live_cells 0.6.1

A better way to manage state in Flutter applications

This package provides a replacement (ValueCell) for ChangeNotifier and ValueNotifier that is simpler to use and more flexible, as well as a library of widgets which expose their properties via ValueCell's replacing the need for controller objects and event handlers.

Features

This package provides a ValueCell interface which offers the following benefits over ChangeNotifier / ValueNotifier:

• Implementing a ValueCell which is an expression of other ValueCell's, e.g. a + b, can be done in a functional manner without manually adding and removing listeners using addListener, removeListener.
• Simpler resource management, no need to call dispose.
• (Still in early stages) A library of widgets which replaces "controller" objects with ValueCell's. This allows for a style of programming which fits in with the reactive paradigm of Flutter.

This package also has the following advantages over other state management libraries:

• Tightly integrated with a widget library replacing the need for event listeners and controller objects. Other libraries ignore this part and leave it up to the user to integrate the state management library with widgets.
• Supports two-way data flow, whereas most other libraries, if not all, only support one-way data flow.

Usage

Basics

The basis of this package is the ValueCell interface which provides the cell interface. Every cell has a value and a set of observers which react to changes in the value.

A MutableCell is a cell which can have its value property set directly:

final cell = MutableCell(0);
...
cell.value = 1;

When the value property is set the observers of the cell react to the change.

A cell which is a function of other cells can be created using the ValueCell.computed constructor, which takes the function that computes the cell's value.

final a = MutableCell(0);
final b = MutableCell(1);

final sum = ValueCell.computed(() => a() + b());

In the above example, cell sum computes the sum of the values of cells a and b. Whenever the value of either a or b changes the value of sum is recomputed using the new values.

NOTE: The values of a and b are accessed using the function call syntax rather than by accessing the value property. This is so that the computed cell can observe changes to the values of those cells and automatically recompute its own value.

Putting it all together let's implement the most trivial of examples, a simple counter:

import 'package:flutter/material.dart';
import 'package:live_cells/live_cells.dart';

class CounterDemo extends StatefulWidget {
  @override
  State<CounterDemo> createState() => _CounterDemoState();
}

class _CounterDemoState extends State<CounterDemo> {
  final counter = MutableCell(0);
  
  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        CellWidget.builder((context) =>
            Text('You clicked the button ${counter()} times')
        ),
        ElevatedButton(
          child: const Text('Increment Counter'),
          onPressed: () => counter.value += 1,
        )
      ],
    );
  }
}

CellWidget.builder is used to create a widget which is rebuilt whenever the value of counter changes. Like ValueCell.computed, the widget is rebuilt when the value of a cell, that is referenced within the widget builder function, changes.

NOTE: ValueCell's do not require manually calling a dispose method once they're no longer used. Disposal is taken care of automatically.

This example can be condensed further by moving the counter cell initialization directly within the build method:

import 'package:flutter/material.dart';
import 'package:live_cells/live_cells.dart';

class CounterExample extends CellWidget with CellInitializer {
  @override
  Widget build(BuildContext context) {
    final counter = cell(() => MutableCell(0));

    return Column(
      children: [
        CellWidget.builder((context) =>
            Text('You clicked the button ${counter()} times')
        ),
        ElevatedButton(
          child: const Text('Increment Counter'),
          onPressed: () => counter.value += 1,
        )
      ],
    );
  }
}

The counter cell is created directly within the build method using the cell method, provided by the CellInitializer mixin, which creates an instance of a ValueCell, using the provided function, on the first build of the widget and retrieves the existing instance in subsequent builds.

