fix(material-experimental/mdc-snack-bar): avoid querying the DOM on each change detection (#24770)

Currently the MDC snack bar container is querying the DOM on each change detection in order to apply the correct class to the content. We don't need to do this so frequently since everything is set after the portal content has been attached.

(cherry picked from commit d3428ba)

Author: crisbeto

src/material-experimental/mdc-snack-bar/snack-bar-container.ts

@@ -14,7 +14,6 @@ import {
   TemplatePortal,
 } from '@angular/cdk/portal';
 import {
-  AfterViewChecked,
   ChangeDetectionStrategy,
   Component,
   ComponentRef,
@@ -65,7 +64,7 @@ const MDC_SNACKBAR_LABEL_CLASS = 'mdc-snackbar__label';
 })
 export class MatSnackBarContainer
   extends BasePortalOutlet
-  implements _SnackBarContainer, AfterViewChecked, OnDestroy
+  implements _SnackBarContainer, OnDestroy
 {
   /** The number of milliseconds to wait before announcing the snack bar's content. */
   private readonly _announceDelay: number = 150;
@@ -157,17 +156,6 @@ export class MatSnackBarContainer
     this._mdcFoundation.setTimeoutMs(-1);
   }
 
-  ngAfterViewChecked() {
-    // Check to see if the attached component or template uses the MDC template structure,
-    // specifically the MDC label. If not, the container should apply the MDC label class to this
-    // component's label container, which will apply MDC's label styles to the attached view.
-    if (!this._label.nativeElement.querySelector(`.${MDC_SNACKBAR_LABEL_CLASS}`)) {
-      this._label.nativeElement.classList.add(MDC_SNACKBAR_LABEL_CLASS);
-    } else {
-      this._label.nativeElement.classList.remove(MDC_SNACKBAR_LABEL_CLASS);
-    }
-  }
-
   /** Makes sure the exit callbacks have been invoked when the element is destroyed. */
   ngOnDestroy() {
     this._mdcFoundation.close();
@@ -202,14 +190,18 @@ export class MatSnackBarContainer
   attachComponentPortal<T>(portal: ComponentPortal<T>): ComponentRef<T> {
     this._assertNotAttached();
     this._applySnackBarClasses();
-    return this._portalOutlet.attachComponentPortal(portal);
+    const componentRef = this._portalOutlet.attachComponentPortal(portal);
+    this._applyLabelClass();
+    return componentRef;
   }
 
   /** Attach a template portal as content to this snack bar container. */
   attachTemplatePortal<C>(portal: TemplatePortal<C>): EmbeddedViewRef<C> {
     this._assertNotAttached();
     this._applySnackBarClasses();
-    return this._portalOutlet.attachTemplatePortal(portal);
+    const viewRef = this._portalOutlet.attachTemplatePortal(portal);
+    this._applyLabelClass();
+    return viewRef;
   }
 
   private _setClass(cssClass: string, active: boolean) {
@@ -269,4 +261,18 @@ export class MatSnackBarContainer
       });
     }
   }
+
+  /** Applies the correct CSS class to the label based on its content. */
+  private _applyLabelClass() {
+    // Check to see if the attached component or template uses the MDC template structure,
+    // specifically the MDC label. If not, the container should apply the MDC label class to this
+    // component's label container, which will apply MDC's label styles to the attached view.
+    const label = this._label.nativeElement;
+
+    if (!label.querySelector(`.${MDC_SNACKBAR_LABEL_CLASS}`)) {
+      label.classList.add(MDC_SNACKBAR_LABEL_CLASS);
+    } else {
+      label.classList.remove(MDC_SNACKBAR_LABEL_CLASS);
+    }
+  }
 }