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Description:

back button interceptor

back_button_interceptor #
In simple cases, when you need to intercept the Android back-button, you usually add WillPopScope
to your widget tree. However, when developing stateful widgets that interact with the back button,
it's more convenient to use the BackButtonInterceptor.
You may add interceptor functions to be called when the back button is tapped. These functions
may perform some useful work, and then, if any of them return true, the default button process
(usually popping a Route) will not be fired.
Android 13 #
To make it work on Android 13 and up, set this to false in the Android manifest:
android:enableOnBackInvokedCallback="false"
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In more detail #
All added functions are called, in order. If any function returns true, the
combined result is true, and the default button process will NOT be fired. Only if all functions
return false (or null), the combined result is false, and the default button process will be
fired.
Optionally, you may provide a z-index. Functions with a valid z-index will be called before
functions with a null z-index, and functions with larger z-index will be called first. When they
have the same z-index, functions added last are called first.
Each function gets the boolean stopDefaultButtonEvent that indicates the current combined result
from all the previous functions. So, if some function doesn't want to fire if some other previous
function already fired, it can do:
if (stopDefaultButtonEvent) return false;
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The same result may be obtained if the optional ifNotYetIntercepted parameter is true. Then the
function will only be called if all previous functions returned false
(that is, if stopDefaultButtonEvent is false).
Notes #

After you've finished you MUST remove each function by calling the remove() method.
Alternatively, you may also provide a name when adding a function, and then later remove it by
calling the removeByName(name) method.

If any of your interceptors throw an error, the error message will be printed to the console, but
the error thrown will be a general error with not much information. You can change the treatment
of errors by changing the static errorProcessing field.

Your functions can also process information about routes by using the function's RouteInfo info
parameter. To get the current route in the navigator, call info.currentRoute(context).
Also, info.routeWhenAdded contains the route that was the current one when the interceptor was
added through the BackButtonInterceptor.add() method.

You can set up some function so that it only runs when the current route is the same route of when
the interceptor was created. To that end, you can use
info.ifRouteChanged() method:
bool myInterceptor(bool stopDefaultButtonEvent, RouteInfo info) {
if (info.ifRouteChanged(context)) return false;
...
}
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This means the interceptor function is temporarily disabled while, for example, a dialog is open.

You can set up some function so that it only runs in certain routes:
bool myInterceptor(bool stopDefaultButtonEvent, RouteInfo info) {
if (["myRoute1", "myRoute2", "myRoute3"]
.contains(info.currentRoute(context))) return false;
...
}
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Both info.routeWhenAdded and info.ifRouteChanged() only work if you passed the context
parameter to the BackButtonInterceptor.add() method. Otherwise, info.routeWhenAdded will
be null and info.ifRouteChanged()
will thrown an error.

The current route can also be obtained by using the static
method BackButtonInterceptor.getCurrentNavigatorRouteName(context).

The interceptor function can return bool or Future<bool>.

Examples #
Intercepting #
@override
void initState() {
super.initState();
BackButtonInterceptor.add(myInterceptor);
}

@override
void dispose() {
BackButtonInterceptor.remove(myInterceptor);
super.dispose();
}

bool myInterceptor(bool stopDefaultButtonEvent, RouteInfo info) {
print("BACK BUTTON!"); // Do some stuff.
return true;
}
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Named function and z-index #
@override
void initState() {
super.initState();
BackButtonInterceptor.add(myInterceptor, zIndex:2, name:"SomeName");
}

@override
void dispose() {
BackButtonInterceptor.removeByName("SomeName");
super.dispose();
}

bool myInterceptor(bool stopDefaultButtonEvent, RouteInfo info) {
print("BACK BUTTON!"); // Do some stuff.
return true;
}
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Runnable Examples #

main

Intercepts the back-button and prints a string to the console.

main

Intercepts the back-button and prints a string to the console, by using an async interceptor
function.

main_complex_example

The first screen has a button which opens a second screen. The second screen has 3 red squares.
By tapping the Android back-button (or the "pop" button) each square turns blue, one by one. Only
when all squares are blue, tapping the back-button once more will return to the previous screen.
Also, if you click the "Open Dialog" button, the interceptors are disabled while the dialog is
open.

main_complex_example_test

This package is test friendly, and this examples shows how to test the back button.

Testing #
For testing purposes, the BackButtonInterceptor.popRoute() method may be called directly, to
simulate pressing the back button. The list of all fired functions and their results is
recorded in the BackButtonInterceptor.results static variable.
Note: You may want to add BackButtonInterceptor.removeAll();
to your test's setUp function, so that you remove old interceptors between tests.
Debugging #
In complex cases, to make it easier for you to debug your interceptors, you may print them to the
console, with their names and z-indexes, by doing:
print(BackButtonInterceptor.describe());
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See also: #

https://docs.flutter.io/flutter/widgets/WillPopScope-class.html
https://stackoverflow.com/questions/45916658/de-activate-system-back-button-in-flutter-app-toddler-navigation

The Flutter packages I've authored:

async_redux
fast_immutable_collections
provider_for_redux
i18n_extension
align_positioned
network_to_file_image
image_pixels
matrix4_transform
back_button_interceptor
indexed_list_view
animated_size_and_fade
assorted_layout_widgets
weak_map
themed
bdd_framework

My Medium Articles:

Async Redux: Flutter’s non-boilerplate version of Redux (
versions:
Português)

i18n_extension (
versions:
Português)

Flutter: The Advanced Layout Rule Even Beginners Must Know (
versions: русский)

The New Way to create Themes in your Flutter App

My article in the official Flutter documentation:

Understanding constraints

Marcelo Glasberg:
https://github.com/marcglasberg
https://linkedin.com/in/marcglasberg/
https://twitter.com/glasbergmarcelo
https://stackoverflow.com/users/3411681/marcg
https://medium.com/@marcglasberg