Chapter 3: Actions

In this chapter, we will learn:

  • How and why to use the command pattern to encapsulate application functionality
  • How to create and use Action
  • How to trigger actions using Button, or by pressing a keyboard shortcut

Introduction: The Command Pattern

As we create more and more complex applications, keeping track of how / when to trigger which functionality gets harder and harder. An application can have hundreds, if not thousands, of functions, all linked to one or more triggers such as buttons, menus, keyboard shortcuts, etc.

Things will get out of hand very quickly, which is why there's a software design pattern just for this purpose: the command pattern.

A command, henceforth called action, is an object that has the following components:

  • A function, which is the action's behavior
  • An ID that uniquely identifies the action
  • An optional shortcut trigger, also often called a keybinding

In Mousetrap, a command is represented by the type Action.


As early as possible, we should drop the habit of defining application behavior inside a global function. Unless a function is used exactly once, it should be an action.

For example, in the previous chapter, we declared a Button with the following behavior:

button = Button()
connect_signal_clicked!(button) do self::Button

In this section, we will learn how to reproduce this behavior using the command pattern and why we should prefer this over connecting a signal handler for signal clicked.

Action IDs

When creating an action, we first need to choose the action's ID. An ID is an identifier that uniquely identifies the action. The ID can only contain the character [a-zA-Z0-9_-.], that is, all roman letters, numbers 0 to 9, _, - and .. The dot is usually reserved to simulate scoping.

For example, one action could be called, while another is called Both actions say what they do, save a file, but the prefix makes it clear which part of the application they act on.

An appropriate ID for our button behavior would therefore be example.print_clicked.

Action Function

Armed with this ID, we can create an action:

action = Action("example.print_clicked", app)

Where app is the application instance from our main.

The second part of an action is its function, also called its callback. We assign an actions function using set_function!:

function on_example_print_clicked(x::Action) ::Nothing

action = Action("example.print_clicked", app)
set_function!(on_example_print_clicked, action)

or, using do-syntax:

action = Action("example.print_clicked", app)
set_function!(action) do x::Action

The function registered using set_function! is required to have the following signature:

(::Action, [::Data_t]) -> Nothing

We see that, much like with signal handlers, the callback is provided the Action instance, along with an optional data argument.

Action also provides a constructor that directly takes the function as its first argument. Using this, we can write the above even more succinctly:

action = Action("example.print_clicked", app) do x::Action

Triggering Actions

At any point, we can call activate! to trigger the actions' callback. This is not the only way to trigger an action, however.

Button provides set_action!, which makes it such that when the button is clicked, the action is triggered:

action = Action("example.print_clicked", app)
set_function!(action) do x::Action

button = Button()
set_action!(button, action)

So far, this doesn't seem to have any upsides over just connecting to signal clicked. This is about to change.

Disabling Actions

Similarly to how blocking signals work, we can disable an action using set_enabled!. If set to false, calling activate! will trigger no behavior. Furthermore, all objects the action is connected to are automatically disabled. This means we do not need to keep track of which button calls which action. To disable all of them, we can simply disable the action.

Action Maps

We recall that Actions constructor requires an instance of our Application as its second argument. This is because the two are linked internally, all actions are registered with the application and are accessible only from within that application. In this way, Application itself acts as an action map, an index of all actions.

Once set_function! was called, we can, at any point, retrieve the action from the application using get_action!:

let action = Action("example.print_clicked", app)
    set_function!(action) do x::Action

# `action` is no longer defined here because of `let`

activate!(get_action(app, "example.print_clicked"))

# but we can retrieve it anyway

Where we used a let-block to create a "hard" scope, meaning at the end of the block, action, the Julia-side object, is no longer defined. We can nonetheless retrieve it by calling get_action! on our Application instance.

This way, we do not have to keep track of actions ourselves; by simply remembering the action's ID, we can, at any point, trigger the action from anywhere in our application.


An action can have a number of optional shortcut triggers, which are aslo called keybindings.

A keybinding is a combination of keyboard keys that, when pressed, trigger an action exactly once. Common keyboard shortcuts familiar to most users of modern operating systems are Control + C to copy,Control + A to "select all", etc.

Most of the time, we will have to implement behavior like this and associate a shortcut with it manually, using actions.

