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EventHandler[expr,{"event1":>action1,"event2":>action2,…}]

displays as expr, evaluating actioni whenever "eventi" occurs in connection with expr.

Details and Options
Details and Options Details and Options
Examples  
Basic Examples  
Scope  
Options  
Method  
PassEventsDown  
PassEventsUp  
Applications  
Properties & Relations  
Neat Examples  
See Also
Tech Notes
Related Guides
History
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EventHandler

EventHandler[expr,{"event1":>action1,"event2":>action2,…}]

displays as expr, evaluating actioni whenever "eventi" occurs in connection with expr.

Details and Options

  • Standard mouse-related event specifications include:
  • "MouseClicked"mouse clicked without moving
    {"MouseClicked",i}mouse button i clicked without moving
    "MouseDown"primary mouse button depressed
    {"MouseDown",i}mouse button i depressed
    "MouseDragged"mouse moving with primary button depressed
    {"MouseDragged",i}mouse moving with button i depressed
    "MouseMoved"mouse moves in the region defined by expr
    "MouseUp"primary mouse button released
    {"MouseUp",i}mouse button i released
  • For mouse events, EventHandler[expr, …] handles events for which the mouse is within the rectangular region defined by the display of expr.
  • Standard keyboard-related event specifications include:
  • "KeyDown"any key on the keyboard depressed
    {"KeyDown","x"}key x depressed
    "ReturnKeyDown" or key depressed
    "EscapeKeyDown" key depressed
    "LeftArrowKeyDown"left arrow key depressed
    "RightArrowKeyDown"right arrow key depressed
    "UpArrowKeyDown"up arrow key depressed
    "DownArrowKeyDown"down arrow key depressed
  • Standard interface events include:
  • {"MenuCommand","name"}menu command with specified name chosen
    "WindowClose"closing of window requested
  • If functions like MousePosition or CurrentValue are evaluated in a particular actioni, the values they give are those associated with the event that triggered that actioni.
  • When EventHandler expressions are nested, events are by default shared by all the expressions, with the innermost expression operating first on a particular event.
  • The following options can be given:
  • Method "Preemptive"the evaluation method to use
    PassEventsDown Automaticwhether to pass events to inner event handlers
    PassEventsUp Truewhether to pass events to outer event handlers
  • Typical possible settings for the Method option include "Preemptive" and "Queued".
  • With the default setting PassEventsDown->Automatic, events handled by an EventHandler are not also passed to built-in event handlers in the Wolfram System front end.

Examples

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Basic Examples  (2)

Create text that turns red when clicked:

Create text that toggles between red and green when clicked repeatedly:

Scope  (1)

Create a graphic that changes color when clicked:

Create a graphic that changes color only while the mouse button is pressed:

Options  (5)

Method  (1)

By default, event handlers are evaluated on a preemptive link and time out after 5 seconds:

Use Method->"Queued" to evaluate button functions on the main link, which never times out:

PassEventsDown  (2)

Allow events to pass down to inner event handlers, changing the color as well as position:

Do not pass mouse events to the inner event handler:

PassEventsUp  (2)

By default, nested EventHandler actions are triggered upon the specified action:

By disabling PassEventsUp, prevent the outer EventHandler action from being triggered:

Allow the outer EventHandler action when the key is pressed:

Applications  (2)

Automatically fit a line to a set of points:

Piecewise interpolation through a set of points:

Properties & Relations  (1)

EventHandler can be used with MousePosition:

Neat Examples  (4)

Click to add points to a BSplineCurve:

Clicking the cone will reflect it across the axis:

Highlight prime numbers in a list when mousing over the list:

Display dynamic graphics and beep simultaneously:

Wolfram Research (2007), EventHandler, Wolfram Language function, https://reference.wolfram.com/language/ref/EventHandler.html (updated 2008).

Text

Wolfram Research (2007), EventHandler, Wolfram Language function, https://reference.wolfram.com/language/ref/EventHandler.html (updated 2008).

CMS

Wolfram Language. 2007. "EventHandler." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2008. https://reference.wolfram.com/language/ref/EventHandler.html.

APA

Wolfram Language. (2007). EventHandler. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/EventHandler.html

BibTeX

@misc{reference.wolfram_2025_eventhandler, author="Wolfram Research", title="{EventHandler}", year="2008", howpublished="\url{https://reference.wolfram.com/language/ref/EventHandler.html}", note=[Accessed: 01-May-2026]}

BibLaTeX

@online{reference.wolfram_2025_eventhandler, organization={Wolfram Research}, title={EventHandler}, year={2008}, url={https://reference.wolfram.com/language/ref/EventHandler.html}, note=[Accessed: 01-May-2026]}