directive install hook

active-rfcs/0000-directive-install-hook.md

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+- Start Date: 2021-08-02
+- Target Major Version: 3.x
+- Reference Issues: 
+- Implementation PR:  https://github.com/vuejs/vue-next/pull/4235
+
+# Summary
+add  `install` hook for a custom directive
+
+# Basic example
+
+```js
+export default {
+   directives: {
+      MyDir: {
+         install(){ return { mounted(){}, unmounted(){} } 
+      } 
+   } 
+}
+```
+
+# Motivation
+
+In current state when a function is passed as a directive, vue will register the function 
+to  `mounted` and `updated` hooks only.
+additionally there is no way to tell which hook invoked the function, or register to other hooks.
+
+having an `install` hook will provide 2 main advantages. 
+1. allows for function based directive, that can target any of the supported directive hooks 
+2. having access to the directive early enough, that a function can be used to create the directive dynamically, 
+with access to the directive binding(instance, value, etc) 
+
+
+
+
+Why are we doing this? What use cases does it support? What is the expected
+outcome?
+
+1. The current function based directive is limited to 2 fixed hooks, and no ability to know the life cycle phase
+when the function is invoked.
+
+2. consider provide/inject api, a parent component can actually provide a directive as an `install` function 
+where the child can then *inject and use. 
+(* currently inject() is not callable in a directive hook, however as a work around, a child can "resolve" the provided value via. `instance.$.provides` ) 
+which make this use case doable. even without full support from the inject functionality.
+
+
+
+# Detailed design
+
+
+https://github.com/vuejs/vue-next/pull/4235
+
+
+```js
+export default {
+   directives: {
+      myDir: {
+          install(binding : DirectiveBinding): ObjectDirective {
+            return { mounted(){}, unmounted(){}, ... } 
+          }
+      }
+   } 
+}
+```
+
+if `install` hook is provided it will be called and the result will be merged into the directive hooks object.
+
+
+# Drawbacks
+
+ a function based directive might have additional overhead, however vue already supports a very limited function based directive.
+
+
+# Alternatives
+n/a
+
+
+# Adoption strategy
+
+The feature is additive and should not have any impact on existing code.
+
+