Fix bug validate observable value

Fix bug nested computed
Remove CancelableThen and use Timer object
Rewrite observable validation thus it can depend on other observable, remove isValid and changed to computed objects valid, error. We also can setError to mark observable invalid. When object changed valid, error status, then it also call notify change
Remove validation async since it require setup UI while validating depend on development context

Improve performance rebuild Computed by use one Future instead create many CancelableThen

Update to support null safety

Add base class ObservableWritable used mixin for Observable, Commission Add property hasValidator used to check has validate value Fix bug notify in Computed Export CancelableThen Check hasListener before execute callbacks

Do not recompute Computed has isValid but there are no listener

Add function toString

Support listener function with parameters
Reuse CancelableThen in ObservableValidator
Spell check

Update follow dart analyze Add example widget in Flutter

Update follow publish suggestions

First release