Center Mat-Menu, Center MatMenu Overlay to Button

问题

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.

---

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?

回答1:

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.

---

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']();
  }
}

来源：https://stackoverflow.com/questions/54129512/center-mat-menu-center-matmenu-overlay-to-button