Small code repo to illustrate by issue
Some links:
I am trying to create a global state management where model changes are broadcasted up
the widget tree using notifications.
Also, I want notifications to be able to propagate down the Navigator stack, and this for an indefinite amount of pages. When we push a page, we want to be able to catch the notifications going up said pushed page despite them being stacked at the MaterialApp's level in the widget tree.
I came up with a simple solution that you can see in the MyPageRoute class.
Code should be fairly simple to read and understand so I hope I was clear enough when
describing my project.
that the code from this repo is an abstraction of the app I am working on.
So yes sometimes the code will be unecessarily complex because it is only an isolatation of the problem.
- Launch app
- You can see an item list
- Tap any item tile
- Tap the floating action button to change the widget's status
- Go back to item list
- Item tile is not updated? (I am expecting it to be rebuild/rerendered)
The item list was definitely modified after catching the ItemChangedNotification in the NotificationListener.
Yet, when the ListView.builder will rebuild its elements, the modified ItemTile will not rerender its child ItemTileTitle who is in charge of showing the item's status.
I am thinking this could be the reason why
but I am not certain because this could also be an issue linked to me passing data to my childrens using a Provider which could lead Flutter to think there was no actual state change...
But I'm not sure.
Also could be because this is an issue related to the widget's lifecycle and how the ItemTile will not rebuild its child ItemTileTitle because nothing has really changed in the widget tree. Only the Provider's value changed.
But then why is the context.select from the build method of ItemTileTitle not rebuilding if the Provider's value changed?
This current part describes my main problem. Anything below are more errors caused by me trying to fix the above problem.
I am considering forcing flutter to rebuild all of my ItemTiles no matter what, by
giving them unconditionally a UniqueKey.
So I changed the following code:
In main.dart at around :68
Widget _itemBuilder(BuildContext context, ItemModel item, int index) {
return ItemTile(key: UniqueKey() /* or ValueKey(item) */, item: item);
}- Launch app
- You can see an item list
- Tap any item tile
- Tap the floating action button to change the widget's status
- Go back to item list
- The item is now updated!
- but...
- Tap any item (again)
- Tap the floating action button MORE THAN ONCE to change the widget's status back and forth
- ERROR, null check value
- error copypasta below
- ???
- Why is this happening?
My understanding of flutter is not advanced enough to accurately understand why this is happening :( so I hope someone can identify the problem!
I know that at least without the changes from this section, the MyPageRoute <> NotificationListener combo can successfully propagate notifications along the Navigation stack so I don't think the issue is directly related to that
════════ Exception caught by gesture ═══════════════════════════════════════════
The following _CastError was thrown while handling a gesture:
Null check operator used on a null value
When the exception was thrown, this was the stack
#0 Element.widget
package:flutter/…/widgets/framework.dart:3229
#1 _NotificationElement.onNotification
package:flutter/…/widgets/notification_listener.dart:128
#2 _NotificationNode.dispatchNotification
package:flutter/…/widgets/framework.dart:3078
#3 Element.dispatchNotification
package:flutter/…/widgets/framework.dart:4375
#4 Notification.dispatch
package:flutter/…/widgets/notification_listener.dart:60
#5 MyPageRoute.onNotification.<anonymous closure>
package:error_notification_listener/my_page_route.dart:12
#6 MyPageRoute.buildPage.<anonymous closure>
package:error_notification_listener/my_page_route.dart:26
#7 _NotificationElement.onNotification
package:flutter/…/widgets/notification_listener.dart:130
#8 _NotificationNode.dispatchNotification
package:flutter/…/widgets/framework.dart:3078
#9 Element.dispatchNotification
package:flutter/…/widgets/framework.dart:4375
#10 Notification.dispatch
package:flutter/…/widgets/notification_listener.dart:60
#11 _ItemDetailsState.build.<anonymous closure>
package:error_notification_listener/item_details_page.dart:61
#12 _InkResponseState._handleTap
package:flutter/…/material/ink_well.dart:1005
#13 GestureRecognizer.invokeCallback
package:flutter/…/gestures/recognizer.dart:198
#14 TapGestureRecognizer.handleTapUp
package:flutter/…/gestures/tap.dart:613
#15 BaseTapGestureRecognizer._checkUp
package:flutter/…/gestures/tap.dart:298
#16 BaseTapGestureRecognizer.handlePrimaryPointer
package:flutter/…/gestures/tap.dart:232
#17 PrimaryPointerGestureRecognizer.handleEvent
package:flutter/…/gestures/recognizer.dart:563
#18 PointerRouter._dispatch
package:flutter/…/gestures/pointer_router.dart:94
#19 PointerRouter._dispatchEventToRoutes.<anonymous closure>
package:flutter/…/gestures/pointer_router.dart:139
#20 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:614:13)
#21 PointerRouter._dispatchEventToRoutes
package:flutter/…/gestures/pointer_router.dart:137
#22 PointerRouter.route
package:flutter/…/gestures/pointer_router.dart:123
#23 GestureBinding.handleEvent
package:flutter/…/gestures/binding.dart:445
#24 GestureBinding.dispatchEvent
package:flutter/…/gestures/binding.dart:425
#25 RendererBinding.dispatchEvent
package:flutter/…/rendering/binding.dart:329
#26 GestureBinding._handlePointerEventImmediately
package:flutter/…/gestures/binding.dart:380
#27 GestureBinding.handlePointerEvent
package:flutter/…/gestures/binding.dart:344
#28 GestureBinding._flushPointerEventQueue
package:flutter/…/gestures/binding.dart:302
#29 GestureBinding._handlePointerDataPacket
package:flutter/…/gestures/binding.dart:285
#33 _invoke1 (dart:ui/hooks.dart:170:10)
#34 PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:331:7)
#35 _dispatchPointerDataPacket (dart:ui/hooks.dart:94:31)
(elided 3 frames from dart:async)
Handler: "onTap"
Recognizer: TapGestureRecognizer#8a772
debugOwner: GestureDetector
state: possible
won arena
finalPosition: Offset(356.8, 798.9)
finalLocalPosition: Offset(44.8, 33.6)
button: 1
sent tap down
════════════════════════════════════════════════════════════════════════════════
When pushing my ItemDetailsPage, I'm using the ItemTile's BuildContext to pass the onNotification callback.
But when rebuilding my ItemTile after a model change in the ItemDetailsPage, the onNotification will still be using the same BuildContext from an old (now unmounted) widget.
Which is why the NotificationListener issues the above error.
Check the fixed branch