Mutate Observer

[pyhinox]

Objective

• Adopts Add OnMutate observer and demonstrate how to use it for UI reactivity #14520 and adds OnMutate observers.
• Provide a mechanism for immediate changes in response to mutate event.

Solution

A EntityChanges which stores the change list is added to World. When Mut::set_changed is called, a change record will be added to the EntityChanges.
When World::flush is called, EntityChanges will be clear and trigger OnMutate event to each relative entity.

Example

world.observe(|trigger: Trigger<OnMutate, Transform>, query: Query<&Transform>| {
    let transform = query.get(trigger.entity()).unwrap();
});

Testing

• Did you test these changes? If so, how?
  Just test for the base function, I don't know which test i should do.
• Are there any parts that need more testing?
  Query::iter and Query::get_mut's benchmark
• How can other people (reviewers) test your changes? Is there anything specific they need to know?
  Just treat the OnMutate as a common observer. Nothing is specific.
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test?
  Mac

Possible Follow-ups

1. Add an OnMutate hook

[github-actions]

Welcome, new contributor!

Please make sure you've read our contributing guide and we look forward to reviewing your pull request shortly ✨

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[pyhinox]

@alice-i-cecile Hi alice, i thought this pr is ready to review now. What should I do next?