Observer.optimized

[subzero911]

I already did a PR #909 but I accidentally deleted the forked repo.
So I made it again and a little bit better.

Mostly you try to make Observer as small as possible. But sometimes you'll want to exclude the widget subtree from re-render and improve the performance.

Real world examples:

1. You have a colored Container with some heavy widgets inside (like ListView with shrinkWrap: true).
   You want to change the color by Observable, but don't want to rebuild the whole ListView.
2. You have some fancy animation driven by Observable. You want to animate the widget and want to preserve its children from being rebuilt.

I introduce the new Observer.optimized which allows you to exclude the child branch.
Usage:

Observer.optimized(
  builderOptimized: (context, child) => FooWidget(foo: foo, child: child),
  child: BarWidget(), // is not rebuilt
),

Solves #551

@amondnet @fzyzcjy

---

Pull Request Checklist

• If the changes are being made to code, ensure the version in pubspec.yaml is updated.
• Increment the major/minor/patch/patch-count, depending on the complexity of change
• Add the necessary unit tests to ensure the coverage does not drop
• Update the Changelog to include all changes made in this PR, organized by version
• Run the melos run set_version command from the root directory
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[codecov]

Codecov Report

Merging #952 (bf2623f) into master (7510ee4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #952   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files          57       57           
  Lines        1992     1998    +6     
=======================================
+ Hits         1972     1978    +6     
  Misses         20       20           

Flag	Coverage Δ
flutter_mobx	100.00% <100.00%> (ø)
mobx	98.55% <ø> (ø)
mobx_codegen	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
flutter_mobx/lib/src/observer.dart	100.00% <100.00%> (ø)

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[pavanpodila]

We should call this withChild to make the intent clear. The idea is to only build parts that are actually changing and keeping rest of the tree intact. I think you would also need a withChildren variant as well which could be a Map<String, Widget>?. This variant will help in plugging in parts of the tree which are not changing. This is useful when you have more than one child.

[subzero911]

@pavanpodila done. I renamed .optimized to .withChild and builderOptimized to builderWithChild
I have no idea how to implement "withChildren" constructor to "exclude subtrees by map keys", and I don't think we actually need it. You can use a Column(children: [...]) as a child, or you can use a composition of Observers.

I applied the same technique which was used in AnimatedBuilder, and provider's Consumer and Selector - they all have the child parameter only. Refer to https://api.flutter.dev/flutter/widgets/AnimatedBuilder-class.html , "Performance optimizations" for details. They don't have any multiple children version.

[pavanpodila]

@all-contributors add @subzero911 for code

[allcontributors]

@pavanpodila

@subzero911 already contributed before to code