Develop Reference

Proper usage of OnBackPressedDispatcher

Sometimes, I want to handle the back button being pressed by the user myself. My (example) code looks something like this:
class TestActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.test)
}
override fun onBackPressed() {
if (checkSomeCondition()) {
// do nothing
} else {
super.onBackPressed()
}
}
private fun checkSomeCondition() = false
}
I get notified when back is pressed, and then I decide if I want to handle it, or let the system handle it by calling super.onBackPressed().
Since onBackPressed() is now deprecated, I replace it by using the OnBackPressedDispatcher:
class TestActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.test)
onBackPressedDispatcher.addCallback(this, object : OnBackPressedCallback(true) {
override fun handleOnBackPressed() {
if (checkSomeCondition()) {
// do nothing
} else {
onBackPressedDispatcher.onBackPressed()
}
}
})
}
private fun checkSomeCondition() = false
}
The problem with this code: by calling onBackPressedDispatcher.onBackPressed(), I call my own callback, creating an infinite loop instead of handing this over to the system like I would with super.onBackPressed().
This can be circumvented when temporarily disabling my callback like:
onBackPressedDispatcher.addCallback(this, object : OnBackPressedCallback(true) {
override fun handleOnBackPressed() {
if (checkSomeCondition()) {
// do nothing
} else {
this.isEnabled = false
onBackPressedDispatcher.onBackPressed()
this.isEnabled = true
}
}
})
but that seems rather awkward and doesn't seem like it was intended that way.
What's the correct usage of the OnBackPressedDispatcher here that lets me either handle the back button press myself or hand it over to the system?
PS: I have seen this question about the deprecation of onBackPressed, but it doesn't answer my much more specific question.

As per the Basics of System Back video, the whole point of the Predictive Back gesture is to know ahead of time what is going to handle back.
That means that you should never have just-in-time logic such as checkSomeCondition as part of your call to handleOnBackPressed.
Instead, as explained in the Custom back navigation documentation, you should be updating the isEnabled state of your OnBackPressedCallback ahead of time whenever the conditions you used to check in checkSomeCondition changed. This ensures that your callback is only invoked when your condition is already true and is disabled when the condition is false, thus allowing the default behavior to occur.
class TestActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.test)
// Instead of always setting enabled to true at the beginning,
// you need to check the state ahead of time to know what the
// initial enabled state should be
val isConditionAlreadySet = checkSomeCondition()
val callback = object : OnBackPressedCallback(
isConditionAlreadySet
) {
override fun handleOnBackPressed() {
// Since you've handled isEnabled correctly, you know
// your condition is set correctly here, so you can
// unconditionally do what you need to do to handle back
}
}
// This is the key part - now you know ahead of time
// when the condition changes, which lets you control
// when you want to handle back or not
setOnSomeConditionChangedListener { isConditionMet ->
callback.isEnabled = isConditionMet
}
onBackPressedDispatcher.addCallback(this, callback)
}
private fun checkSomeCondition() = false
private fun setOnSomeConditionChangedListener(
(isConditionMet: Boolean) -> Unit
) {
// The implementation here will depend on what
// conditions checkSomeCondition() actually depends on
}
}

How to start another activity in function? Kotlin

class MainActivity : ComponentActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContent {
val navigate = Intent(this,Activity2::class.java)
startActivity(navigate)
}
}
}
fun switchActivity(){
val navigate = Intent(this,Activity2::class.java)
startActivity(navigate)
}
I want to start an activity from a function and not from the main activity..
I can start an activity from main class but in function, the code doesnt work..
Please help.. I'm new to kotlin android programming..

You can also try like this
class MainActivity : AppCompatActivity() {
#SuppressLint("MissingInflatedId")
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
nextActivity(this, MainActivity2())
}
private fun nextActivity(context: Context, activity: Activity) {
startActivity(Intent(context, activity::class.java))
}
}

Just use it
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
switchActivity()
}
}
fun switchActivity(){
val navigate = Intent(this#MainActivity, Activity2::class.java)
startActivity(navigate)
}

Actually i don't get your question clearly. But i'll try to answer your question based on my understanding of yours.
class MainActivity : ComponentActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContent {
switchActivity()
// if you want to pass the class through the paramater,
// can try this
// TODO: Another Way
// switchActivityCustom(Activity2::class.java)
}
}
fun switchActivity(){
val navigate = Intent(this, Activity2::class.java)
startActivity(navigate)
}
// TODO: Another Function
// fun switchActivityCustom(destination: Class<*>,){
// val navigate = Intent(this, destination)
// startActivity(navigate)
// }
}
And the last one is, put your function inside the class (in this case: MainActivity Class)

If your function is declared at the top level, without a reference to this, you'd need to pass an instance of Activity to your function:
fun switchActivity(activity: Activity) {
val navigate = Intent(activity, Activity2::class.java)
activity.startActivity(navigate)
}

How to handle the back button DrawerNavigation on android with kotlin

I'm using the DrawerLayout with fragments inside and every fragment navigates to another fragment. I was able to handle the physical back button using :
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val callback = requireActivity().onBackPressedDispatcher.addCallback(this) {
findNavController().navigate(R.id.action_user_validation_to_make_money);
}
}
But every time that I go inside another fragment, the toolbar shows a back button :
I would like to know how can I handle that back button. Thanks!

