BUGFIX: opening CanteenFragment.kt more than once would disallow refreshing from working properly

BUGFIX: fragments launch slowly when quickly clicking through the bottom navigation view

app/src/main/java/com/denizk0461/studip/activity/MainActivity.kt

@@ -3,7 +3,6 @@ package com.denizk0461.studip.activity
 import android.os.Bundle
 import androidx.appcompat.app.AppCompatActivity
 import androidx.core.view.WindowCompat
-import androidx.fragment.app.Fragment
 import androidx.fragment.app.FragmentTransaction
 import androidx.lifecycle.Lifecycle
 import com.denizk0461.studip.R
@@ -21,14 +20,12 @@ class MainActivity : AppCompatActivity() {
 
     // View binding
     private lateinit var binding: ActivityMainBinding
+
     /*
-     * ID of the bottom navigation view button that launched the currently active fragment; set on
-     * activity launch to the fragment that will be first shown
+     * Tag for the currently active fragment; set on activity launch to the fragment that will be
+     * shown first.
      */
-    private var currentFragment: Int = R.id.plan
-    // All fragments are instantiated at app launch
-    private val fragments: List<Fragment> =
-        listOf(EventFragment(), CanteenFragment(), SettingsFragment())
+    private var currentFragment: String = "event"
 
     override fun onCreate(savedInstanceState: Bundle?) {
         WindowCompat.setDecorFitsSystemWindows(window, false)
@@ -44,12 +41,15 @@ class MainActivity : AppCompatActivity() {
         binding = ActivityMainBinding.inflate(layoutInflater)
         setContentView(binding.root)
 
-        /*
-         * Set up navigation view bar to launch fragments.
-         * TODO: why do fragments launch slowly when quickly clicking between them?
-         */
+        // Set up navigation view bar to launch fragments
         binding.contentMain.navView.setOnItemSelectedListener { item ->
-            loadFragment(item.itemId)
+
+            // Launch fragment based on the item that has been clicked
+            loadFragment(when (item.itemId) {
+                R.id.food -> "canteen"
+                R.id.settings -> "settings"
+                else -> "event"
+            })
         }
 
         // Launch the fragment defined in currentFragment
@@ -59,56 +59,38 @@ class MainActivity : AppCompatActivity() {
     /**
      * Loads a given fragment.
      *
-     * @param id    the ID of the bottom navigation view button that was clicked
+     * @param type  tag for the fragment that is to be opened
      * @return      true on success, false if another fragment is currently instantiating
      */
-    private fun loadFragment(id: Int): Boolean {
-        // Get fragment to be loaded
-        val fragment = getFragment(id)
+    private fun loadFragment(type: String): Boolean {
 
-        // Check if another fragment exists and is not fully initialised
+        // Find the fragment that is to be opened, or create it if it hasn't been instantiated yet
+        val fragment = supportFragmentManager.findFragmentByTag(type) ?: getFragment(type)
+
+        // Don't interrupt the current fragment's lifecycle if it isn't fully initialised
         if (fragment.lifecycle.currentState != Lifecycle.State.INITIALIZED) {
-            // Return false to avoid interrupting the fragment's lifecycle
             return false
         }
 
-        // Prepare and commit a transaction between the current and the next fragment
+        // Start the transaction to show the new fragment
         supportFragmentManager.beginTransaction()
-            .replace(R.id.nav_host_fragment_content_main, fragment, id.toString())
+            .replace(R.id.nav_host_fragment_content_main, fragment, type)
             .setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE)
-//            .setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN)
             .commit()
 
-        // Set text of the app bar accordingly
-//        binding.contentMain.appTitleBar.text = getTitleString(id)
-        // Note down the new current fragment
-        currentFragment = id
-
-        // Transaction successful
+        // Success
         return true
     }
 
     /**
-     * Retrieve fragment by the ID of the bottom navigation view's button.
+     * Retrieve fragment by its tag.
      *
-     * @param id    the ID of the bottom navigation button
+     * @param type  tag for the corresponding fragment
      * @return      the corresponding fragment
      */
-    private fun getFragment(id: Int) = when (id) {
-        R.id.food -> fragments[1] // CanteenFragment.kt
-        R.id.settings -> fragments[2] // SettingsFragment.kt
-        else -> fragments[0] // on unknown value or R.id.plan, launch EventFragment.kt
-    }
-
-    /**
-     * Retrieve app bar text by the ID of the bottom navigation view's button.
-     *
-     * @param id    the ID of the bottom navigation button
-     * @return      the corresponding fragment's title
-     */
-    private fun getTitleString(id: Int) = when (id) {
-        R.id.food -> getString(R.string.title_food)
-        R.id.settings -> getString(R.string.title_settings)
-        else -> getString(R.string.title_schedule) // on unknown value or R.id.plan, get text for EventFragment.kt
+    private fun getFragment(type: String) = when (type) {
+        "canteen" -> CanteenFragment() // CanteenFragment.kt
+        "settings" -> SettingsFragment() // SettingsFragment.kt
+        else -> EventFragment() // on unknown value or "event", launch EventFragment.kt
     }
 }