Simplify PageElement API

src/page/background/background.ts

@@ -60,16 +60,14 @@ export class PageBackground extends PageElement {
     requestAnimationFrame(this.drawIfNecessary.bind(this));
   }
 
-  private parent?: HTMLElement;
-  protected setParent(parent: PageElement) {
-    this.parent = parent.htmlRoot;
+  protected initialize() {
+    super.initialize();
     requestAnimationFrame(this.drawIfNecessary.bind(this));
-    super.setParent(parent);
   }
 
   private getSiblings(): Array<HTMLElement> {
     return Array.prototype.slice
-      .call(this.parent!.childNodes)
+      .call(this.htmlRoot.parentElement!.childNodes)
       .filter((n: HTMLElement) => n !== this.htmlRoot);
   }
 

src/page/basics/preview/preview.ts

@@ -10,14 +10,14 @@ export class Preview extends PageElement {
     this.query('.start-button').addEventListener('click', this.loadContent.bind(this));
   }
 
-  protected setParent(parent: PageElement) {
+  protected initialize() {
     new IntersectionObserver((e) => {
       if (!e[0].isIntersecting) {
         this.unloadContent();
       }
     }).observe(this.htmlRoot.parentElement!);
 
-    super.setParent(parent);
+    super.initialize();
   }
 
   public loadContent() {

src/page/page-element.ts

@@ -6,30 +6,25 @@ export class PageElement {
 
   public attachToDOM(target: HTMLElement) {
     target.appendChild(this.htmlRoot);
-    this.setParent(null);
+    this.initialize();
   }
 
-  protected setParent(_parent?: PageElement | null): void {
-    this.children.forEach((c) => c.setParent(this));
+  protected initialize(): void {
+    this.children.forEach((c) => c.initialize());
   }
 
   protected query(query: string): HTMLElement {
     return this.htmlRoot.querySelector(query) as HTMLElement;
   }
 
+  protected attachElement(element: PageElement) {
+    this.htmlRoot.appendChild(element.htmlRoot);
+    this.children.push(element);
+  }
+
   protected attachElementByReplacing(query: string, element: PageElement) {
     const old = this.query(query);
     old.parentElement!.replaceChild(element.htmlRoot, old);
     this.children.push(element);
   }
-
-  protected attachElementAsChildOf(query: string, element: PageElement) {
-    this.query(query).appendChild(element.htmlRoot);
-    this.children.push(element);
-  }
-
-  protected attachElement(element: PageElement) {
-    this.htmlRoot.appendChild(element.htmlRoot);
-    this.children.push(element);
-  }
 }