The following example demonstrates computed cells:

import 'package:flutter/material.dart';
import 'package:live_cells/live_cells.dart';

class ComputedExample extends CellWidget with CellInitializer {
  @override
  Widget build(BuildContext context) {
    final a = cell(() => MutableCell(0));
    final b = cell(() => MutableCell(0));

    final sum = cell(() => ValueCell.computed(() => a() + b()));

    return Column(
      children: [
        Row(
          children: [
            Expanded(
              child: TextField(
                onChanged: (value) {
                  a.value = int.tryParse(value) ?? 0;
                },
                keyboardType: const TextInputType.numberWithOptions(decimal: false),
              ),
            ),
            const SizedBox(width: 5),
            const Text('+'),
            const SizedBox(width: 5),
            Expanded(
              child: TextField(
                onChanged: (value) {
                  b.value = int.tryParse(value) ?? 0;
                },
                keyboardType: const TextInputType.numberWithOptions(decimal: false),
              ),
            ),
          ],
        ),
        const SizedBox(height: 10),
        CellWidget.builder((_) => Text(
            '${a()} + ${b()} = ${sum()}',
            style: const TextStyle(
                fontWeight: FontWeight.bold,
                fontSize: 20
            )
        ))
      ],
    );
  }
}

The above example demonstrates how the value of a computed cell, created using ValueCell.computed, is recomputed whenever the values of the referenced argument cells change.

The definition of the sum cell above can be simplified further to the following:

final sum = cell(() => a + b);

The arithmetic and relational operators are overloaded for ValueCell's holding num values, so that a computed cell can be defined as an expression of ValueCell's. This is not only simpler but more efficient since the argument cells are determined at compile time.

User Input

So far we've used the onChanged callback with the stock TextField provided by Flutter. This has two disadvantages:

• The content of the TextField cannot be set externally without a TextEditingController.
• You have to manually synchronize the state of the cells with the state of the text field, in an event handler.

Live cells provides a CellTextField widget which allows its content to be accessed and controlled by a ValueCell.

Example:

import 'package:flutter/material.dart';
import 'package:live_cells/live_cell_widgets.dart';
import 'package:live_cells/live_cells.dart';

class CellTextFieldDemo extends CellWidget with CellInitializer {
  @override
  Widget build(BuildContext context) {
    final input = cell(() => MutableCell(''));

    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        const SizedBox(height: 10),
        CellTextField(content: input),
        const SizedBox(height: 10),
        const Text(
            'You wrote:',
            style: TextStyle(
                fontWeight: FontWeight.bold,
                fontSize: 20
            )
        ),
        const SizedBox(height: 10),
        CellWidget.builder((_) => Text(input())),
        ElevatedButton(
          child: const Text('Clear'),
          onPressed: () {
            input.value = '';
          },
        )
      ],
    );
  }
}

The CellTextField constructor takes a content cell parameter, in this example the cell input is provided. The value of the provided content cell, which must be a MutableCell, is updated to reflect the content of the text field whenever it is changed by the user. Whatever the user writes in the field is reflected in the widget below.

The "Clear" button clears the text field by setting the input cell to the empty string. The benefits of this approach are:

• No need for a TextEditingController
• No need for event handlers allowing for a declarative style of programming
• The content of the text field is a cell and can be referenced by a computed cell

Two-way data flow

Whilst the above is an improvement over what is offered by the stock Flutter TextField, it is still quite limited in that the content cell has to be a string cell. You'll run into difficulties, if instead of string input you require numeric input, as in the sum example.

Mutable computed cells are MutableCell's which ordinarily function like normal computed cells, created with ValueCell.computed, in that they compute a value out of one or more argument cells. However, a mutable computed cell can also have its value changed by setting its value property as though it is a MutableCell. When the value of a mutable computed cell is set, it reverses the computation by setting the argument cells to a value such that when the mutable computed cell is recomputed, the same value will be produced as the value that was set. Thus mutable computed cells support two-way data flow, which is what sets Live Cells apart from other reactive state management libraries.

Mutable computed cells can be created using the MutableCell.computed constructor, which takes the computation function and reverse computation function. The computation function computes the cell's value as a function of argument cells, like ValueCell.computed. The reverse computation function reverses the computation by assigning a value to the argument cells. It is given the value that was assigned to the value property.

