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EventHandler—Wolfram Documentation

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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
Cite this Page

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:

Wolfram Language code: DynamicModule[{col = Green}, EventHandler[Style["text", FontColor -> Dynamic[col]], {"MouseClicked" :> (col = Red)}]]

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

Wolfram Language code: DynamicModule[{col = Green}, EventHandler[Style["text", FontColor -> Dynamic[col]], {"MouseClicked" :> (col = col /. {Red -> Green, Green -> Red})}]]

Scope  (1)

Create a graphic that changes color when clicked:

Wolfram Language code: DynamicModule[{col = Green}, EventHandler[Dynamic[Graphics[{col, Disk[]}, ImageSize -> Tiny]], {"MouseClicked" :> (col = col /. {Red -> Green, Green -> Red})}]]

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

Wolfram Language code: DynamicModule[{col = Green}, EventHandler[Dynamic[Graphics[{col, Disk[]}, ImageSize -> Tiny]], {"MouseDown" :> (col = col /. {Red -> Green, Green -> Red}), "MouseUp" :> (col = col /. {Red -> Green, Green -> Red})}]]

Options  (5)

Method  (1)

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

Wolfram Language code: DynamicModule[{a = "start"}, {EventHandler[Panel["does time out"], "MouseClicked" :> (Pause[6];a = "end")], Dynamic[a]}]

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

Wolfram Language code: DynamicModule[{a = "start"}, {EventHandler[Panel["does not time out"], "MouseClicked" :> (Pause[6];a = "end"), Method -> "Queued"], Dynamic[a]}]

PassEventsDown  (2)

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

Wolfram Language code: DynamicModule[{p = {0, 0}, c = Green}, EventHandler[EventHandler[Framed@Dynamic[Graphics[{c, Disk[p, 0.2]}, PlotRange -> 2]], {"MouseDown" :> (c = c /. {Red -> Green, Green -> Red})}], {"MouseDown" :> (p = MousePosition["Graphics"])}, PassEventsDown -> True]]

Do not pass mouse events to the inner event handler:

Wolfram Language code: DynamicModule[{p = {0, 0}, c = Green}, EventHandler[EventHandler[Framed@Dynamic[Graphics[{c, Disk[p, 0.2]}, PlotRange -> 2]], {"MouseDown" :> (c = c /. {Red -> Green, Green -> Red})}], {"MouseDown" :> (p = MousePosition["Graphics"])}, PassEventsDown -> False]]

PassEventsUp  (2)

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

Wolfram Language code: DynamicModule[{i = 1, j = 1}, Style[EventHandler[Framed@{Dynamic[i], EventHandler[Framed[Dynamic[j]], "MouseDown" :> ++j]}, "MouseDown" :> ++i], 36]]

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

Wolfram Language code: DynamicModule[{i = 1, j = 1}, Style[EventHandler[Framed@{Dynamic[i], EventHandler[Framed[Dynamic[j]], "MouseDown" :> ++j, PassEventsUp -> False]}, "MouseDown" :> ++i], 36]]

Allow the outer EventHandler action when the key is pressed:

Wolfram Language code: DynamicModule[{i = 1, j = 1}, Style[EventHandler[Framed@{Dynamic[i], EventHandler[Framed[Dynamic[j]], "MouseDown" :> ++j, PassEventsUp :> CurrentValue["ShiftKey"]]}, "MouseDown" :> ++i], 36]]

Applications  (2)

Automatically fit a line to a set of points:

Wolfram Language code: lineFit[points_, {{xmin_, xmax_}, {ymin_, ymax_}}] := Module[{x, lf}, lf = Function@@{{x}, Fit[points, {x, 1}, {x}]}; Line[{{xmin, lf[xmin]}, {xmax, lf[xmax]}}]];
Wolfram Language code: DynamicModule[{p = {}, l = {}}, EventHandler[Dynamic@Graphics[{Point[p], l}, PlotRange -> 1, Frame -> True], "MouseDown" :> (AppendTo[p, MousePosition["Graphics"]]; If[Length[p] ≥ 2, l = lineFit[p, {{-1, 1}, {-1, 1}}]])]]

Piecewise interpolation through a set of points:

Wolfram Language code: interpolationCurve[p_, n_] := Module[{f = Interpolation[p, InterpolationOrder -> n ]}, First@Plot[Evaluate@f[x], {x, Min[p[[All, 1]]], Max[p[[All, 1]]]}]];
Wolfram Language code: DynamicModule[{n = 2, p = {}, c = {}}, EventHandler[Dynamic@Graphics[{Point[p], c}, PlotRange -> 1, Frame -> True], "MouseDown" :> (p = Union[Sort@Append[p, MousePosition["Graphics"]], SameTest -> (First[#1] == First[#2]&)]; If[Length[p] ≥ n + 1, c = interpolationCurve[p, n]])]]

Properties & Relations  (1)

EventHandler can be used with MousePosition:

Wolfram Language code: DynamicModule[{pos = {0, 0}}, EventHandler[Framed@Graphics[{Disk[Dynamic[pos], 0.2]}, PlotRange -> 2], {"MouseDown" :> (pos = MousePosition["Graphics"])}]]

Neat Examples  (4)

Click to add points to a BSplineCurve:

Wolfram Language code: DynamicModule[{pts = {}, down = False}, EventHandler[ Graphics[{Thickness[.03], Red, BSplineCurve[Dynamic[pts]]}, PlotRange -> 1, Frame -> True, FrameTicks -> False], {"MouseDown" :> (AppendTo[pts, MousePosition["Graphics"]]; down = True), "MouseMoved" :> If[down, AppendTo[pts, MousePosition["Graphics"]]], "MouseUp" :> (down = False)}]]

Clicking the cone will reflect it across the axis:

Wolfram Language code: Graphics3D[DynamicModule[{b = False}, EventHandler[Cone[Dynamic[{{4, 4, 0}, {4, 4, If[b, -2, 2]}}]], {"MouseDown" :> (b = !b)}]], PlotRange -> {Automatic, Automatic, {-2, 2}}]

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

Wolfram Language code: DynamicModule[{f = Identity}, (EventHandler[Dynamic[Row[f[#1]& /@ Range[15], ","]], {"MouseEntered" :> (f = Function[{y}, If[PrimeQ[y], Style[y, Red], y]]), "MouseExited" :> (f = Identity)}])]

Display dynamic graphics and beep simultaneously:

Wolfram Language code: DynamicModule[{obj = {}, x}, EventHandler[ EventHandler[Dynamic[If[x < 1, x = x + .01];Graphics[{obj, Orange, Disk[{0, 0}, .2]}, PlotRange -> 1]], {"MouseClicked" :> (obj = {Dynamic[Table[{Circle[{0, 0}, n Dynamic[x]]}, {n, 3}]]};Beep[])}], {"MouseDown" :> (x = 0)}, PassEventsDown -> False]]
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_2026_eventhandler, author="Wolfram Research", title="{EventHandler}", year="2008", howpublished="\url{https://reference.wolfram.com/language/ref/EventHandler.html}", note=[Accessed: 14-November-2026]}

BibLaTeX

@online{reference.wolfram_2026_eventhandler, organization={Wolfram Research}, title={EventHandler}, year={2008}, url={https://reference.wolfram.com/language/ref/EventHandler.html}, note=[Accessed: 14-November-2026]}