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How do I add functionality to the back button?
How do I add functionality to the back button in Android without reimplementing the back button entirely?
Prior to last year, I would just call onBackPressed() and then simply override it:
override fun onBackPressed() {
super.onBackPressed()
doMyStuff()
}
Now this is deprecated, and it's recommended to use OnBackPressedCallback objects. It's simple enough to replace onBackPressed() with onBackPressedDispatcher.onBackPressed(), but I can't figure out how to recreate the override.
I can replace the functionality easy enough:
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
…
onBackPressedDispatcher.addCallback(this, object : OnBackPressedCallback(true) {
override fun handleOnBackPressed() {
doMyStuff()
}
})
}
However, this replaces all back button behavior. It doesn't just add to it, despite the function name. I still want the back button to go back, but don't want to have to try to reinvent the wheel. Is there some equivalent to super.onBackPressed() with this new API or another way to achieve this?
1 answer
TL;DR : end your callback with a call to finish().
According to this article, addCallback really does add a callback. However, the way Android handles these callbacks is this:
Iterator<OnBackPressedCallback> iterator =
mOnBackPressedCallbacks.descendingIterator();
while (iterator.hasNext()) {
OnBackPressedCallback callback = iterator.next();
if (callback.isEnabled()) {
callback.handleOnBackPressed();
return;
}
}
In other words, as soon as a callback is enabled, that callback will be the one to be executed, and any callbacks added earlier will be ignored.
This leads to a counterintuitive situation where having even a single callback enabled, causes that callback to be executed instead of the code that actually takes you back to a previous Activity!
This is intentional. The official Android guide gives an example of overriding the "Back" behavior where the developer doesn't want the normal "Back" behavior to take place. The reason seems to be that this new way of working, supports Android's Predictive Back Gesture.
The simplest solution is to end your callback with a call to finish(). Note that this will call onDestroy() for your Activity.
If that is not an option, you may be able to make calls on the onBackInvokedDispatcher directly, rather than using the onBackPressedDispatcher. As of this writing, I have not yet done that, and it seems a rather dirty way of getting what you want.
Further sources used: "onBackPressed() deprecated, What is the alternative?" on Stack Overflow.
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