Here's a simple example:

final a = MutableCell<num>(0);
final strA = MutableCell.computed(() => a().toString(), (value) {
  a.value = num.tryParse(value) ?? 0;
});

The above mutable computation cell converts the value of its argument cell a, which is a num in this case, to a string. The reverse computation function parses a num from the string which was assigned to the cell. Assigning a string value to strA will result in the num parsed from the string being assigned to cell a.

strA.value = '100';
print(a.value + 1); // Prints 101

The above definition will prove useful when implementing a text field for numeric input. In-fact, this library already provides a definition for this cell with the mutableString extension method on MutableCell's holding int, double and num values.

final a = MutableCell<num>(0);
final strA = a.mutableString();

We can now reimplement the sum example from earlier using CellTextField and mutableString:

import 'package:flutter/material.dart';
import 'package:live_cells/live_cell_widgets.dart';
import 'package:live_cells/live_cells.dart';

class ComputedExample extends CellWidget with CellInitializer {
  @override
  Widget build(BuildContext context) {
    final a = cell(() => MutableCell<num>(0));
    final b = cell(() => MutableCell<num>(0));

    final strA = cell(() => a.mutableString());
    final strB = cell(() => b.mutableString());

    final sum = cell(() => a + b);

    return Column(
      children: [
        Row(
          children: [
            Expanded(
              child: TCellTextField(
                content: strA,
                keyboardType: TextInputType.number,
              ),
            ),
            const SizedBox(width: 5),
            const Text('+'),
            const SizedBox(width: 5),
            Expanded(
              child: CellTextField(
                content: strB,
                keyboardType: TextInputType.number,
              ),
            ),
          ],
        ),
        const SizedBox(height: 10),
        CellWidget.builder((_) => Text(
            '${a()} + ${b()} = ${sum()}',
            style: const TextStyle(
                fontWeight: FontWeight.bold,
                fontSize: 20
            )
        )),
        ElevatedButton(
          child: const Text('Reset'),
          onPressed: () {
            a.value = 0;
            b.value = 0;
          },
        )
      ],
    );
  }
}

In the above example two mutable computed cells strA and strB are created using mutableString, which are used as the content cells for the text fields for a and b respectively. There is also a "Reset" button which resets the values of cells a and b to 0 when pressed. When the values of a and b are set, the value of sum is automatically recomputed and the content of the text fields is updated to reflect the new values of a and b.

The benefits of using CellTextField and mutable computed cells are:

• No need for a TextEditingController which you have to remember to dispose.
• No manual synchronization of state between the TextEditingController and the widget State / ChangeNotifier object. Your state is instead stored in one place and in one representation.
• No need to use StatefulWidget or make ugly empty calls to setState(() {}) to force the widget to update when the text property of the TextEditingController is updated.

NOTE:

The reverse computation functions of mutable computed cells are called in a batch update, which means that all cell value updates performed within the function will be reflected, in the observers of the cells, only after the function returns.

A batch update can be done outside of a reverse computation function using Mutable.batch. In-fact the proper implementation of the "Reset" button is:

ElevatedButton( 
  child: const Text('Reset'),
  onPressed: () {
    MutableCell.batch(() {
      a.value = 0;
      b.value = 0;
    });
  },
)

With the above implementation cells a and b are both set to 0 simultaneously after the batch update function returns. With this implementation the sum cell, which is an observer of both a and b will only be recomputed once after both a and b are set to 0.

Fun with mutable computed cells

Let's say we want the user to be able to enter the result of the addition and have the values for a and b automatically computed and displayed in the corresponding fields:

We can do this with another mutable computed cell, this time with two arguments:

final sum = MutableCell.computed(() => a() + b(), (sum) {
  final half = sum / 2;

  a.value = half;
  b.value = half;
});

The reverse computation cell assigns the sum divided by two to both cells a and b.

