Zif: A Drop-in Framework for DragonRuby GTK

Zif is a collection of features commonly required in 2D games. The name is a reference to Zero Insertion Force connectors used on the original Nintendo - You can drop it in your project and it should just work. Everything is namespaced to Zif so any existing classes or namespaces you have should be preserved.

Example App

This repo is an example app - the Zif library is entirely contained within the app/lib directory.

Running the Example App:

  1. Create a fresh copy of DragonRuby GTK - you can call this folder dragonruby-zif-testbed and it should contain the dragonruby executable.
  2. Change directory in your terminal to dragonruby-zif-testbed
  3. Clone this entire dragonruby-zif repo inside the dragonruby-zif-testbed directory - git clone https://github.com/danhealy/dragonruby-zif.git.
  4. Run using ./dragonruby dragonruby-zif/

Installation in your app:

  1. Create a lib directory inside the app directory, and then copy the app/lib/zif directory into it.
  2. In your main.rb, require the parts of Zif you need. To require everything:
# Require all of the Zif library:
require 'app/lib/zif/require.rb'

def tick
  # ....

If you only want to require some parts, please see the require.rb file for more information.




The Zif module itself has a collection of frequently used helpers, like Zif.sub_positions for subtracting one array with two elements (e.g. [x, y]) from another.

  • .boomerang(i, max): returns i if it's under max, or max - (i - max) otherwise, clamped to 0
  • .add_positions(a, b) adds two 2-element arrays together. [10, 20] + [3, 4] = [13, 24]
  • .sub_positions(a, b) subtracts two 2-element arrays together. [10, 20] + [3, 4] = [7, 16]
  • .position_math(op, a, b) arbitrary operation on two 2-element arrays. Will send op to a elements with b elements as arguments. .position_math(:plus, a, b) is equivalent to .add_positions(a, b)
  • .relative_rand(x) returns a random integer around 0 to a magnitude of x/2.
  • .rand_rgb(base, max) returns a random 3-element array of integers of at least base and at most max
  • .hsv_to_rgb(h, s, v) expects hues 0-359, saturation and value 0-100 and returns a 3-element array of integers representing the red, green and blue values.
  • .distance(x1, y1, x2, y2) implements the distance formula.
  • .radian_angle_between_points(x1, y1, x2, y2)
  • .roll(given_dice) expects given_dice to be a hash {dice: X, sides: Y, modifier: Z}. Rolls X dice of Y sides each and adds Z to the total.
  • .roll_raw(die, sides) rolls die dice of sides sides and returns the total.

Some methods will expect array arguments and others expect individual elements (add_positions vs distance), remember that you can splat arrays: .distance(*a, *b) works if a and b are 2-element arrays.


These modules are designed to be included in another class.

  • Zif::Serializable: Works with any class. Automatically defines #serialize #to_s and #inspect based on the instance variables defined on the class. As you will encounter using DRGTK, if these methods are defined, DRGTK will use them to automatically print info to the console when exceptions occur.
  • Zif::Actionable: Works with any class. Defines the required attributes & methods to allow this class to be used with the ActionService, namely #run, #new_action, #stop_action etc
  • Zif::Assignable: Works with classes which implement sprite attributes (Zif::Sprite or any class with attr_sprite). Allows you to #assign sprite attributes to the object with a hash, e.g. obj.assign({x: 10, y: 10, w: 32...
  • Zif::Animatable: Works with classes which implement #path and #path=. Helpers for generating Sequences which change the path over time (animations).
  • Zif::Traceable: Works with any class but expects $services. Allows you to set a variable @tracer_service_name to the registered name of the Zif::TickTraceService, and afterwards you can simply use #mark to mark a section of code in this class for performance tracing.

Basic Classes


This class is the basis for most of the framework. It's a class which uses attr_sprite and defines some basic helper methods: #xy returns an array [@x, @y], and so on. Includes all the mixins described above.

Example usage:

@dragon = Zif::Sprite.new.tap do |s|
  s.x = 300
  s.y = 300
  s.w = 100
  s.h = 80
  s.path = "sprites/dragon_0.png"

You can give Sprites a name - this is used directly by Zif::SpriteRegistry, etc.

