[IMP] components: add hooks mechanism

part of #194

src/hooks.ts

@@ -0,0 +1,56 @@
+import { Component } from "./component/component";
+import { Observer } from "./core/observer";
+
+/**
+ * Owl Hook System
+ *
+ * This file introduces the concept of hooks, similar to React or Vue hooks.
+ * We have currently an implementation of:
+ * - useState (reactive state)
+ * - onMounted
+ * - onWillUnmount
+ */
+
+
+/**
+ * useState hook
+ *
+ * This is the main way a component can be made reactive.  The useState hook
+ * will return an observed object (or array).  Changes to that value will then
+ * trigger a rerendering of the current component.
+ */
+export function useState<T>(state: T): T {
+  const component: Component<any,any> = Component._current;
+  const __owl__ = component.__owl__;
+  if (!__owl__.observer) {
+    __owl__.observer = new Observer();
+    __owl__.observer.notifyCB = component.render.bind(component);
+  }
+  return __owl__.observer.observe(state);
+}
+
+/**
+ * Mounted hook. The callback will be called when the current component is
+ * mounted.  Note that the component mounted method is called first.
+ */
+export function onMounted(cb) {
+  const component = Component._current;
+  const current = component.mounted;
+  component.mounted = function() {
+    current.call(component);
+    cb();
+  };
+}
+
+/**
+ * willUnmount hook. The callback will be called when the current component is
+ * willUnmounted.  Note that the component mounted method is called last.
+ */
+export function onWillUnmount(cb) {
+  const component = Component._current;
+  const current = component.willUnmount;
+  component.willUnmount = function() {
+    cb();
+    current.call(component);
+  };
+}