Deprecate the (action) template helper and modifier

text/1006-deprecate-action-template-helper.md

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+---
+stage: accepted
+start-date: 2024-02-13T00:00:00.000Z
+release-date:
+release-versions:
+teams: # delete teams that aren't relevant
+  - cli
+  - data
+  - framework
+  - learning
+  - steering
+  - typescript
+prs:
+  accepted: https://github.com/emberjs/rfcs/pull/1006
+project-link:
+---
+
+<!---
+Directions for above:
+
+stage: Leave as is
+start-date: Fill in with today's date, 2032-12-01T00:00:00.000Z
+release-date: Leave as is
+release-versions: Leave as is
+teams: Include only the [team(s)](README.md#relevant-teams) for which this RFC applies
+prs:
+  accepted: Fill this in with the URL for the Proposal RFC PR
+project-link: Leave as is
+-->
+
+# Deprecate `(action)` template helper and `{{action}}` modifier. 
+
+## Summary
+
+The `(action)` template helper and `{{action}}` modifier was common pre-Octane. Now that we have native classes and the `{{on}}` modifier, we no longer need to use `(action)` or `{{action}}`
+
+## Motivation
+
+Remove legacy code with confusing semantics.
+
+This is a part of _[Deprecating Ember Classic (pre-Octane)](https://github.com/emberjs/rfcs/issues/832)_.
+
+## Transition Path
+
+This was written in the [Octave vs Classic cheatsheet](https://ember-learn.github.io/ember-octane-vs-classic-cheat-sheet/#component-properties__ddau)
+
+<details><summary>that content here</summary>
+
+### Before (pre-Octane)
+
+```js
+// parent-component.js
+import Component from '@ember/component';
+
+export default Component.extend({
+  count: 0
+});
+
+```
+```hbs
+{{!-- parent-component.hbs --}}
+{{child-component count=count}}
+Count: {{this.count}}
+
+```
+```js
+// child-component.js
+import Component from '@ember/component';
+
+export default Component.extend({
+  actions: {
+    plusOne() {
+      this.set('count', this.get('count') + 1);
+    }
+  }
+});
+```
+```hbs
+{{!-- child-component.hbs --}}
+<button type="button" {{action "plusOne"}}>
+  Click Me
+</button>
+```
+
+### After (post-Octane)
+```js
+// parent-component.js
+import Component from '@glimmer/component';
+import { tracked } from '@glimmer/tracking';
+import { action } from '@ember/object';
+
+export default class ParentComponent extends Component {
+  @tracked count = 0;
+
+  @action plusOne() {
+    this.count++;
+  }
+}
+
+```
+```hbs
+{{!-- parent-component.hbs --}}
+<ChildComponent @plusOne={{this.plusOne}} />
+Count: {{this.count}}
+
+```
+```hbs
+{{!-- child-component.hbs --}}
+<button type="button" {{on "click" @plusOne}}>
+  Click Me
+</button>
+
+```
+
+</details>
+
+But what we could put in the deprecation app:
+
+### Scenario: `action` is passed a string
+
+Before:
+```hbs
+<button type="button" {{action "plusOne"}}>
+  Click Me
+</button>
+```
+
+After
+
+```hbs
+<button type="button" {{on 'click' this.plusOne}}>
+  Click Me
+</button>
+```
+or, if `plusOne` is passed in as an argument 
+```hbs
+<button type="button" {{on 'click' @plusOne}}>
+  Click Me
+</button>
+```
+
+If the `plusOne` action is in an actions object, it needs to move out:
+
+Before:
+```js
+import Component from '@glimmer/component';
+
+export default class Demo extends Component {
+    actions = {
+        plusOne() {
+           /* ... */ 
+        }
+    }
+}
+```
+or
+```js
+import Component from '@ember/component';
+
+export default class Demo extends Component {
+    actions = {
+        plusOne() {
+           /* ... */ 
+        }
+    }
+}
+```
+or
+```js
+import Component from '@ember/component';
+
+export default Component.extend({
+    actions: {
+        plusOne() {
+           /* ... */ 
+        }
+    }
+})
+```
+
+After:
+```js
+import Component from '@glimmer/component';
+import { action } from '@ember/object';
+
+export default class Demo extends Component {
+    @action
+    plusOne() {
+       /* ... */ 
+    }
+}
+```
+
+Note that `@action` is completely different from `(action)` or `{{action}}` (and is partly a motivator for deprecating `(action)` and `{{action}}`, to reduce ambiguity).
+
+`@action` is binds the `this` on the method to the instance of the class. 
+
+### Scenario: `action` is passed a function reference
+
+Before:
+```hbs
+<SomeComponent @update={{action this.plusOne}} />
+```
+
+After
+
+```hbs
+<SomeComponent @update={{this.plusOne}} />
+```
+
+### Scenario: `action` is passed parameters
+
+Before:
+```hbs
+<SomeComponent @update={{action this.plus 1}} />
+```
+
+After:
+```hbs
+<SomeComponent @update={{fn this.plus 1}} />
+```
+
+### Scenario: `action` is used with `mut` 
+
+Before:
+```hbs
+<SomeComponent @update={{action (mut @value.property)}} />
+```
+After:
+```js
+// parent.js
+import Component from '@glimmer/component';
+import { action } from '@ember/object';
+
+export default class SomeComponent extends Component {
+    @action
+    handleUpdate(value) {
+        this.args.property = value; 
+    }
+}
+```
+```hbs
+{{! parent.hbs }}
+<SomeComponent @update={{this.handleUpdate}} />
+```
+
+Related, [Combining function arguments with action functions](https://guides.emberjs.com/release/components/component-state-and-actions/#toc_combining-arguments-and-actions)
+
+### Related: `send`
+
+When removing `(action)` or `{{action}}` with a _string_ name, you'll also need to verify that there are no [`send`](https://api.emberjs.com/ember/5.6/classes/Component/methods/send?anchor=send) calls with that same string.
+
+## How We Teach This
+
+The guides already cover how to invoke functions in the modern way.
+
+Remove: https://api.emberjs.com/ember/5.6/classes/Ember.Templates.helpers/methods/action?anchor=action
+
+## Drawbacks
+
+Older code will stop working once the deprecated code is removed.
+
+## Alternatives
+
+- adding an import so folks can keep using action in gjs.
+  I don't think we should do this because we want to clean up antiquated patterns, rather than encourage their continued existence.
+
+## Unresolved questions
+
+- Could there be a codemod?
+  _Potentially_ for action usage that references `this.properties`. For string actions, it's impossible.
+