# Screens and Python

## Screen Functions

The following functions support various operations related to screens.

## Actions

Many of the displayables created in the screen language take actions as arguments. An action is one of three things:

- A callable Python object (like a function or bound method) that takes no arguments.

- An object of a class that inherits from the Action class.

- A list of other Actions.

The advantage to inheriting from the Action class is that it allows you to override the methods that determine when a button should be sensitive, and when it is selected.

class Action

  To define a new action, inherit from this class. Override the methods in this class to change the behavior of the action.

  __call__(self)

    This is the method that is called when the action is activated. In many cases, returning a non-None value from the action will cause the current interaction to end.

    This method must be overridden, as the default method will raise a NotImplemented exception (and hence cause Ren'Py to report an error).

  get_sensitive(self)

    This is called to determine if the button with this action should be sensitive. It should return true if the button is sensitive.

    Note that __call__ can be called, even if this returns False.

    The default implementation returns True.

  get_selected(self)

    This should return true if the button should be rendered as a selected button, and false otherwise.

    The default implemention returns False.

  get_tooltip(self)

    This gets a default tooltip for this button, if a specific tooltip is not assigned. It should return the tooltip value, or None if a tooltip is not known.

    This defaults to returning None.

  periodic(self, st)

    This method is called once at the start of each interaction, and then is called periodically thereafter. If it returns a number, it will be called before that many seconds elapse, but it might be called sooner.

    The main use of this is to call `renpy.restart_interaction()` if the value of get_selected or get_sensitive should change.

    It takes one argument:

    st

      The number of seconds since the screen or displayable this action is associated with was first shown.

  unhovered(self)

    When the action is used as the hovered parameter to a button (or similar object), this method is called when the object loses focus.

To run an action from Python, use `renpy.run()`.

## BarValues

When creating a bar, vbar, or hotbar, a BarValue object can be supplied as the value property. Methods on the BarValue object are called to get the adjustment and styles.

class BarValue

  To define a new BarValue, inherit from this class and override some of the methods.

  get_adjustment(self)

    This method is called to get an adjustment object for the bar. It should create the adjustment with `ui.adjustment()`, and then return the object created this way.

    This method must be overridden, as the default method will raise NotImplemented (and hence cause Ren'Py to report an error).

  get_style(self)

    This is used to determine the style of bars that use this value. It should return a tuple of two style names or style objects. The first is used for a bar, and the second for vbar.

    This defaults to ("bar", "vbar").

  get_tooltip(self)

    This gets a default tooltip for this button, if a specific tooltip is not assigned. It should return the tooltip value, or None if a tooltip is not known.

    This defaults to returning None.

  replaces(self, other)

    This is called when a BarValue replaces another BarValue, such as when a screen is updated. It can be used to update this BarValue from the other. It is called before get_adjustment.

    Note that other is not necessarily the same type as self.

  periodic(self, st)

    This method is called once at the start of each interaction. If it returns a number of seconds, it will be called before that many seconds elapse, but it might be called sooner. It is called after get_adjustment.

    It can be used to update the value of the bar over time, like `AnimatedValue()` does. To do this, get_adjustment should store the adjustment, and periodic should call the adjustment's changed method.

## InputValue

When creating an input, an InputValue object can be supplied as the value property. Methods on the InputValue object are called to get and set the text, determine if the input is editable, and handle the enter key being pressed.

class InputValue

  To define a new InputValue, inherit from this class, override some or all of the methods, and set the value of the default field.

  editable

    If not true, disables the input field from being editable at all.

  default

    If true, the input is eligible to be editable by default. (That is, it may be given the caret when the screen is shown.)

  get_text(self)

    Returns the default text of the input. This must be implemented.

  set_text(self, s)

    Called when the text of the input is changed, with the new text. This must be implemented.

  enter(self)

    Called when the user presses enter. If this returns a non-None value, that value is returned from the interacton. This may also raise renpy.IgnoreEvent() to ignore the press. Otherwise, the enter-press is propagated to other displayables.

  The following actions are available as methods on InputValue:

  Enable()

    Returns an action that enables text editing on the input.

  Disable()

    Returns an action that disables text editing on the input.

  Toggle()

    Returns an action that toggles text editing on the input.

## Creator-Defined Screen Language Statements

Ren'Py supports defining custom screen language statements. Creator-defined screen language statements are wrappers for the screen language use statement. Positional arguments remain positional arguments, properties become keyword arguments, and if the statement takes a block, so does the use statement. For example, the custom screen language statement:

    titledwindow "Test Window":
        icon "icon.png"

        text "This is a test."

becomes:

    use titledwindow("Test Window", icon="icon.png"):
        text "This is a test."

Creator-defined screen language statements must be registered in a `python early` block. What's more, the filename containing the creator-defined statement must be be loaded earlier than any file that uses it. Since Ren'Py loads files in the Unicode sort order of their paths, it generally makes sense to prefix the name of any file registering a user-defined statement with 01, or some other small number.

Creator-defined screen language statements are registered with the renpy.register_sl_statement function:

As an example of a creator-defined screen language statement, here's an implementation of the `titledwindow` statement given above. First, the statement must be registered in a `python early` block in a file that is loaded early – a name like 01custom.rpy will often load soon enough. The registration call looks like:

    python early:
        renpy.register_sl_statement("titledwindow", children=1).add_positional("title").add_property("icon").add_property("pos")

Then, we define a screen that implements the custom statement. This screen can be defined in any file. One such screen is:

    screen titledwindow(title, icon=None, pos=(0, 0)):
        drag:
            pos pos

            frame:
                background "#00000080"

                has vbox

                hbox:
                    if icon is not None:
                        add icon

                    text title

                null height 15

                transclude

When are used large property groups like a add_property_group, it makes sense to use the **properties syntax with a properties keyword in some place. For example:

    screen titledwindow(title, icon=None, **properties):
        frame:
            # When background not in properties it will use it as default value.
            background "#00000080"

            properties properties

            has vbox

            hbox:
                if icon is not None:
                    add icon

                text title

            null height 15

            transclude