WiP: feat(mixin): support for lazy mixins

@xen-orchestra/mixin/index.js

@@ -2,7 +2,12 @@
 
 const camelCase = require('lodash/camelCase')
 
-const { defineProperties, defineProperty, keys } = Object
+const {
+  defineProperties,
+  defineProperty,
+  hasOwn = Function.prototype.call.bind(Object.prototype.hasOwnProperty),
+  keys,
+} = Object
 const noop = Function.prototype
 
 const MIXIN_CYCLIC_DESCRIPTOR = {
@@ -13,23 +18,49 @@ const MIXIN_CYCLIC_DESCRIPTOR = {
 }
 
 module.exports = function mixin(object, mixins, args) {
+  const importing = { __proto__: null }
+  const importers = { __proto__: null }
+
+  function instantiateMixin(name, Mixin) {
+    defineProperty(object, name, MIXIN_CYCLIC_DESCRIPTOR)
+    const instance = new Mixin(object, ...args)
+    defineProperty(object, name, {
+      value: instance,
+    })
+    return instance
+  }
+
   // add lazy property for each of the mixin, this allows mixins to depend on
   // one another without any special ordering
-  const descriptors = {}
+  const descriptors = {
+    loadMixin(name) {
+      if (hasOwn(this, name)) {
+        return Promise.resolve(this[name])
+      }
+
+      let promise = importing[name]
+      if (promise === undefined) {
+        const clean = () => {
+          delete importing[name]
+        }
+        promise = importers[name]().then(Mixin => instantiateMixin(name, Mixin))
+        promise.then(clean, clean)
+        importing[name] = promise
+      }
+      return promise
+    },
+  }
   keys(mixins).forEach(name => {
     const Mixin = mixins[name]
     name = camelCase(name)
 
-    descriptors[name] = {
-      configurable: true,
-      get: () => {
-        defineProperty(object, name, MIXIN_CYCLIC_DESCRIPTOR)
-        const instance = new Mixin(object, ...args)
-        defineProperty(object, name, {
-          value: instance,
-        })
-        return instance
-      },
+    if (Mixin.prototype === undefined) {
+      importers[name] = Mixin(name)
+    } else {
+      descriptors[name] = {
+        configurable: true,
+        get: () => instantiateMixin(name, Mixin),
+      }
     }
   })
   defineProperties(object, descriptors)

@xen-orchestra/mixin/package.json

@@ -16,7 +16,7 @@
   },
   "preferGlobal": false,
   "engines": {
-    "node": ">=6"
+    "node": ">=7.6"
   },
   "dependencies": {
     "bind-property-descriptor": "^2.0.0",