A Zif::Sprite has attribtues for logical_x/y. These can be used to assign a position to the sprite independent of the pixel size. For example, Zif::TiledLayer uses these attributes to position a tile relative to other tiles, independently of tile pixel size.

It has a z attribute, on_mouse_[down/changed/up] attributes, and it implements #clicked? - These are used by the Zif::InputService.

To support Zif::RenderTarget, a render_target attribute is set on a Sprite which is a "containing sprite" for a render target (meaning the path is assigned to a RenderTarget name).


This class acts like a (inherits from) Sprite but can itself accept a list of @sprites and @labels, like $gtk.args.outputs. Sprites and labels added to these arrays will be drawn using the CompoundSprite#draw_override method, which is checked by DragonRuby GTK during the draw cycle.

The CompoundSprite itself is not drawn directly, per se (@path is ignored), but has x/y/w/h/source_x/source_y/source_w/source_h values which act as modifiers to the @sprites it is drawing. A sprite which has been added to a CompoundSprite's @sprites array will be drawn in the following way:

  • The CompoundSprite's x/y/w/h act as a viewable rectangle on the main screen and are absolute values compared to the game resolution. Sprites which would be drawn completely outside of this rect will be ignored. Important! This is unlike Render Targets or regular sprites, which cut off the image cleanly at these boundaries. The CompoundSprite is a virtual effect which can't slice a sprite in half. Therefore, the entire sprite is rendered if even a portion is visible in the viewable rect.
  • The CompoundSprite's source_x/source_y/source_w/source_h act like these attributes would if displaying a normal image instead of a collection of sprites. They are relative values of the @sprites it is drawing.
    • source_x/source_y reposition the origin of the viewport into the @sprites array. E.g. If you have a sprite @ 0x/0y with 10w/10h it will not be drawn if the CompoundSprite's source_x/source_y exceeds 10/10
    • source_w/source_h describe the extent of the viewport into the @sprites array. E.g. If the CompoundSprite has 0x/0y/20w/20h/0 source_x/0 source_y/10 source_w/10 source_h, the example 0x/0y/10w/10h sprite will be displayed twice as large as normal.
    • Important! As above, unlike a normal sprite, changing the source_x/source_y/source_w/source_h will not cause sprites drawn this way to be sliced in any way. It will simply zoom and pan the complete sprites, and possibly ignore them if they exceed the extent of the viewable rectangle or the source viewport.

You can use CompoundSprite to draw several sprites which need to move together or relative to each other. You can move the entire collection by changing the CompoundSprite's x and y values, or you can move the component sprites relative to each other individually.

This class is the basis for ActiveLayer and many of the UI elements (TwoStageButton, ProgressBar, NinePanel, etc).


A render target is a way to programmatically create a sprite. It acts just like $gtk.args.outputs in that it accepts an array of sprites and other primitives. It gets rendered on the tick where its width and height are defined (allocated). To display it, you need to create a sprite and reference the name of the render target as the path.

Render targets are great for increasing the performance of your game by rendering a large collection of sprites once, and then just reusing that render later. It's also good for displaying the same image composed of many sprites multiple times (think minimap or mirrors).

This class handles this for you: it accepts sprites labels and primitives arrays. You can force it to #redraw. It produces a #containing_sprite (a Zif::Sprite) which references itself as the path and sets the source_* attributes to match the width and height. You can use #project_to and #project_from to control panning and zooming of the containing sprite. It can be used with the InputService as it responds to #clicked? - and passes the click down to the component sprites and primitives.

Example usage:

paint_canvas = Zif::RenderTarget.new(:my_paint_canvas, :white, 1000, 500)
paint_canvas.sprites << @all_current_brushstrokes
$gtk.args.outputs.static_sprites << paint_canvas.containing_sprite

Although an already rendered RenderTarget is cheap to display, one drawback of using RenderTargets is that they take a little longer to process in the first place, compared to simply drawing sprites the normal way. This can become an issue if you need to frequently update the RenderTarget due to changes on the source sprites. Say you have a large tile map you pregenerate as a RenderTarget when the game loads. If you need to change a single tile, like if that tile represents a door and the door opens, normally you would need to regenerate the entire RenderTarget using all of the source sprites.

