[REF] component: large cleanup of concurrency branch

We remove here old comments, add some tests and documentation, and in
general, make sure the state of the code is in a good shape

part of #330

src/component/scheduler.ts

@@ -1,14 +1,29 @@
 import { Fiber } from "./fiber";
 
-// scheduler
+/**
+ * Owl Scheduler Class
+ *
+ * The scheduler is the part of Owl that will effectively apply a rendering
+ * whenever a fiber is ready.
+ *
+ * Briefly, it can be used to register root fibers.  Whenever there is an
+ * active root fiber, it will poll continuously each animation frame (so, about
+ * once every 16ms) and whenever a root fiber is ready, it will apply it.
+ */
+
 interface Task {
   fiber: Fiber;
   callback: () => void;
 }
 
-export const scheduler = {
-  tasks: [] as Task[],
-  isRunning: false,
+export class Scheduler {
+  tasks: Task[] = [];
+  isRunning: boolean = false;
+  requestAnimationFrame: typeof window.requestAnimationFrame;
+
+  constructor(requestAnimationFrame) {
+    this.requestAnimationFrame = requestAnimationFrame;
+  }
 
   addFiber(fiber, callback) {
     this.tasks.push({ fiber, callback });
@@ -16,7 +31,12 @@ export const scheduler = {
       return;
     }
     this.scheduleTasks();
-  },
+  }
+
+  /**
+   * Process all current tasks. This only applies to the fibers that are ready.
+   * Other tasks are left unchanged.
+   */
   flush() {
     let tasks = this.tasks;
     this.tasks = [];
@@ -31,20 +51,17 @@ export const scheduler = {
       return true;
     });
     this.tasks = tasks.concat(this.tasks);
-  },
-  processTasks() {
-    this.flush();
-    if (this.tasks.length > 0) {
-      this.scheduleTasks();
-    } else {
-      this.isRunning = false;
-    }
-  },
+  }
 
   scheduleTasks() {
     this.isRunning = true;
-    this.requestAnimationFrame(() => this.processTasks());
-  },
-
-  requestAnimationFrame: requestAnimationFrame.bind(window)
-};
+    this.requestAnimationFrame(() => {
+      this.flush();
+      if (this.tasks.length > 0) {
+        this.scheduleTasks();
+      } else {
+        this.isRunning = false;
+      }
+    });
+  }
+}