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perf(cdk/a11y): only check for high contrast mode once #22561

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merged 1 commit into from
May 3, 2021
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perf(cdk/a11y): only check for high contrast mode once #22561

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8 changes: 7 additions & 1 deletion src/cdk/a11y/high-contrast-mode/high-contrast-mode-detector.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,11 @@ export const HIGH_CONTRAST_MODE_ACTIVE_CSS_CLASS = 'cdk-high-contrast-active';
*/
@Injectable({providedIn: 'root'})
export class HighContrastModeDetector {
/**
* Figuring out the high contrast mode and adding the body classes can cause
* some expensive layouts. This flag is used to ensure that we only do it once.
*/
private _hasCheckedHighContrastMode: boolean;
private _document: Document;

constructor(private _platform: Platform, @Inject(DOCUMENT) document: any) {
Expand Down Expand Up @@ -80,12 +85,13 @@ export class HighContrastModeDetector {

/** Applies CSS classes indicating high-contrast mode to document body (browser-only). */
_applyBodyHighContrastModeCssClasses(): void {
if (this._platform.isBrowser && this._document.body) {
if (!this._hasCheckedHighContrastMode && this._platform.isBrowser && this._document.body) {
const bodyClasses = this._document.body.classList;
// IE11 doesn't support `classList` operations with multiple arguments
bodyClasses.remove(HIGH_CONTRAST_MODE_ACTIVE_CSS_CLASS);
bodyClasses.remove(BLACK_ON_WHITE_CSS_CLASS);
bodyClasses.remove(WHITE_ON_BLACK_CSS_CLASS);
this._hasCheckedHighContrastMode = true;

const mode = this.getHighContrastMode();
if (mode === HighContrastMode.BLACK_ON_WHITE) {
Expand Down