Center Mat-Menu, Center MatMenu Overlay to Button

Per Material Issue 9631 centering the mat-menu to the button is as follows.

the issue is that supporting it deviates from the spec and may end up looking weird.

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I have a need for this functionality... Because writing my own CDK overlay would be more time intensive than overriding the mat-menu component... I am not interested in re-creating the mat-menu, and simply need the menu to be centered... I am also not interested in any other library's to accomplish this, I want to use Material mat-menu, so my question is as follows.

Question:

Utilizing an angular directive, how can I override the private variables and methods of the MatMenuTrigger to center the CDK overlay to the button?

I chose a Directive because MatMenuTrigger is a Directive, it made sense to replicate the logic used by the Material source.

So far this is the approach I have come up with, I am not sure if reaching into the class like this is "acceptable" per se, or if there are any negative ramifications for doing it this way.

I am open to constructive discussion on this approach for any recommendations or improvements.

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Stackblitz

https://stackblitz.com/edit/angular-jpgjdc-nqmyht?embed=1&file=app/center-matmenu.directive.ts

Essentially I am decoupling the matMenuTrigger from the button by placing it on a div wrapping the mat-menu... this is so I can programmatically open the menu from the directive and not the button.

• I am also assigning menuTrigger to a templateRef on the div and passing it as an input to my center-mat-menu selector

<button mat-button [center-mat-menu]="menuTrigger">Menu</button>
<div #menuTrigger="matMenuTrigger" [matMenuTriggerFor]="menu">
 <mat-menu #menu="matMenu">

From there I am creating a listener via @HostListener

@HostListener('click', ['$event'])

And then replicating the logic of the MatMenuTrigger directive to manipulation the placement, initialize the menu, and fire the open animation on click.

This is basically a replication of the openMenu() method in the menu-trigger.ts source except I am manipulating the left and top styles after the menu is initialized, and before I call this.menuTrigger.menu['_startAnimation'](); to open the menu.

menu-trigger.ts

I store the dimension of the source button into variables and use that information to calculate the center point of the button, I then use that with the width of the initialized menu to calculate left

@Directive({
    selector: "[center-mat-menu]"
})
export class CenterMatmenuDirective {
    overlayRef: OverlayRef;
    overlayConf: OverlayConfig;
    dropDown: HTMLElement;
    overlayPositionBox: HTMLElement;
    menu: MatMenuPanel;
    button: HTMLElement;
    buttonWidth: number;
    buttonLeft: number;
    buttonBottom: number;

    @Input("center-mat-menu") private menuTrigger: MatMenuTrigger;

    constructor(private _menuButton: ElementRef, private _renderer: Renderer2) {}

    @HostListener("click", ["$event"])
    onclick(e) {
        this._setVariables();
        //menu not opened by keyboard down arrow, have to set this so MatMenuTrigger knows the menu was opened with a mouse click
        this.menuTrigger["_openedBy"] = e.button === 0 ? "mouse" : null;

        this._overrideMatMenu();

        this.dropDown = this.overlayRef.overlayElement.children[0].children[0] as HTMLElement;
        this.overlayPositionBox = this.overlayRef.hostElement;

        setTimeout(() => {
            this._styleDropDown(this.dropDown);
            this._setOverlayPosition(this.dropDown, this.overlayPositionBox);
            this._openMenu();
        });
    }

    private _setVariables() {
        const config = this.menuTrigger["_getOverlayConfig"]();
        this.menuTrigger["_overlayRef"] = this.menuTrigger["_overlay"].create(config);
        this.overlayRef = this.menuTrigger["_overlayRef"];
        this.overlayConf = this.overlayRef.getConfig();
        this.overlayRef.keydownEvents().subscribe();
        this.menu = this.menuTrigger.menu;
        this._setButtonVars();
    }

    private _setButtonVars() {
        this.button = this._menuButton.nativeElement;
        this.buttonWidth = this.button.getBoundingClientRect().width;
        this.buttonLeft = this.button.getBoundingClientRect().left;
        this.buttonBottom = this.button.getBoundingClientRect().bottom;
    }

    private _overrideMatMenu() {
        let strat = this.overlayConf.positionStrategy as FlexibleConnectedPositionStrategy;
        this.menuTrigger["_setPosition"](strat);
        strat.positionChanges.subscribe(() => {
            this._setButtonVars();
            this._setOverlayPosition(this.dropDown, this.overlayPositionBox);
        });
        this.overlayConf.hasBackdrop =
            this.menu.hasBackdrop == null ? !this.menuTrigger.triggersSubmenu() : this.menu.hasBackdrop;
        this.overlayRef.attach(this.menuTrigger["_getPortal"]());

        if (this.menu.lazyContent) {
            this.menu.lazyContent.attach();
        }

        this.menuTrigger["_closeSubscription"] = this.menuTrigger["_menuClosingActions"]().subscribe(() => {
            this.menuTrigger.closeMenu();
        });
        this.menuTrigger["_initMenu"]();
    }

    private _styleDropDown(dropDown: HTMLElement) {
        this._renderer.setStyle(this._renderer.parentNode(dropDown), "transform-origin", "center top 0px");
    }

    private _setOverlayPosition(dropDown: HTMLElement, overlayPositionBox: HTMLElement) {
        let dropDownleft = this.buttonWidth / 2 + this.buttonLeft - dropDown.offsetWidth / 2;

        this._renderer.setStyle(overlayPositionBox, "top", this.buttonBottom + 5 + "px");
        this._renderer.setStyle(overlayPositionBox, "left", dropDownleft + "px");
        this._renderer.setStyle(overlayPositionBox, "height", "100%");
    }

    private _openMenu() {
        this.menuTrigger.menu["_startAnimation"]();
    }
}