Have you ever come across this error when using a BuildContext after awaiting for some work to complete?
This is a common mistake that can lead to unexpected behavior.
So let's learn what causes it and how to fix it (thread). 🧵
In Flutter, the BuildContext identifies the element that corresponds to our widget in the element tree.
At any point (and especially after an asynchronous operation), our widget may be no longer mounted.
For example, consider what happens when:
- we tap on the button that triggers the
onPressedcallback - we immediately navigate away using the back button
- when the Future completes, we try to use
Navigator.of(context)
This leads to the big scary error that we see here. 👇
This happens because our context is stale and no longer points to a valid element by the time the Future completes.
To work around this, we need to check that the widget is mounted before attempting to use the context:
Here's another example where we try to show a SnackBar after an asynchronous operation completes.
In this case, checking if the widget is mounted will prevent an error.
But it also means that the user won't see the SnackBar.
So how can we ensure the SnackBar is presented?
In this case, we should get the resource we need (ScaffoldMessenger) before the asynchronous gap.
And this makes it safe to use it after the asynchronous gap. 👇
Note that if you use flutter_lints 2.0.0 or above, the analyzer will warn you if you use a BuildContext across an asynchronous gap.
This makes it a lot easier to spot places where you need to fix your code before running your app, rather than bumping into errors at runtime.
For more info about using BuildContext synchronously, check out this excellent video by the Flutter team:
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