actionButtons is an array you assign on the element. Each entry is select-all, clear-all, or custom — the built-ins wire their own behaviour, while custom buttons run your onClick(ms).

AB01 Built-in + custom actions

Select all and Clear come wired; a custom button selects the first three.

Controls
Quick usage 
 
 
  • Built-ins auto-disable smartly: Select all disables once every option is
						selected, Clear disables while nothing is selected — no callback needed.
    •  
  • The action bar is laid out with attributes: actions-position, actions-align and actions-layout (try the controls).
    •  
  • Custom buttons receive the live element as ms; here ms.setSelected(values, { notify: true }) fires change.
 
 
AB02 Dynamic buttons
 
Text, visibility and disabled state driven by callbacks that re-evaluate on every selection change.
 
  
Open the dropdown and select items: the Clear button appears with a live count, and Select all disables once everything is picked.
 
 
Quick usage 
 
 
 
  • getTextCallback(ms) — compute the label (here a live selected count).
    •  
  • getIsVisibleCallback(ms) — show/hide the button (Clear only appears when
						something is selected).
    •  
  • getIsDisabledCallback(ms) — enable/disable reactively (Select all off at
						max).
    •  
  • getClassCallback(ms) — swap CSS classes by state (inject the styles via customStylesCallback, since the button lives in the shadow root).
    •  
  • Callbacks always win over the matching static property (text, cssClass, …).
 
Dynamic actionButtons 
 
 
 
 
AB03 The standardized action context (v2.2.0)
 
Callbacks now get a typed ActionContext — live state + a controller facade — as their 2nd argument.
 
  
Open the dropdown and select items: the first button’s label tracks selectedCount / optionCount and its click toasts via ctx.controller; “Invert” flips the selection using ctx.options + ctx.selectedValues.
 
 
Quick usage 
 
 
 
  • v2.2.0 hands every action-button callback a typed ActionContext<T> as an additive 2nd argument — onClick, getTextCallback, getIsVisibleCallback, getIsDisabledCallback, getClassCallback and getTooltipCallback. Existing one-argument (ms) => … code is
						unaffected.
    •  
  • It's a snapshot of live state — selectedCount, optionCount, selectedValues, selectedOptions, options, searchTerm, isOpen — plus ctx.controller, a MultiSelectController facade that mirrors the element API
						(setSelected, showMessage, open/close, search, …).
    •  
  • It extends PresentationContext, so a button can branch on ctx.isFullscreen / ctx.isModal — the same contract the render
						callbacks use. ActionContext and MultiSelectController are exported
						from the package entry.
 
Reading the ActionContext