A technique the DragonRuby community (specifically Islacrusez, oeloeloel) has identified to overcome this performance issue is to build another RenderTarget using the previously rendered one, plus whatever sprites are changing. See this example implementation of this technique.

There are two downsides to this technique:

  • It's inherently an additive process, so erasing data is tricky. Zif has chosen a simple approach where you may delete a single rectangle per redraw.
  • Although Zif::RenderTarget keeps track of a single @containing_sprite which references it's @name (path), you are allowed to reuse that path in any number of other sprites this class might not know about.

This technique is built into Zif::RenderTarget using the #redraw_from_buffer method. This method accepts three arguments:

  • An array of sprites to be added to the new render target
  • An optional [x, y, width, height] 4-element array rectangle to be used to remove data from the old render target (if you need to delete something, mitigating the first issue above.
  • An optional array of sprites besides the containing_sprite which reference this RenderTarget's @name and therefore need to have their @path changed, mitigating the second issue above.

See the DoubleBufferRenderTest scene in the example app for a working example.

Example usage:

# building on previous example

minimap = # ... a sprite referencing the RenderTarget
new_brushstroke = # ... a new Sprite
erase_rect = [200, 200, 10, 10] # Let's say you erased something, too

paint_canvas.redraw_from_buffer([new_brushstroke], erase_rect, [minimap])


Inspried by SpriteKit's Actions and Squirrel Eiserloh's GDC talk on nonlinear transformations. An Action is a transition of a set of attributes over time using an easing function (aka tweening, easing). On #initialize you define:

  • an object to perform the action on (generally done for you via #new_action on Zif::Actionable)
  • a finish state which is a hash of attributes and their desired final values
  • a duration in ticks
  • the easing function to use (see Zif::Action::EASING_FUNCS)
  • the rounding function to use (Zif::Action::ROUNDING_FUNCS)
  • the number of times to repeat the action, either an integer value or :forever
  • Finally, it accepts a block to be used as a callback when the action finishes.

An Actionable can have several Actions running simultaneously.

Example usage:

# @dragon is a Zif::Sprite and therefore an Actionable
# Move from starting position to 1000x over 1 second, starting slowly, then flip the sprite at the end
@dragon.run(@dragon.new_action({x: 1000}, 1.seconds, :smooth_start) { @dragon.flip_horizontally = true })


A Sequence is a series of Actions to be run in order.

Example usage:

# Run some action sequences on this sprite
@dragon.run(@dragon.fade_out_and_in_forever) # fade_out_and_in_forever is an example sequence, check out the source
      # Move from starting position to 1000x over 1 second, starting slowly, then flip the sprite at the end
      @dragon.new_action({x: 1000}, 1.seconds, :smooth_start) { @dragon.flip_horizontally = true },
      # Move from the new position (1000x) back to the start 600x over 2 seconds, stopping slowly, then flip again
      @dragon.new_action({x: 600}, 2.seconds, :smooth_stop) { @dragon.flip_horizontally = false }

# Register the flying animation by name.
# Tell it to use the 4 images 1 through 4, then reverse back to 1.
# Hold each image for 4 ticks.
  [1,2,3,4,3,2].map { |i| ["dragon_#{i}", 4] }



A Scene is a full-screen view of your game. The concept in Zif::Game is to show one Scene at a time. So each Scene in your game should be a subclass of Zif::Scene which overrides #perform_tick. Using the structure in Zif::Game, #perform_tick comes after input handling and before updating Actionables. So your subclass should use #perform_tick to add/remove clickables/Actionables, and respond to any detected input. Switching scenes is handled in Zif::Game, based on the return value of #perform_tick. You can optionally define #prepare_scene - a method invoked prior to the first tick it is the active scene, and #unload_scene which is invoked after the Scene has been switched.


This is designed to be the base class for your game. It's mainly responsible for Scene switching and yielding to Scene#perform_tick as described above, but also it automatically registers all the Zif services and provides standard functionality around this, including exception handling.