To navigate using up button, as mentioned in the official docs, override onSupportNavigateUp() in your activity class
override fun onSupportNavigateUp(): Boolean {
return navController.navigateUp(drawerLayout)
}

Using androix backButtonPressedCallback

I need to implement a custom onBackButtonPress method for Fragments. example in LogoutFragment, after logout is handled. user cannot go backStack but a message is shown like press again to exist and is exited. I used this solution. But is not working. Then I saw this Android Doc with onBackPressedDispatcher callback method CODE BELOW. I guess this will work. I added dependencies, but how to implement this in a different fragments, with only 1 activity and fragment container. Kotlin version.
class LogoutFragment : DaggerFragment() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val callback = requireActivity().onBackPressedDispatcher.addCallback(this) {
...
}
...
}
...
}

onBackPressedDispatcher gives an opportunity to handle back press differently in every fragment. Hence you may have to give this piece of code in every fragment.
In the fragment:
var doubleBackToExitPressedOnce = false
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
requireActivity().onBackPressedDispatcher.addCallback(this, callback)
}
val callback = object : OnBackPressedCallback(true ) {
override fun handleOnBackPressed() {
if (doubleBackToExitPressedOnce)
requireActivity().finish()
Toast.makeText(requireContext(), "Press again to go back",
Toast.LENGTH_LONG).show()
doubleBackToExitPressedOnce = true
}
}
If you want to handle back presses of all fragments in one place, you can do so by manipulating the doBack() method in your reference: How to implement onBackPressed() in Fragments?
//pseudocode
fun doBack()
{
//find the fragment
val fragment = supportFragmentManager.findFragmentByTag(
//your fragment tag
)
if(fragment is FragmentA)
{
//do something
}
else if(fragment is FragmentB)
{
//do something else
}
else
{
activity.getSupportFragmentManager().popBackStack(null,
FragmentManager.POP_BACK_STACK_INCLUSIVE);
}
}

How to save data when i press back button

I have made an app in kotlin through the android studio, Now I have used ViewModels to save UI data while phone rotation(configuration change), i also used onSaveInstanceState to save data while pressing back button but it's not working.
The code is below
fragOne.kt
class fragOne : Fragment() {
private lateinit var viewModel: fragViewModel
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
if(savedInstanceState!=null){
with(savedInstanceState) {
viewModel.num=getInt("number")
}
}
// Inflate the layout for this fragment
var binding = DataBindingUtil.inflate<FragmentFragoneBinding>(
inflater,
R.layout.fragment_fragone,
container,
false
)
viewModel = ViewModelProviders.of(this).get(fragViewModel::class.java)
// function to update number
fun updateNumber()
{
binding.number.text="${viewModel.num}"
}
updateNumber()
// setting on Click listener for add button
binding.add.setOnClickListener()
{
viewModel.addFive()
updateNumber()
}
// setting on on Click Listener for minus button
binding.minus.setOnClickListener()
{
viewModel.minusOne()
updateNumber()
}
return binding.root
}
override fun onSaveInstanceState(outState: Bundle) {
// Save the user's current game state
outState?.run {
putInt("number",viewModel.num)
}
// Always call the superclass so it can save the view hierarchy state
if (outState != null) {
super.onSaveInstanceState(outState)
}
}
}
ViewModelclass
class fragViewModel:ViewModel()
{
// Initializing num=0
var num=0
// Functions to add five or subtract one
fun addFive()
{
num=num+5
}
fun minusOne()
{
num=num-1
}
}
please tell me because data is not saved when I press back

You can override onBackPressed to do your state saving:
How to implement onBackPressed() in Fragments?
Remember to call super, so that is does also do the back command!
You could also do like the below:
// This callback will only be called when MyFragment is at least Started.
val callback = requireActivity().onBackPressedDispatcher.addCallback(this) {
// Handle the back button event
}
Really good read: https://developer.android.com/guide/navigation/navigation-custom-back
Back navigation is how users move backward through the history of screens they previously visited. All Android devices provide a Back button for this type of navigation, so you should not add a Back button to your app’s UI. Depending on the user’s Android device, this button might be a physical button or a software button.
Ref:
How to show warning message when back button is pressed in fragments
Example:
Ensure your Activity extends AppCompatActivity
class MyFragment : Fragment() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewModel = ViewModelProviders.of(this).get(fragViewModel::class.java)
val prefs = activity.getSharedPreferences("Key")
int num = prefs.get("number", -999)
if(num != -999) {
viewModel.num = num
}
val callback = requireActivity().onBackPressedDispatcher.addCallback(this) {
prefs.edit().putInt("number", viewModel.num).apply()
}
}
...
}