Deprecation guide for (action) / {{action}}

content/ember/v5/deprecate-template-action.md

@@ -0,0 +1,170 @@
+---
+title: Action helper and modifier 
+until: 6.0.0
+since: 5.9.0
+---
+
+### Scenario: `action` is passed a string
+
+Before:
+```handlebars
+<button type="button" {{action "plusOne"}}>
+  Click Me
+</button>
+```
+
+After
+
+```handlebars
+<button type="button" {{on 'click' this.plusOne}}>
+  Click Me
+</button>
+```
+or, if `plusOne` is passed in as an argument 
+```handlebars
+<button type="button" {{on 'click' @plusOne}}>
+  Click Me
+</button>
+```
+
+If the `plusOne` action is in an actions object, it needs to move out:
+
+Before:
+```javascript
+import Component from '@glimmer/component';
+
+export default class Demo extends Component {
+    actions = {
+        plusOne() {
+           /* ... */ 
+        }
+    }
+}
+```
+After:
+```javascript
+import Component from '@glimmer/component';
+import { action } from '@ember/object';
+
+export default class Demo extends Component {
+    @action
+    plusOne() {
+       /* ... */ 
+    }
+}
+```
+
+or
+
+Before:
+```javascript
+import Component from '@ember/component';
+
+export default class Demo extends Component {
+    actions = {
+        plusOne() {
+           /* ... */ 
+        }
+    }
+}
+```
+
+After:
+```javascript
+import Component from '@ember/component';
+import { action } from '@ember/object';
+
+export default class Demo extends Component {
+    @action
+    plusOne() {
+       /* ... */ 
+    }
+}
+```
+or
+
+Before:
+```javascript
+import Component from '@ember/component';
+
+export default Component.extend({
+    actions: {
+        plusOne() {
+           /* ... */ 
+        }
+    }
+})
+```
+
+After:
+```javascript
+import Component from '@ember/component';
+import { action } from '@ember/object';
+
+export default Component.extend({
+  plusOne: action(function() {
+      /* ... */ 
+  }),
+})
+```
+
+If `(action)` or `{{action}}` is passed a string, it's _possible_ that the referenced method is declared on the caller, and _not_ the immediate component -- that is, `(action)` and `{{action}}` bubble up the render tree.
+
+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:
+```handlebars
+<SomeComponent @update={{action this.plusOne}} />
+```
+
+After
+
+```handlebars
+<SomeComponent @update={{this.plusOne}} />
+```
+
+### Scenario: `action` is passed parameters
+
+Before:
+```handlebars
+<SomeComponent @update={{action this.plus 1}} />
+```
+
+After:
+```handlebars
+<SomeComponent @update={{fn this.plus 1}} />
+```
+
+### Scenario: `action` is used with `mut` 
+
+Before:
+```handlebars
+<SomeComponent @update={{action (mut @value.property}} />
+```
+After:
+```javascript
+// 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; 
+    }
+}
+```
+```handlebars
+{{! 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)
+
+
+For more background, read the [RFC](https://github.com/emberjs/rfcs/pull/1006)