feat(decorate-with): perInstance helper

@vates/decorate-with/README.md

@@ -70,6 +70,25 @@ decorateMethodsWith(Foo, {
 })
 ```
 
+### `perInstance(fn, ...args)`
+
+Helper to decorate the method by instance instead of for the whole class.
+
+This is often necessary for caching or deduplicating calls.
+
+```js
+import { perInstance } from '@vates/decorateWith'
+
+class Foo {
+  @decorateWith(perInstance, lodash.memoize)
+  bar() {
+    // body
+  }
+}
+```
+
+Because it's a normal function, it can also be used with `decorateMethodsWith`, with `compose` or even by itself.
+
 ## Contributions
 
 Contributions are _very_ welcomed, either on the documentation or on

@vates/decorate-with/USAGE.md

@@ -51,3 +51,22 @@ decorateMethodsWith(Foo, {
   ])
 })
 ```
+
+### `perInstance(fn, ...args)`
+
+Helper to decorate the method by instance instead of for the whole class.
+
+This is often necessary for caching or deduplicating calls.
+
+```js
+import { perInstance } from '@vates/decorateWith'
+
+class Foo {
+  @decorateWith(perInstance, lodash.memoize)
+  bar() {
+    // body
+  }
+}
+```
+
+Because it's a normal function, it can also be used with `decorateMethodsWith`, with `compose` or even by itself.

@vates/decorate-with/index.js

@@ -19,3 +19,15 @@ exports.decorateMethodsWith = function decorateMethodsWith(klass, map) {
   }
   return klass
 }
+
+exports.perInstance = function perInstance(fn, decorator, ...args) {
+  const map = new WeakMap()
+  return function () {
+    let decorated = map.get(this)
+    if (decorated === undefined) {
+      decorated = decorator(fn, ...args)
+      map.set(this, decorated)
+    }
+    return decorated.apply(this, arguments)
+  }
+}