Shortcut Trigger Syntax

Before we can learn about keybindings, we need to talk about keys. In Mousetrap, keyboard keys are split into two groups: modifiers and non-modifiers.

A modifier is one of the following:

  • Shift
  • Control
  • Alt

Additional modifiers include CapsLock, AltGr, Meta, Apple and Win. These are keyboard-layout and/or OS-specific. See here for more information.

A non-modifier, then, is any key that is not a modifier.

A keybinding, or shortcut trigger, henceforth also called "shortcut", is the combination of any number of modifiers, along with exactly one non-modifier key. A few examples:

  • a (that is the A keyboard key) is a shortcut
  • <Control><Shift>plus (that is the + keyboard key, along with the Control and Shift modifiers) is a shortcut
  • <Alt><Control><Shift> is not a shortcut, because it does not contain a non-modifier
  • <Control>xy (that is the X key and the Y key) is not a shortcut, because it contains more than one non-modifier key

Shortcuts are represented as strings, which have a specific syntax. As seen above, each modifier is enclosed in <>, with no spaces in between. After the group of modifiers, the non-modifier key is placed after the last modifiers >. Some more examples:

  • "Control + C" is written <Control>c
  • "Alt + LeftArrow" is written as <Alt>Left (sic, L is capitalized)
  • "Shift + 1" is written as exclam

That last one requires explanation. On most keyboard layouts, to type !, the user has to press the shift modifier key, then press the 1 key. When "Shift + 1" is pressed, Mousetrap does not receive this keyboard key event as-is, instead, it receives a single key event for the ! key with no modifiers. The identifier of ! is exclam, hence why "Shift + 1" is written as exclam.

Looking up Key Identifiers

An example of how to look up the identifier of any key will be performed here.

Let's say we want to write the shortcut "Control + Space". We know that we can write "Control" as <Control>. Next, we navigate to, which has a list of all keys recognized by Mousetrap. In line 1039, we find that the constant for the space key is called KEY_space. The identifier of a key used for shortcuts is this name, without the KEY_ prefix. For the space bar key, the enum value is KEY_space, the identifier is therefore space.

One more obscure example: to write "Alt + Beta", that is, the β key on the somewhat rare greek keyboard layout, we find the constant named KEY_Greek_BETA in line 3034. Erasing KEY_ again, the keys' identifier is Greek_BETA. "Alt + Beta" is therefore written as <Alt>Greek_BETA

If we make an error and use the wrong identifier, a soft warning will be printed at runtime, informing us of this.

To access a list of key codes from within the REPL, we can search the vector Mousetrap.key_codes, which contains the symbols of all key codes recognized by Mousetrap, with the KEY_ prefix already removed:

julia> Mousetrap.key_codes
    2278-element Vector{Symbol}:
Operating System Priority

Depending on the operating system, some shortcuts will already be assigned. If this is the case, we should take care not to use them in our application. For example, the above mentioned <Control>space shortcut is reserved for changing input sources on macOS, while on Windows <Control><Alt>Delete will always open the task manager.

Assigning Shortcuts to Actions

Now that we know how to write a shortcut as a shortcut trigger string, we can assign it to our actions. For this, we use add_shortcut!:

shortcut_action = Action("example.shortcut_action", app) do self::Action
    println("shortcut action called")
add_shortcut!(shortcut_action, "<Control>M")

An action can have multiple shortcuts, and one shortcut can be associated with two or more actions, though the latter is usually not recommended.

We need one more thing before we can trigger our action: an object that can receive keyboard key events. We will learn much more about the event model in its own dedicated chapter. For now, we can use set_listens_for_shortcut_action! on our top-level window. This makes the window instance listen for any keyboard presses. If it recognizes that a keybinding associated with an action it is listening for was pressed, it will trigger that action.

A complete main.jl file showing how to trigger an action using a shortcut is given here:

using Mousetrap
main() do app::Application

    # create a window
    window = Window(app)

    # create an action that prints `shortcut action called`
    action = Action("example.shortcut_action", app) do action::Action
        println("shortcut action called")

    # add the shortcut `Control + M`
    add_shortcut!(action, "<Control>M")
    # make `window` listen for all shortcuts of `action`
    set_listens_for_shortcut_action!(window, action)

    # show the window to the user

Pressing "Control + M", we get:

shortcut action called