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Efficient by default zero-dependency declarative state/dependency management for flutter

Inheritable #

Efficient by default, zero-dependency, declarative state/dependency management for flutter.

Examples #

Consider the following mutable value of User

class User {
    String fname;
    String lname;

    // <proper Object.hashCode & Object.== implementations>

    String toString() {
        return 'User($fname $lname)';

Assume that there is widget to show the user's full name in ALL_CAPS.

class UserName extends StatelessWidget {
    Widget build(BuildContext context) {
        final fullName = context.aspect((User u) => '${u.fname} ${u.lname}'.trim().toUpperCase());
        return Text(fullName);

This UserName widget is being used something like

    Widget build(BuildContext context) {
        return Inheritable(
            value: User()..fname = 'John'..lname = 'Doe ',
            child: const UserName(),

For a User(John Doe ), the Widget would display JOHN DOE. The widget is currently listening for changes to the nearest User, if the user changes to User(JOHN DOE) which is essentially the same as what the widget would display anyway, Inheritable skips informing the widget about that change. However if the user were to change to User(John Doe2), the final result display to the user would change to JOHN DOE2, which is different from the last value, here Inheritable will rebuild the UserName widget.

In the previous example the UserName widget was using all available aspects of User i.e. fname and lname. Now consider,

class UserFirstName extends StatelessWidget {

    Widget build(BuildContext context) {
        final fName = context.aspect((User u) => u.fname.trim().toUpperCase());
        return Text(fName);

Similar to UserName, UserFirstName is being used something like

    Widget build(BuildContext context) {
        return Inheritable(
            value: User()..fname = 'John'..lname = 'Doe ',
            child: const UserFirstName(),

Now UserFirstName would display JOHN, if the user were to change to User(John Doe2), the UserFirstName would not be notified, since it only cares about the first name and Inheritable know about this. Not only that, Inheritable also knows that UserFirstName doesn't care about being notified until the trimmed and ALL_CAPS value of User.fname changes to something other than JOHN.

Checkout the tests for more examples.

Description #

Inheritable was initially made for an internal app, but extracted out to be a separate open-source package.

Inheritable is based on the pre-existing InheritableModel of flutter. The concepts are very similar.

Inheritable however has some advantages

  • Moves the decision of whether or not a widget should rebuild to itself. Even if the value held by Inheritable changes, along with the aspect, it's still up to the dependent to decide whether to rebuild.

  • Optionally allow sending updates e.g. context.aspect.update(User()..fname = 'Josh' ..lname = 'Doe')

    Notice that update directly takes the value you want to update, If there is a Inheritable.mutable available, it supplies the new value to it. That's all. Just like whether to rebuild or not is up to the dependent, whether to update the value or not is up to the owner of that value.

  • Update a value without depending on it. From the above examples UserFirstName could send an update for User.lname to be changed. However UserFirstName has only declared User.fname as a dependency. So User.lname will be updated without causing rebuild for UserFirstName. This allows for interesting scenarios such as sending data to siblings, parents or children widgets

  • Dynamically add aspects to listen for changes

  • Stop listening to changes at a later point.

  • Reuse aspects in multiple widgets. One could create the following aspect and pass it around to multiple widgets var fname = Aspect((Useru) => u.fname) The widget would then simply do fname.of(context) to get the value. Generally these would be dumb widgets that are only used for presentation purposes of a certain value type for example AllCapitalText(fname) would be a widget that requires some string aspect of some value, and display that in ALL_CAPS

  • Replace existing dependencies using Key. e.g. var fname = Aspect((Useru) => u.fname, key: Key('user.fname')) A widget using fname could later replace it by simply doing context.aspect((User u) => u.lname, key: fname.key); The widget would then stop listening for fname changes and start listening for lname changes. This works because the keys for both aspects are same.

  • Chaining aspects. One could also do

    var result  = Aspect((User u) => u.lname)
                  .where((lname) => lname!=null)
                  .map((lname) => lname.trim().toUpperCase())

    In the above case, you are filtering the values of User.lname to not be null. When it's null you simply won't be notified, it short-circuits the chain, so the map won't execute. When it's not null, only then it will be mapped, compared to last value, and you'd be notified if it was different. You would then pass the result around or immediately use it like result.of(context)

  • Composable: create/remove/reuse Inheritables and Aspects. It is encouraged to create custom implementations of InheritableAspect, such as a custom SpreadsheetCellAspect rebuilds a cell if any one of the cells in it's formula changes, So for a cell with value A1 + B1, it will only rebuild if A1 | B1 changes. And if you aren't building offstage widgets, it would make your spreadsheet even more efficient.

  • User definable behaviour for aspects. See Aspect and NoAspect implementations of InheritableAspect

  • Get asynchronous values in a synchronous fashion

  • Get Static/Compile-time errors for aspects that couldn't exist on a value. Contrary to InheritableModel's example use case instead of specifying "fname" aspect you specify (User u) => u.fname which wouldn't work if User didn't have fname, whereas "fname" would've silently been allowed, until you get a runtime error.

  • Short-circuit unnecessary work.

The idea behind Inheritable is that you specify your dependencies "declaratively" in a type-safe manner. More often than not I see dependencies being registered/declared/requested in a non-auto-completable fashion. If it's type-safe, it can be auto-completed.

Allow Presentation of a value via multiple widgets without causing unnecessary rebuilds. A User could be presented by UFistName & ULastName widgets. While it's possible to create InheritedWidget for User.fname and User.lname separately both of them will have same runtimeType. Which turns out to be a limitation of InheritedWidget. It doesn't allow multiple InheritedWidgets of same runtimeType to be available at the same time. Which, if you think about it, is fine, because the users often don't know how to distinctively request for 1 of them and not the other.

Since you'd either have to create 1 InheritedWidget<String> subclass for supplying String values to widgets (which would fail, since the last widget in hierarchy overrides the String value) or create 2 separate classes InheritedWidget<UserFirstName> & InheritedWidget<UserLastName> which is too verbose and very little reusability.

A single default implementation of Inheritable would be enough in this case. However it would be possible to even go further and define custom behaviour using custom implementations of Inheritable to allow various hierarchies, but it remains to be decided whether I want to support that and to what extent. Custom implementations of InheritableAspect should be enough in most cases. Allowing custom Inheritable implementations would only complicate things.

Notes #

  • Keys are useful but not required in most common cases

  • "Dependency" is used in the sense of state management, and not in the sense of dependency-injection. However is could be made possible to use Inheritable for such use cases

  • Using Inheritable won't magically make you app perform better and rebuild efficiently, However it will allow you to do just that. It depends on how well you understand flutter, dart, inheritable & most importantly your use case.

Roadmap #

  • Complete test suite
  • Whether to support custom implementations of Inheritable
  • Add examples
  • Update README with more examples and use cases

License #


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Efficient by default zero-dependency declarative state/dependency management for flutter

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