perf(item): reorder is only added to the DOM if needed

I have measured the performance impact of this change, since we use the push change detector strategy, the *ngIf is only evaluated once.

Items wrapped around an element with the ListReorder directive will receive a hidden `<ion-reorder>` in their DOM, but items that are not wrapped (i.e. they CAN NOT be reordered) will not even have the `<ion-reorder>` element in their DOM.

fixes #9065

src/components/item/item-reorder-gesture.ts

@@ -47,7 +47,7 @@ export class ItemReorderGesture {
       console.error('ion-reorder does not contain $ionComponent');
       return false;
     }
-    this.reorderList.reorderPrepare();
+    this.reorderList._reorderPrepare();
 
     let item = reorderMark.getReorderNode();
     if (!item) {
@@ -63,13 +63,13 @@ export class ItemReorderGesture {
     this.lastToIndex = indexForItem(item);
 
     this.windowHeight = window.innerHeight - AUTO_SCROLL_MARGIN;
-    this.lastScrollPosition = this.reorderList.scrollContent(0);
+    this.lastScrollPosition = this.reorderList._scrollContent(0);
 
     this.offset = pointerCoord(ev);
     this.offset.y += this.lastScrollPosition;
 
     item.classList.add(ITEM_REORDER_ACTIVE);
-    this.reorderList.reorderStart();
+    this.reorderList._reorderStart();
     return true;
   }
 
@@ -97,7 +97,7 @@ export class ItemReorderGesture {
           this.lastToIndex = toIndex;
           this.lastYcoord = posY;
           this.emptyZone = false;
-          this.reorderList.reorderMove(fromIndex, toIndex, this.selectedItemHeight);
+          this.reorderList._reorderMove(fromIndex, toIndex, this.selectedItemHeight);
         }
       } else {
         this.emptyZone = true;
@@ -133,7 +133,7 @@ export class ItemReorderGesture {
     } else {
       reorderInactive();
     }
-    this.reorderList.reorderEmit(fromIndex, toIndex);
+    this.reorderList._reorderEmit(fromIndex, toIndex);
   }
 
   private itemForCoord(coord: PointerCoordinates): HTMLElement {
@@ -142,9 +142,9 @@ export class ItemReorderGesture {
 
   private scroll(posY: number): number {
     if (posY < AUTO_SCROLL_MARGIN) {
-      this.lastScrollPosition = this.reorderList.scrollContent(-SCROLL_JUMP);
+      this.lastScrollPosition = this.reorderList._scrollContent(-SCROLL_JUMP);
     } else if (posY > this.windowHeight) {
-      this.lastScrollPosition = this.reorderList.scrollContent(SCROLL_JUMP);
+      this.lastScrollPosition = this.reorderList._scrollContent(SCROLL_JUMP);
     }
     return this.lastScrollPosition;
   }

src/components/item/item-reorder.ts

@@ -133,19 +133,10 @@ export interface ReorderIndexes {
 })
 export class ItemReorder {
 
-  /** @private */
   _enableReorder: boolean = false;
-
-  /** @private */
   _visibleReorder: boolean = false;
-
-  /** @private */
   _reorderGesture: ItemReorderGesture;
-
-  /** @private */
   _lastToIndex: number = -1;
-
-  /** @private */
   _element: HTMLElement;
 
   /**
@@ -196,10 +187,7 @@ export class ItemReorder {
     }
   }
 
-  /**
-   * @private
-   */
-  reorderPrepare() {
+  _reorderPrepare() {
     let ele = this._element;
     let children: any = ele.children;
     for (let i = 0, ilen = children.length; i < ilen; i++) {
@@ -209,18 +197,12 @@ export class ItemReorder {
     }
   }
 
-  /**
-   * @private
-   */
-  reorderStart() {
+  _reorderStart() {
     this.setElementClass('reorder-list-active', true);
   }
 
