chore: use computed radius size for checkbox ripple during 2018 material spec update transition

Author: josephperrott

src/lib/checkbox/checkbox.scss

@@ -438,7 +438,7 @@ $_mat-checkbox-mark-stroke-size: 2 / 15 * $mat-checkbox-size !default;
   left: calc(50% - #{$_mat-checkbox-ripple-radius});
   top: calc(50% - #{$_mat-checkbox-ripple-radius});
   height: $_mat-checkbox-ripple-radius * 2;
-  width: $_mat_checkbox-ripple-radius * 2;
+  width: $_mat-checkbox-ripple-radius * 2;
   z-index: 1;
   pointer-events: none;
 }

src/lib/checkbox/checkbox.ts

@@ -50,6 +50,11 @@ import {MAT_CHECKBOX_CLICK_ACTION, MatCheckboxClickAction} from './checkbox-conf
 // Increasing integer for generating unique ids for checkbox components.
 let nextUniqueId = 0;
 
+// TODO(josephperrott): Revert to constants for ripple radius once 2018 Checkbox updates have
+// landed.
+// The radius for the checkbox's ripple, in pixels.
+let calculatedRippleRadius = 0;
+
 /**
  * Provider Expression that allows mat-checkbox to register as a ControlValueAccessor.
  * This allows it to support [(ngModel)].
@@ -210,6 +215,10 @@ export class MatCheckbox extends _MatCheckboxMixinBase implements ControlValueAc
       .subscribe(focusOrigin => this._onInputFocusChange(focusOrigin));
   }
 
+  ngAfterViewChecked() {
+    this._calculateRippleRadius();
+  }
+
   ngOnDestroy() {
     this._focusMonitor.stopMonitoring(this._inputElement);
   }
@@ -457,4 +466,19 @@ export class MatCheckbox extends _MatCheckboxMixinBase implements ControlValueAc
 
     return `mat-checkbox-anim-${animSuffix}`;
   }
+
+  // TODO(josephperrott): Revert to constants for ripple radius once 2018 Checkbox updates have
+  // landed.
+  /**
+   * Calculate the radius for the ripple based on the ripple elements width.  Only calculated once
+   * for the application.
+   */
+  private _calculateRippleRadius() {
+    if (!calculatedRippleRadius) {
+      const rippleWidth =
+          this._elementRef.nativeElement.querySelector('.mat-checkbox-ripple').clientWidth || 0;
+      calculatedRippleRadius = rippleWidth / 2;
+    }
+    this.ripple.radius = calculatedRippleRadius;
+  }
 }