RFC to deprecate component#sendAction

text/0000-deprecate-send-action.md

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+- Start Date: 2018-05-29
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# Deprecate `.sendAction`
+
+## Summary
+
+In old versions of Ember (< 1.13) `component#sendAction` was the only way for a component to call an
+action on a parent scope. In 1.13 with the so called _closure actions_ a more intuitive and flexible
+way of calling actions was introduced, yielding the old way redundant.
+
+## Motivation
+
+With the new _closure actions_ being the recommended way, `component#sendAction` is not even
+mentioned in the guides.
+With the goal of simplifying the framework I think we should remove what is not considered the
+current best practice.
+_Closure actions_ have been available since 1.13. That is 3 years ago, so deprecating `sendAction`
+should not cause too much pain and yet addons can support still support the last version of the 1.X
+cycle if they really want to.
+
+It is out of the scope of this RFC to enumerate the reasons why _closure actions_ are preferred over
+_sendAction_ but you can find an in depth explanation of _closure actions_ in [this blog post from 2016](http://miguelcamba.com/blog/2016/01/24/ember-closure-actions-in-depth).
+
+## Detailed design
+
+A deprecation message will appear when `sendAction` is invoked. The feature will be removed in
+Ember 4.0. The deprecation message will use the arguments passed to `sendAction` to generate a dynamic
+explanation that will make super-easy for developers to migrate to closure actions.
+
+As it is mandatory with new deprecations, a new entry in the deprecation guides will be added
+explaining the migration path in depth.
+
+To refresh what the migration path would look like in the typical use case.
+
+**BEFORE**
+```js
+// parent-component.js
+export default Component.extend({
+  actions: {
+    sayHi() {
+      alert('Hello user!');
+    }
+  }
+})
+```
+
+```hbs
+{{!-- parent-component.hbs --}}
+{{child-component salute="sayHi"}}
+```
+
+```js
+// child-component.js
+export default Component.extend({
+  actions: {
+    sendSalute() {
+      this.sendAction('salute');
+    }
+  }
+});
+```
+
+```hbs
+{{!-- child-component.hbs --}}
+<button {{action "sendSalute"}}>Send salute</button>
+```
+
+**AFTER**
+```js
+// parent-component.js
+export default Component.extend({
+  actions: {
+    sayHi() {
+      alert('Hello user!');
+    }
+  }
+})
+```
+
+```hbs
+{{!-- parent-component.hbs --}}
+{{child-component salute=(action "sayHi")}}
+```
+
+```js
+// child-component.js
+export default Component.extend({
+  actions: {
+    sendSalute() {
+      this.salute();
+      // if the salute action is optional you'll have to guard in case it's undefined:
+      // if (this.salute) {
+      //   this.salute()
+      // }
+      // 
+      // Alternatively, you can also define a noop salute function:
+      // salute() {}
+      //
+      // This allows you to remove the guard while provinding an obvious place to add
+      // docs for that action.
+    }
+  }
+});
+```
+
+```hbs
+{{!-- child-component.hbs --}}
+<button {{action "sendSalute"}}>Send salute</button>
+```
+
+However _closure actions_ allow to be less verbose, so the same behavior could be attained using
+less intermediate calls
+
+```js
+// parent-component.js
+export default Component.extend({
+  actions: {
+    sayHi() {
+      alert('Hello user!');
+    }
+  }
+})
+```
+
+```hbs
+{{!-- parent-component.hbs --}}
+{{child-component salute=(action "sayHi")}}
+```
+
+```hbs
+{{!-- child-component.hbs --}}
+<button onclick={{@salute}}>Send salute</button>
+```
+
+## How we teach this
+
+There are no new concepts to teach, but the removal of an old concept now considered outdated.
+
+## Drawbacks
+
+There might be some churn following the deprecation, specially comming from addons that haven't been
+updated in a while.
+Addons that want to support the latest versions of Ember without deprecation messages and still work
+past Ember 1.13 will have to do some gymnastics to do so.
+
+## Alternatives
+
+Wait longer to deprecate it and keep `sendAction` undocumented until it's usage is yet more minoritary
+than it is today, to lower the churn.
+