-  /**
-   * @private
-   */
-  reorderEmit(fromIndex: number, toIndex: number) {
-    this.reorderReset();
+  _reorderEmit(fromIndex: number, toIndex: number) {
+    this._reorderReset();
     if (fromIndex !== toIndex) {
       this._zone.run(() => {
         this.ionItemReorder.emit({
@@ -231,21 +213,15 @@ export class ItemReorder {
     }
   }
 
-  /**
-   * @private
-   */
-  scrollContent(scroll: number) {
+  _scrollContent(scroll: number) {
     let scrollTop = this._content.getScrollTop() + scroll;
     if (scroll !== 0) {
       this._content.scrollTo(0, scrollTop, 0);
     }
     return scrollTop;
   }
 
-  /**
-   * @private
-   */
-  reorderReset() {
+  _reorderReset() {
     let children = this._element.children;
     let len = children.length;
 
@@ -257,10 +233,7 @@ export class ItemReorder {
     this._lastToIndex = -1;
   }
 
-  /**
-   * @private
-   */
-  reorderMove(fromIndex: number, toIndex: number, itemHeight: number) {
+  _reorderMove(fromIndex: number, toIndex: number, itemHeight: number) {
     if (this._lastToIndex === -1) {
       this._lastToIndex = fromIndex;
     }

src/components/item/item-sliding-gesture.ts

@@ -10,6 +10,7 @@ const DRAG_THRESHOLD = 10;
 const MAX_ATTACK_ANGLE = 20;
 
 export class ItemSlidingGesture extends PanGesture {
+
   private preSelectedContainer: ItemSliding = null;
   private selectedContainer: ItemSliding = null;
   private openContainer: ItemSliding = null;

src/components/item/item.ts

@@ -1,11 +1,12 @@
-import { ChangeDetectionStrategy, Component, ContentChild, ContentChildren, Directive, ElementRef, Input, QueryList, Renderer, ViewChild, ViewEncapsulation } from '@angular/core';
+import { ChangeDetectionStrategy, Component, ContentChild, ContentChildren, Directive, ElementRef, Input, QueryList, Renderer, Optional, ViewChild, ViewEncapsulation } from '@angular/core';
 
 import { Button } from '../button/button';
 import { Config } from '../../config/config';
 import { Form } from '../../util/form';
 import { Icon } from '../icon/icon';
 import { Ion } from '../ion';
 import { Label } from '../label/label';
+import { ItemReorder } from './item-reorder';
 
 
 /**
@@ -283,7 +284,7 @@ import { Label } from '../label/label';
         '<ng-content select="ion-select,ion-input,ion-textarea,ion-datetime,ion-range,[item-content]"></ng-content>' +
       '</div>' +
       '<ng-content select="[item-right],ion-radio,ion-toggle"></ng-content>' +
-      '<ion-reorder></ion-reorder>' +
+      '<ion-reorder *ngIf="_shouldHaveReorder"></ion-reorder>' +
     '</div>' +
     '<div class="button-effect"></div>',
   host: {
@@ -297,6 +298,7 @@ export class Item extends Ion {
   _inputs: Array<string> = [];
   _label: Label;
   _viewLabel: boolean = true;
+  _shouldHaveReorder: boolean = false;
 
   /**
    * @private
@@ -324,9 +326,15 @@ export class Item extends Ion {
     this._setMode('item', val);
   }
 
-  constructor(form: Form, config: Config, elementRef: ElementRef, renderer: Renderer) {
+  constructor(
+    form: Form,
+    config: Config,
+    elementRef: ElementRef,
+    renderer: Renderer,
+    @Optional() reorder: ItemReorder
+  ) {
     super(config, elementRef, renderer);
-
+    this._shouldHaveReorder = !!reorder;
     this.mode = config.get('mode');
     this.id = form.nextId().toString();
   }

src/navigation/nav-controller-base.ts

@@ -24,6 +24,7 @@ import { TransitionController } from '../transitions/transition-controller';
  * This class is for internal use only. It is not exported publicly.
  */
 export class NavControllerBase extends Ion implements NavController {
+
   _children: any[] = [];
   _ids: number = -1;
   _init = false;