {
                    service.identityManager.signOut()
                    updateNavigationDrawer(service.identityManager.isUserSignedIn)
                } else {
                    service.identityManager.login(this@MainActivity, object : DefaultSignInResultHandler() {
                        /**
                         * Called when the user has obtained an identity by signing in with a provider.
                         *
                         * @param callingActivity the calling activity that should be finished.
                         * @param provider the provider or null if succeeded with an unauthenticated identity.
                         */
                        override fun onSuccess(callingActivity: Activity?, provider: IdentityProvider?) {
                            if (service.identityManager.isUserSignedIn) {
                                analyticsService.recordSuccessfulLogin(service.identityManager.cachedUserID)
                                updateNavigationDrawer(true)
                            } else {
                                analyticsService.recordFailedLogin()
                                updateNavigationDrawer(false)
                            }
                        }

                        /**
                         * User pressed back from the sign-in Activity.
                         *
                         * @return true if the activity should be finished, otherwise false.
                         */
                        override fun onCancel(callingActivity: Activity?): Boolean = true
                    })

                    val authConfig = AuthUIConfiguration.Builder()
                            .userPools(true)
                            .build()
                    SignInActivity.startSignInActivity(this@MainActivity, authConfig)
                }
            }
        }
        drawer_layout.closeDrawer(GravityCompat.START)
        return true
    }