Here's the full example with a CellTextField for the result of the addition:

import 'package:flutter/material.dart';
import 'package:live_cells/live_cell_widgets.dart';
import 'package:live_cells/live_cells.dart';

class ComputedExample extends CellWidget with CellInitializer {
  @override
  Widget build(BuildContext context) {
    final a = cell(() => MutableCell<num>(0));
    final b = cell(() => MutableCell<num>(0));

    final sum = cell(() => MutableCell.computed(() => a() + b(), (sum) {
      final half = sum / 2;

      a.value = half;
      b.value = half;
    }));

    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        const SizedBox(height: 10),
        Row(
          children: [
            Expanded(
              child: CellTextField(
                content: cell(() => a.mutableString()),
                keyboardType: TextInputType.number,
              ),
            ),
            const SizedBox(width: 5),
            const Text('+'),
            const SizedBox(width: 5),
            Expanded(
              child: CellTextField(
                content: cell(() => b.mutableString()),
                keyboardType: TextInputType.number,
              ),
            ),
            const SizedBox(width: 5),
            const Text('='),
            const SizedBox(width: 5),
            Expanded(
              child: CellTextField(
                content: cell(() => sum.mutableString()),
                keyboardType: TextInputType.number,
              ),
            )
          ],
        ),
        const SizedBox(height: 10),
        CellWidget.builder((_) => Text(
           '${a()} + ${b()} = ${sum()}', 
            style: const TextStyle(
                fontWeight: FontWeight.bold,
                fontSize: 20
            )
        )),
        ElevatedButton(
          child: const Text('Reset'),
          onPressed: () {
            MutableCell.batch(() {
              a.value = 0;
              b.value = 0;
            });
          },
        )
      ],
    );
  }
}

For brevity the definitions of the string conversion cells are placed directly in the CellTextField constructor. When a value for a and b is entered, the result of the addition is displayed in the result text field. When a value for the result is entered in the result text field, the values of a and b and reflected in their corresponding text fields. This example also demonstrates how mutable computed cells can be chained.

Other Widgets

We've already covered CellTextField. The live_cell_widgets library also provides the following widgets which expose their properties via ValueCell's:

• CellCheckbox - A Checkbox with the value property controlled by a cell
• CellCheckboxListTile - A CheckboxListTile with the value property controlled by a cell
• CellRadio - A Radio with the groupValue property controlled by a cell
• CellRadioListTile - A RadioListTile with the groupValue property controlled by a cell`
• CellSlider - A Slider with the value property controlled by a cell
• CellSwitch - A Switch with the value property controlled by a cell
• CellSwitchListTile - A SwitchListTile with the value property controlled by a cell

Error Handling

The user might enter text in the text field from which a number cannot be parsed. mutableString, as used in the previous examples, handles this by assigning a default value to its argument cell, which is controlled by the errorValue argument.

Example:

final strA = a.mutableString(
  errorValue: -1.cell
);

strA.vaue = 'not a valid number';

print(a.value); // Prints -1

In the above example a default vaue of -1 was set in the case that a number cannot be parsed from the value of the string cell.

NOTE: The errorValue argument is a ValueCell, which allows the default value to be changed dynamically. The cell extension property was used in the above example to create a constant value cell.

Usually, however, we want to handle the error rather than assigning a default value. This can be done with Maybe cells. A Maybe object either holds a value or an exception that was thrown while computing a value.

A Maybe cell, a cell holding a Maybe, can easily be created from a MutableCell with the maybe() method. The resulting Maybe cell is a mutable computed cell with the following behaviour:

• Its computed value is the value of the argument cell wrapped in a Maybe.
• When the cell's value is set, it sets the value of the argument cell to the value wrapped in the Maybe if it is holding a value.

The Maybe cell provides an error property which retrieves a ValueCell that evaluates to the exception held in the Maybe or null if the Maybe is holding a value. This can be used to determine whether an error occurred while computing a value.

To handle errors while parsing a number, mutableString should be called on a cell containing a Maybe<num> rather than a num. We can then check whether the error cell is non-null to check if an error occurred.

Putting it all together the cell definition for a now becomes:

final a = cell(() => MutableCell<num>(0));
final maybeA = cell(() => a.maybe());
final strA = cell(() => maybeA.mutableString());
final errorA = cell(() => maybeA.error);

• maybeA holds the the value of a wrapped in a Maybe.
• strA will be used as the content cell for a which binds the value of the text field to maybeA.
• errorA holds the error which occurred while parsing a number from strA.