Example usage: in main.rb

class MyGame < Zif::Game
  def initialize
    MyOneTime.setups # do anything here, like register services...
    register_scene(:rainbow_road, RainbowRoadScene) # (RainbowRoadScene is a Zif::Scene subclass)
    @scene = OpeningScene.new # (this is a Zif::Scene subclass)

def tick(args)
  if args.tick_count == 2 # Some things can't be initialized in DR on the first tick.
    $game = MyGame.new
    $game.scene.prepare_scene # if needed on first scene


Classes for 2D Scrolling Games

If your game's play area extends beyond the screen resolution, you likely want some way of managing this! These classes are designed for this use case.


Creates a set of overlapping play area layers and handles redrawing them.

Has a concept of "logical" position as a multiple of "tile" width/height. (For example, if your tiles are 16px wide, the 4th tile is at @logical_x==4 but @x==64)

As an example, you can have a "tiles" layer which gets redrawn only at the start of the game, an "interactive objects" layer which gets redrawn whenever objects appear or disappear, and then an "avatar" layer which gets redrawn every time the avatar moves. The advantage of using RenderTargets here is to keep the positioning consistent across all of the layers. You can just pass all of the RT containing sprites to Zif::Camera and it will pan them all in unison.

You setup and configure these layers via #new_simple_layer and #new_tiled_layer.

Performance notes:

  • Since the memory requirements here are based on the number of layers * area of each layer, consider other approaches if you have a lot of layers with few sprites in them (maybe use sprites directly but with more math to keep positions in sync)
  • It is expensive to redraw a RT with thousands of sprites. Consider - 1280x720 / 16x16 -> 80*45 = 3600 tiles. Of course it's more expensive to draw these every tick (not using render_target), but you will see noticable hiccups if you do this often. Try not to redraw RTs with lots of sprites while action is happening.
    • You can use the RenderTarget's #redraw_from_buffer strategy indirectly by setting the @rerender_rect attribute. If this attribute is set when the layer is redrawn, it'll remove that rect from the last rendered version of the layer and redraw any sprites which intersect that rect.

Example usage:

map_layer_render_target_prefix = "map"
tile_width_and_height = 64 # pixels
map_width_and_height_in_tiles = 100 # 64 * 100 = 6400x6400 pixels

@map = Zif::LayeredTileMap.new(
@map.force_refresh # Force it to set up the render targets for the first time

# The should_render attribute will be checked each tick to decide if the RT should render
@map.layers[:avatar].should_render = true
@map.layers[:avatar].source_sprites = [@dragon]

# ----
# Add a bunch of tiles over a few ticks
a_new_tile = Zif::Sprite.new....
@map.add_positioned_sprite(x, y, a_new_tile)
# ----

# When all the tiles have been added
@map.layers[:tiles].should_render = true

@camera = Zif::Camera.new(

@map.layers[:tiles].should_render = false # Really just want to render the tiles once.

$gtk.args.outputs.static_sprites << @camera.layers


Designed to be used with Zif::LayeredTileMap, this is an extension of RenderTarget where source_sprites is a simple flat array. Uses natural x/y positioning, and visible_sprites is a simple select of sprites which intersect_rect?.

Defines the #redraw_from_buffer method described above.


Acts like SimpleLayer but is implemented using CompoundSprite instead of RenderTarget. The source_sprites will be drawn on every tick, so use this for layers which have low sprite counts but which need to be redrawn frequently (like the player character).


A subclass of SimpleLayer, this redefines source_sprites as a 2-dimensional array, indexed by logical (tile) position.


A TiledLayer where the sprites are chosen via bitmasked adjacency rules on the presence data layer - otherwise known as Autotiling. This class expects that you've registered your autotile images using a SpriteRegistry available at $services[:sprite_registry]. See Zif::SpriteRegistry#register_autotiles


Designed to work with Zif::LayeredTileMap, the Camera is initialized with a set of layer sprites, typically these are the containing_sprites of large render targets. It zooms these sprites to fit the viewable area of the screen. It is responsible for directing the layers to reposition based on camera movements.

