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1-js/07-object-oriented-programming/13-mixins/article.md

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+# Mixins
+
+In JavaScript we can only inherit from a single object. There can be only one `[[Prototype]]`.
+
+But sometimes we need such kind of thing. For instance, we have a code that implements events exchange or templating, and we'd like to be able to add these capabilities to any class easily.
+
+What can help here is *mixins*.
+
+As [defined in Wikipedia](https://en.wikipedia.org/wiki/Mixin), a *mixin* is a class that contains methods for use by other classes without having to be the parent class of those other classes.
+
+In other words, a *mixin* is a class that implements a certain behavior. But we do not use it alone, we use it to add the behavior to other classes.
+
+## A mixin example
+
+The simplest way to make a mixin in JavaScript -- is to make an object with useful methods, that we can just copy into the prototype.
+
+For instance here the mixin `sayHiMixin` is used to add some "speech" for `User`:
+
+```js run
+*!*
+// mixin
+*/!*
+let sayHiMixin = {
+  sayHi() {
+    alert("Hello " + this.name);
+  },
+  sayBye() {
+    alert("Bye " + this.name);
+  }
+};
+
+*!*
+// usage:
+*/!*
+class User {
+  constructor(name) {
+    this.name = name;
+  }
+}
+
+// copy the methods
+Object.assign(User.prototype, sayHiMixin);
+
+// now User can say hi
+new User("Dude").sayHi(); // Hi Dude!
+```
+
+There's no inheritance, there's a simple method copying. So `User` may extend some other class and also include the mixin to "mix-in" the additional methods.
+
+Mixins also can also make use of inheritance.
+
+For instance, here `sayHiMixin` inherits from `sayMixin`:
+
+```js run
+let sayMixin = {
+  say(phrase) {
+    alert(phrase);
+  }
+};
+
+let sayHiMixin = {
+  // can use any way of prototype setting here
+  __proto__: sayMixin,
+
+  sayHi() {
+    *!*
+    // call parent method
+    */!*
+    super.say("Hello " + this.name);
+  },
+  sayBye() {
+    super.say("Bye " + this.name);
+  }
+};
+
+class User {
+  constructor(name) {
+    this.name = name;
+  }
+}
+
+// copy the methods
+Object.assign(User.prototype, sayHiMixin);
+
+// now User can say hi
+new User("Dude").sayHi(); // Hi Dude!
+```
+
+Please note that the call to the parent method `super.say()` from `sayHiMixin` looks for the method in the prototype of that mixin, not the class.
+
+![](mixin-inheritance.png)
+
+That's because methods from `sayHiMixin` have `[[HomeObject]]` set to it. So `super` actually means `sayHiMixin.__proto__`, not `User.__proto__`.
+
+## EventMixin
+
+Now a mixin for the real life.
+
+The important feature of many objects is working with events.
+
+That is: an object should have a method to "generate an event" when something important happens to him, and other objects should be able to "subscribe" to receive such notifications.
+
+An event must have a name and, if necessary, the attached data.
+
+For instance, an object `user` can generate an event `"login"` when the visitor logs in. And an object `calendar` may want to receive such notifications and load the information about that visitor.
+
+Or, the object `menu` can generate the event `"select"` when a menu item is selected, and other objects may want to get that information and react on that event.
+
+Events is a way to "share information" with anyone who wants it.
+
+The `eventMixin` to implement the corresponding methods:
+
+```js run
+let eventMixin = {
+
+  /**
+   * Subscribe to event, usage:
+   *  menu.on('select', function(item) { ... }
+  */
+  on(eventName, handler) {
+    if (!this._eventHandlers) this._eventHandlers = {};
+    if (!this._eventHandlers[eventName]) {
+      this._eventHandlers[eventName] = [];
+    }
+    this._eventHandlers[eventName].push(handler);
+  },
+
+  /**
+   * Cancel the subscription, usage:
+   *  menu.off('select', handler)
+   */
+  off(eventName, handler) {
+    let handlers = this._eventHandlers && this._eventHandlers[eventName];
+    if (!handlers) return;
+    for(let i = 0; i < handlers.length; i++) {
+      if (handlers[i] == handler) {
+        handlers.splice(i--, 1);
+      }
+    }
+  },
+
+  /**
+   * Generate the event and attach the data to it
+   *  this.trigger('select', data1, data2);
+   */
+  trigger(eventName, ...args) {
+    if (!this._eventHandlers || !this._eventHandlers[eventName]) {
+      return; // no handlers for that event name
+    }
+
+    // call the handlers
+    this._eventHandlers[eventName].forEach(handler => handler.apply(this, args));
+  }
+};
+```
+
+There are 3 methods here:
+
+1. `.on(eventName, handler)` -- assigns the function `handler` to run when the event with that name happens. The handlers are stored in `_eventHandlers` property.
+2. `.off(eventName, handler)` -- removes the function from the handlers list.
+3. `.trigger(eventName, ...args)` -- generates the event: all assigned handlers are called and `args` are passed as arguments to them.
+
+
+Usage:
+
+```js run
+// Make a class
+class Menu {
+  choose(value) {
+    this.trigger("select", value);
+  }
+}
+// Add the mixin
+Object.assign(Menu.prototype, eventMixin);
+
+let menu = new Menu();
+
+// call the handler on selection:
+*!*
+menu.on("select", value => alert("Value selected: " + value));
+*/!*
+
+// triggers the event => shows Value selected: 123
+menu.choose("123"); // value selected
+```
+
+Now if we have the code interested to react on user selection, we can bind it with `menu.on(...)`.
+
+And the `eventMixin` can add such behavior to as many classes as we'd like, without interfering with the inheritance chain.
+
+## Summary
+
+*Mixin* -- is a generic object-oriented programming term: a class that contains methods for other classes.
+
+In JavaScript that can be implemented as copying them into the prototype.
+
+We can use mixins as a way to augment a class by multiple behaviors like event-handling that we overlooked above.
+
+Mixins may become a point of conflict if they occasionally overwrite native class methods. So generally one should think well about the naming for a mixin, to minimalize such possibility.