The definition of the text field for a becomes:

CellTextField(
  content: strA,
  keyboardType: TextInputType.number,
  decoration: InputDecoration(
    errorText: errorA() != null
      ? 'Please enter a valid number'
      : null
    ),
)

Here we're testing whether errorA is non-null, that is whether an error occurred while parsing a number from strA and providing an error message in the errorText of the InputDecoration. NOTE: The value of errorA is accessed directly in the CellWidget, this can be done but it will cause the entire CellWidget to be rebuilt which may be inefficient.

The error message can be made more descriptive by also checking whether the field is empty, or not.

For example:

final isEmptyA = cell(() => ValueCell.computed(() => strA().isEmpty));

...

CellTextField(
  content: strA,
  keyboardType: TextInputType.number,
  decoration: InputDecoration(
    errorText: isEmpty()
        ? 'Cannot be empty'
        : error() != null
        ? 'Not a valid number'
        : null
    ),
  );
)

Here we've created a new cell isEmptyA which depends directly on strA (the content cell) and has a value of true if the strA holds an empty string.

You'll notice the cell definitions are becoming a bit unwieldy. To clean things up the definition for the text field, along with its related cells, can be packaged in a function:

Widget inputField(MutableCell<num> cell) {   
  return CellWidget.builder((context) {
    final maybe = context.cell(() => cell.maybe());
    final content = context.cell(() => maybe.mutableString());
    final error = context.cell(() => maybe.error);

    final isEmpty = context.cell(() => ValueCell.computed(() => content().isEmpty));

    return CellTextField(
      content: content,
      keyboardType: TextInputType.number,
      decoration: InputDecoration(
         errorText: isEmpty() 
              ? 'Cannot be empty'
              : error() != null
              ? 'Not a valid number'
              : null
      ),
    );
  }); 
}

Note we've used CellWidget.builder to create a CellWidget without subclassing and the cells are defined using the cell method of the context object provided to the builder function. This is identical to the cell method provided by the CellInitializer mixin. NOTE: This method may only be called on the BuildContext of a CellWidget with the CellInitializer mixin or a CellWidget created using CellWidget.builder.

The UI definition now becomes the following:

Widget build(BuildContext context) {   
  final a = cell(() => MutableCell<num>(0));
  final b = cell(() => MutableCell<num>(0));

  final sum = cell(() => a + b);

  return Column(
    children: [
      Row(
        children: [
          Expanded(
            child: inputField(a),
          ),
          const SizedBox(width: 5),
          const Text('+'),
          const SizedBox(width: 5),
          Expanded(
            child: inputField(b),
          ),
        ],
      ),
      const SizedBox(height: 10),
      CellWidget.builder((_) => Text(
         '${a()} + ${b()} = ${sum()}',
         style: const TextStyle(
             fontWeight: FontWeight.bold,
             fontSize: 20
         )
      )),
      ElevatedButton(
        child: const Text('Reset'),
        onPressed: () {
          MutableCell.batch(() {
            a.value = 0;
            b.value = 0;
          });
        },
      )
    ],
  );
}

Advanced

Optimization

ValueCell.computed and MutableCell.computed determine their dependencies, the argument cells referenced by the value computation function, at runtime. This allows you to conveniently create a computed cell without having to list out the referenced cells beforehand. However it also introduces additional overhead at runtime.

There are two ways to avoid this:

1. Use the overloaded operators or compose your computations out of the functions offered by the library, rather than implementing a value computation function.
2. Specify the argument cells manually

We've already seen an example of option 1, in the definition of the sum cell from earlier. For option 2 a computed cell can be defined with an explicit argument list using the computeCell extension method on List.

