UIElement:update_tree

Computes layout and runs update logic for this element and all its descendants. You should call this once per frame on the root of each UI element tree your game uses.

The outline of everything happening within this call is:

1. UIElement:update is called on every element inside this tree. Notably, this ticks all scripts owned by Widget elements.
2. The element tree is traversed again so every element can compute its desired size (compute_desired_size is called on every element).
3. The element tree is traversed one last time to assign each element its actual size (set_relative_position is called on every element).

This method can only be called on elements that have no parent.

Usage

ui_element:update_tree(delta_time)

Arguments

Name	Type	Description
delta_time	number	Delta time in seconds.

When calling this variant, without explicit width and height arguments, the element will be sized to its desired size.

Usage

ui_element:update_tree(delta_time, width, height)

Arguments

Name	Type	Description
delta_time	number	Delta time in seconds.
width	number	Available width for the widget layout, in pixels.
height	number	Available height for the widget layout, in pixels.

Examples

This example defines a scene drawing a UI tree that covers the whole screen:

local TitleScreenUI = Class("TitleScreenUI", crystal.Overlay);

TitleScreenUI.init = function(self)
  TitleScreenUI.super.init(self);
  local text = self:add_child(crystal.Text:new());
  text:set_text("Legend of Sword");
  text:set_alignment("center", "center");
end

local TitleScreenScene = Class("TitleScreenScene", crystal.Scene);

TitleScreenScene.init = function(self)
  self.ui = TitleScreenUI:new();
end

TitleScreenScene.update = function(self, dt)
  local width, height = crystal.window.viewport_size();
  self.ui:update_tree(dt, width, height);
end

TitleScreenScene.draw = function(self)
  self.ui:draw_tree();
end

crystal.scene.replace(TitleScreenScene:new());