refactor(list): convert MatLineSetter into a function

Currently the `MatLineSetter` is a class, however it doesn't really do anything aside from calling a couple of functions inside the constructor. The following changes turn it into a function and make it a bit more compact. This also avoids having to turn off a tslint rule in a couple of places.

Author: crisbeto

src/lib/core/line/line.ts

@@ -12,6 +12,7 @@ import {
   ElementRef,
   QueryList,
 } from '@angular/core';
+import {startWith} from 'rxjs/operators';
 import {MatCommonModule} from '../common-behaviors/common-module';
 
 
@@ -30,38 +31,38 @@ export class MatLine {}
  * Helper that takes a query list of lines and sets the correct class on the host.
  * @docs-private
  */
-export class MatLineSetter {
-  constructor(private _lines: QueryList<MatLine>, private _element: ElementRef<HTMLElement>) {
-    this._setLineClass(this._lines.length);
-
-    this._lines.changes.subscribe(() => {
-      this._setLineClass(this._lines.length);
-    });
-  }
+export function setLines(lines: QueryList<MatLine>, element: ElementRef<HTMLElement>) {
+  // Note: doesn't need to unsubscribe, because `changes`
+  // gets completed by Angular when the view is destroyed.
+  lines.changes.pipe(startWith<QueryList<MatLine>>(lines)).subscribe(({length}) => {
+    setClass(element, 'mat-2-line', false);
+    setClass(element, 'mat-3-line', false);
+    setClass(element, 'mat-multi-line', false);
 
-  private _setLineClass(count: number): void {
-    this._resetClasses();
-    if (count === 2 || count === 3) {
-      this._setClass(`mat-${count}-line`, true);
-    } else if (count > 3) {
-      this._setClass(`mat-multi-line`, true);
+    if (length === 2 || length === 3) {
+      setClass(element, `mat-${length}-line`, true);
+    } else if (length > 3) {
+      setClass(element, `mat-multi-line`, true);
     }
-  }
+  });
+}
 
-  private _resetClasses(): void {
-    this._setClass('mat-2-line', false);
-    this._setClass('mat-3-line', false);
-    this._setClass('mat-multi-line', false);
-  }
+/** Adds or removes a class from an element. */
+function setClass(element: ElementRef<HTMLElement>, className: string, isAdd: boolean): void {
+  const classList = element.nativeElement.classList;
+  isAdd ? classList.add(className) : classList.remove(className);
+}
 
-  private _setClass(className: string, isAdd: boolean): void {
-    if (isAdd) {
-      this._element.nativeElement.classList.add(className);
-    } else {
-      this._element.nativeElement.classList.remove(className);
-    }
+/**
+ * Helper that takes a query list of lines and sets the correct class on the host.
+ * @docs-private
+ * @deprecated Use `setLines` instead.
+ * @breaking-change 8.0.0
+ */
+export class MatLineSetter {
+  constructor(lines: QueryList<MatLine>, element: ElementRef<HTMLElement>) {
+    setLines(lines, element);
   }
-
 }
 
 @NgModule({

src/lib/grid-list/grid-tile.ts

@@ -19,7 +19,7 @@ import {
   ChangeDetectionStrategy,
   Inject,
 } from '@angular/core';
-import {MatLine, MatLineSetter} from '@angular/material/core';
+import {MatLine, setLines} from '@angular/material/core';
 import {coerceNumberProperty} from '@angular/cdk/coercion';
 import {MAT_GRID_LIST, MatGridListBase} from './grid-list-base';
 
@@ -70,17 +70,12 @@ export class MatGridTile {
   encapsulation: ViewEncapsulation.None,
 })
 export class MatGridTileText implements AfterContentInit {
-  /**
-   *  Helper that watches the number of lines in a text area and sets
-   * a class on the host element that matches the line count.
-   */
-  _lineSetter: MatLineSetter;
   @ContentChildren(MatLine) _lines: QueryList<MatLine>;
 
   constructor(private _element: ElementRef<HTMLElement>) {}
 
   ngAfterContentInit() {
-    this._lineSetter = new MatLineSetter(this._lines, this._element);
+    setLines(this._lines, this._element);
   }
 }
 

src/lib/list/list.ts

@@ -22,7 +22,7 @@ import {
   CanDisableRipple,
   CanDisableRippleCtor,
   MatLine,
-  MatLineSetter,
+  setLines,
   mixinDisableRipple,
 } from '@angular/material/core';
 
@@ -136,9 +136,7 @@ export class MatListItem extends _MatListItemMixinBase implements AfterContentIn
   }
 
   ngAfterContentInit() {
-    // TODO: consider turning the setter into a function, it doesn't do anything as a class.
-    // tslint:disable-next-line:no-unused-expression
-    new MatLineSetter(this._lines, this._element);
+    setLines(this._lines, this._element);
   }
 
   /** Whether this list item should show a ripple effect when clicked. */

src/lib/list/selection-list.ts

@@ -33,7 +33,7 @@ import {
 import {
   CanDisableRipple, CanDisableRippleCtor,
   MatLine,
-  MatLineSetter,
+  setLines,
   mixinDisableRipple,
 } from '@angular/material/core';
 import {ControlValueAccessor, NG_VALUE_ACCESSOR} from '@angular/forms';
@@ -161,9 +161,7 @@ export class MatListOption extends _MatListOptionMixinBase
   }
 
   ngAfterContentInit() {
-    // TODO: consider turning the setter into a function, it doesn't do anything as a class.
-    // tslint:disable-next-line:no-unused-expression
-    new MatLineSetter(this._lines, this._element);
+    setLines(this._lines, this._element);
   }
 
   ngOnDestroy(): void {