To create a computed cell with a static dependencies, call computeCell on a List containing the arguments cells referenced within the computation function:

final sum = [a, b].computeCell(() => a.value + b.value);

The example above shows a definition for the sum cell using the computeCell method. The arguments referenced within the computation function, passed to computeCell, are specified in the list on which the method is called. Note the value property is accessed directly rather than using the function call syntax since the cell dependencies are not determined at runtime.

This definition has two major differences from the definition using ValueCell.computed:

1. The argument cells are known at compile-time, eliminating the overhead of determining the argument cells at runtime.
2. A lightweight computed cell is created.

Cells created using computeCell are lightweight which means they neither store their own value nor track their own observers. Instead their value is computed on demand whenever the value property is accessed and all observers added to the cell are added directly to the argument cells.

The store method creates a cell which stores the value of a lightweight cell, created using computeCell, in memory so that it is not recomputed when the value property is is accessed again. This is useful for time-intensive computations.

final sum = [a, b].computeCell(() => a.value + b.value).store()

The above definition of the sum cell uses the store method to create a cell which stores its value instead of computing it on demand whenever the value property is accessed.

Mutable computed cells with static dependencies can be created using the mutableComputeCell method on List, which takes the computation and reverse computation functions. Like computeCell the arguments referenced within the computation function are specified in the list on which the method is called. Unlike computeCell the returned cell does store its own value and track its own listeners, so store is unnecessary.

final sum = [a, b].mutableComputeCell(() => a.value + b.value, (sum) {
  final half = sum / 2;
  a.value = half;
  b.value = half;
});

The above example shows the definition for the mutable sum cell, from the example in Fun with mutable computed cells, using mutableComputeCell and a static dependency list.

When should you use computeCell and mutableComputeCell? The ValueCell.computed and MutableCell.computed constructors are preferred since they are easier to use and reduce the risk of bugs caused by omitting an argument cell from the depency list. Use computeCell and mutableComputeCell if you'd like to optimize your code after it is working correctly and even then it is preferred you limit their usage to within reusable components, such as functions or methods which create cells.

Writing your own cells

Rarely will you need to write your own ValueCell subclasses but should the need arise, Live Cells can be extended.

To subclass ValueCell, the following methods have to be implemented:

• The get value property accessor to return the cell's value.
• The addObserver and removeObserver methods to add and remove observers, respectively.
• The equality comparison methods eq and neq.

The CellEquality mixin provides implementations of the equality comparison methods using the == and != operators for a given type.

To implement a lightweight cell which performs a computation on the values of one or more argument cells, you should extend DependentCell. This class already implements addObserver and removeObserver, and provides a constructor which takes a list of the argument cells on which the value of the cell depends. You're only required to implement the value property accessor in which the value of the cell is computed.

Example:

class ClampCell<T extends num> extends DependentCell<T> with CellEquality<T> {
  final ValueCell<T> argValue;
  final ValueCell<T> argMin;
  final ValueCell<T> argMax;
  
  ClampCell(this.argValue, this.argMin, this.argMax) :
    super([argValue, argMin, argMax]);

  @override
  T get value => min(argMax.value, max(argMin.value, argValue.value));
}

The above ValueCell subclass implements a cell of which the value is the value of an argument cell clamped between a minimum and maximum which are also supplied in argument cells.

NOTE: The argument cell containing the value to be clamped as well as the cells containing the minimum and maximum are all passed to the constructor of DependentCell so that the observers of the ClampCell are called whenever the values of the argument cells change.

If you need to implement a cell which initiates changes to its value you will need to subclass NotifierCell. This class provides implementations of addObserver, removeObserver and the get value property accessor. This class also provides a protected set value property accessor for setting the cell's value. When the value of the cell is set via set value the observers of the cell are notified, if the new value is not equal to the previous value.

Resource Management

In Flutter resources are typically acquired in a constructor or init method and are released by calling a dispose method. You'll notice that there are no calls to dispose anywhere in any of these examples. This package takes a slightly different approach. The cells which require manual resource management implement the ManagedCell interface, which NotifierCell extends.

