perf(tabs): reduce amount of reflows when aligning the ink bar (#2372)

src/lib/tabs/tab-header.html

@@ -8,7 +8,7 @@
 
 <div class="md-tab-label-container" #tabListContainer
      (keydown)="_handleKeydown($event)">
-  <div class="md-tab-list" #tabList role="tablist" (cdkObserveContent)="_updatePagination()">
+  <div class="md-tab-list" #tabList role="tablist" (cdkObserveContent)="_onContentChanges()">
     <ng-content></ng-content>
     <md-ink-bar></md-ink-bar>
   </div>

src/lib/tabs/tab-header.ts

@@ -5,7 +5,13 @@ import {
   NgZone,
   QueryList,
   ElementRef,
-  ViewEncapsulation, ContentChildren, Output, EventEmitter, Optional
+  ViewEncapsulation,
+  ContentChildren,
+  Output,
+  EventEmitter,
+  Optional,
+  AfterContentChecked,
+  AfterContentInit,
 } from '@angular/core';
 import {RIGHT_ARROW, LEFT_ARROW, ENTER, Dir, LayoutDirection} from '../core';
 import {MdTabLabelWrapper} from './tab-label-wrapper';
@@ -44,7 +50,7 @@ const EXAGGERATED_OVERSCROLL = 60;
     '[class.md-tab-header-rtl]': "_getLayoutDirection() == 'rtl'",
   }
 })
-export class MdTabHeader {
+export class MdTabHeader implements AfterContentChecked, AfterContentInit {
   @ContentChildren(MdTabLabelWrapper) _labelWrappers: QueryList<MdTabLabelWrapper>;
 
   @ViewChild(MdInkBar) _inkBar: MdInkBar;
@@ -106,12 +112,12 @@ export class MdTabHeader {
       this._tabLabelCount = this._labelWrappers.length;
     }
 
-
     // If the selected index has changed, scroll to the label and check if the scrolling controls
     // should be disabled.
     if (this._selectedIndexChanged) {
       this._scrollToLabel(this._selectedIndex);
       this._checkScrollingControls();
+      this._alignInkBarToSelectedTab();
       this._selectedIndexChanged = false;
     }
 
@@ -123,18 +129,6 @@ export class MdTabHeader {
     }
   }
 
-  /**
-   * Waits one frame for the view to update, then updates the ink bar and scroll.
-   * Note: This must be run outside of the zone or it will create an infinite change detection loop.
-   */
-  ngAfterViewChecked(): void {
-    this._zone.runOutsideAngular(() => {
-      window.requestAnimationFrame(() => {
-        this._alignInkBarToSelectedTab();
-      });
-    });
-  }
-
   _handleKeydown(event: KeyboardEvent) {
     switch (event.keyCode) {
       case RIGHT_ARROW:
@@ -149,6 +143,21 @@ export class MdTabHeader {
     }
   }
 
+  /**
+   * Aligns the ink bar to the selected tab on load.
+   */
+  ngAfterContentInit() {
+    this._alignInkBarToSelectedTab();
+  }
+
+  /**
+   * Callback for when the MutationObserver detects that the content has changed.
+   */
+  _onContentChanges() {
+    this._updatePagination();
+    this._alignInkBarToSelectedTab();
+  }
+
   /**
    * Updating the view whether pagination should be enabled or not
    */
@@ -178,7 +187,7 @@ export class MdTabHeader {
   _isValidIndex(index: number): boolean {
     if (!this._labelWrappers) { return true; }
 
-    const tab = this._labelWrappers.toArray()[index];
+    const tab = this._labelWrappers ? this._labelWrappers.toArray()[index] : null;
     return tab && !tab.disabled;
   }
 
@@ -282,7 +291,10 @@ export class MdTabHeader {
    * should be called sparingly.
    */
   _scrollToLabel(labelIndex: number) {
-    const selectedLabel = this._labelWrappers.toArray()[labelIndex];
+    const selectedLabel = this._labelWrappers
+        ? this._labelWrappers.toArray()[labelIndex]
+        :  null;
+
     if (!selectedLabel) { return; }
 
     // The view length is the visible width of the tab labels.
@@ -359,6 +371,11 @@ export class MdTabHeader {
     const selectedLabelWrapper = this._labelWrappers && this._labelWrappers.length
         ? this._labelWrappers.toArray()[this.selectedIndex].elementRef.nativeElement
         : null;
-    this._inkBar.alignToElement(selectedLabelWrapper);
+
+    this._zone.runOutsideAngular(() => {
+      requestAnimationFrame(() => {
+        this._inkBar.alignToElement(selectedLabelWrapper);
+      });
+    });
   }
 }