perf(item): improve performance of sliding item (#9005)

- Using new NativeRafDebouncer
- Events are not zone wrapped unless strictly necessary

Author: manucorporat

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

@@ -4,6 +4,7 @@ import { List } from '../list/list';
 import { GesturePriority } from '../../gestures/gesture-controller';
 import { PanGesture } from '../../gestures/drag-gesture';
 import { pointerCoord } from '../../util/dom';
+import { NativeRafDebouncer } from '../../util/debouncer';
 
 const DRAG_THRESHOLD = 10;
 const MAX_ATTACK_ANGLE = 20;
@@ -19,6 +20,8 @@ export class ItemSlidingGesture extends PanGesture {
     super(list.getNativeElement(), {
       maxAngle: MAX_ATTACK_ANGLE,
       threshold: DRAG_THRESHOLD,
+      zone: false,
+      debouncer: new NativeRafDebouncer(),
       gesture: list._gestureCtrl.create('item-sliding', {
         priority: GesturePriority.SlidingItem,
       })

src/components/item/item-sliding.ts

@@ -402,7 +402,10 @@ export class ItemSliding {
     }
 
     this.item.setElementStyle(CSS.transform, `translate3d(${-openAmount}px,0,0)`);
-    this._zone.run(() => this.ionDrag.emit(this));
+    let ionDrag = this.ionDrag;
+    if (ionDrag.observers.length > 0) {
+      this._zone.run(ionDrag.emit.bind(ionDrag, this));
+    }
   }
 
   private _setState(state: SlidingState) {