ManagedCell provides an init method where resources should be acquired and a dispose method where resources should be released. The init method is called before the first observer is added to the cell and dispose is called after the last observer is removed. The init method may be called again after dispose if a new observer is added after the last one is removed. Therefore implementations of ManagedCell should be written in such a way that the cell can be reused after dispose is called.

Below is an example of a NotifierCell subclass which overrides the ManagedCell methods:

import 'dart:async';

import 'package:live_cells/live_cells.dart';

class CountCell extends NotifierCell<int> {
  final int end;
  final Duration interval;

  Timer? _timer;

  CountCell(this.end, {
    this.interval = const Duration(seconds: 1)
  }) : super(0);

  @override
  void init() {
    super.init();

    _timer = Timer.periodic(interval, _timerTick);
  }

  @override
  void dispose() {
    _timer?.cancel();
    _timer = null;

    super.dispose();
  }
  
  void _timerTick(Timer timer) {
    if (value >= end) {
      timer.cancel();
    }
    
    value++;
  }
}

The CountCell class above implements a cell which increments its value by 1 every second. The initial value of 0 is given in the call to the NotifierCell constructor super(0). A timer is initialized in init which increments the value property directly every time the timer callback is called, hence the cell starts "counting" after the first observer is added. The timer is cancelled in the dispose method to stop the cell from incrementing its value after the last observer is removed.

Observing Cells

Observers of cells implement the CellObserver interface which has the following methods:

• willUpdate -- Called before the value of a cell changes.
• update -- Called after the value of a cell has changed.
• shouldNotifyAlways -- Should the observer be notified if the new value of the observed cell is equal to the previous value.

When a cell's value is changed first its observers are notified that its value will changed, by calling the willUpdate method and then after its value is set, its observers are notified that the value has changed, by calling the update method.

If you're implementing a ValueCell subclass which observes and reacts to changes in the values of other cells you'll have to properly implement both willUpdate and update.

The correct behaviour of willUpdate is to mark the cell's value as stale and call the willUpdate method of the cell's observers. When the cell's value is referenced while it is marked as stale, the value should be recomputed even if it is referenced before the update method is called.

The correct behaviour of update is to recompute the cell's value, if it hasn't been recomputed already, and call the update method of the cell's observers.

The ObserverCell mixin already provides definitions of willUpdate and update for a cell which reacts to changes in the value of other cells. You should mixin ObserverCell into your NotifierCell subclass and add the cell as an observer of the cells it should observer changes in. The mixin also provides a stale property which is true when the value of the cell should be recomputed. Override the get value property accessor and recompute the cell'a value if stale is true.

NOTE:

The cell must add itself as an observer to its dependent cells in the init method and remove itself in the dispose method to ensure that no resources are leaked:

class MyCell<T> extends NotifierCell<T> with ObserverCell<T> {
  final ValueCell<T> arg1;
  final ValueCell<T> arg2;
  
  MyCell(super.value, {
    required this.arg1,
    required this.arg2
  });
  
  @override
  T get value {
    if (stale) {
      // recompute value
      ...
      stale = false;
    }
    
    return super.value;
  }
  
  @override
  void init() {
    super.init();
    
    arg1.addObserver(this);
    arg2.addObserver(this);
  }
  
  @override
  void dispose() {
    arg1.removeObserver(this);
    arg2.removeObserver(this);
    
    super.dispose();
  }
  
  ...
}

The shouldNotifyAlways property should be true if the observer should be notified when an observed cell is assigned a new value that is equal to the previous value. Note: This only applies when observing a MutableCell and then only when its value property is set directly. By default this is false.

ValueListenable Interface

If you're not implementing a ValueCell subclass and you're only interested in the update method of the CellObserver interface, you can use the listenable property of ValueCell to retrieve a ValueListenable object which calls its listeners, whenever the value of the cell changes. Listeners are added using addListener and removed using removeListener.

NOTE:

Every call to addListener has to be matched by a call to removeListener for the same listener function when the listener is no longer required.

Additional information

If you discover any issues or have any feature requests, please open an issue on the package's Github repository.

Take a look at the example directory for more complete examples of how to use this library.