The Camera is an example of an Actionable class which isn't a Sprite. It defines #pos_x and #pos_x= methods to act like a single accessor for each layer's source_x values. In this way, we can ease the panning of all the layers by creating an Action for a final pos_x value.

Example usage: (also see usage under Zif::LayeredTileMap)

@camera.start_following(@dragon) if @dragon.walking

This implements #scrolled? and interprets it as a directive to zoom the layer sprites. A zoom factor of 1.0 indicates the native screen size, you can retrieve this with #zoom_unit. Min and max zoom factor are controlled by @max_zoom_in/@max_zoom_out. By default, it will center the zoom at the last followed location @last_follow or the center of the screen.

#translate_pos(given) translates a point in the window with the corresponding point on the map layers, by considering the zoom factor and current viewport.

TODO: Camera could be extended to support parallax effects...

Panels, Components, Labels

These are classes which help create UI elements. TODO: More documentation.


Services are game features which can be accessed from any context within your game. If you use Zif::Game, each service will be registered in the Game's ivar @services by a symbol, e.g. @services.register(:action_service, Zif::ActionService.new) is run during initialization and thereafter you can access the ActionService by game.services[:action_service]. Additionally by convention, both $game and $services are available as global variables.


Register your sprite as something to check for running Actions by using #register_actionable. Call #run_all_actions once per tick (handled by Zif::Game automatically), which will invoke #perform_actions on each registered @actionable.


On each tick, #process_click should be run, which will detect clicks and pass them on to each sprite which has been registered via #register_clickable. It expects each clickable object to define a #clicked?(point, kind) method. If the sprite decides it has been clicked, it should return itself from this method. Clicks are passed through to sprites based on their z value - if this value is nil, the service considers it to be at the bottom. If the argument absorb_click is true, it directs the service to stop bubbling the click through any other clickables lower in the z order.

Zif::Sprite defines #clicked? and a set of ivars which are expected to contain callback lambdas: @on_mouse_down, @on_mouse_up, @on_mouse_changed. These callbacks will receive as arguments the sprite itself and the location of the mouse in point. If it can't handle the click but it knows it is the containing_sprite for a Zif::RenderTarget, it passes the click through.

Zif::RenderTarget also defines #clicked?, and passes clicks down to the component @sprites and @primitives of the render target.

Zif::SimpleLayer defines #clicked? and uses its #visible_sprites method to decide which component @source_sprites need to be checked for clicks.

#register_scrollable, is analogous to #register_clickable but for the scroll wheel. #scrolled? is expected to be defined, and it receives the mouse point and the direction of scrolling as arguments. Only Zif::Camera defines #scrolled? out of the box.


This service allows you to register prototypes of assets as Zif::Sprites.

Example usage:

The following code will create a Zif::Sprite with w/h of 82px and 66px, referencing a path of sprites/dragon_1.png. The second line demonstrates getting a fresh Zif::Sprite copy with these settings.

$services[:sprite_registry].register_basic_sprite("dragon_1", 82, 66)
@dragon = $services[:sprite_registry].construct("dragon_1")

#register_basic_sprite accepts a block: this is a convenience, it sets the block to the constructed Sprite's @on_mouse_up callback.

You can setup an alias for a registered sprite by using #alias_sprite

You can automatically register images used for autotiling / Zif::BitmaskedTiledLayer using the #register_autotiles method. See the comments at Zif::BitmaskedTiledLayer and Zif::SpriteRegistry#register_autotiles.


Generally, you want your game to run at a full 60fps. If your tick takes longer than 16.6ms, you'll drop below that number. The TickTrace service is designed to report when a tick has taken longer than a threshold (20ms by default), and hopefully narrow down the slowest section of code. #reset_tick must be called at the beginning of a tick, and then #finish at the end. If you use Zif::Game, this is done for you, all you need to do is include Traceable in any class you want to mark, set the @tracer_service_name ivar to :tracer, and then mark('a section of code'). Since backtraces are not supplied in DRGTK, the best it can do is tell you the name of the class it was invoked in. By convention, you should include the name of the method which calls #mark: mark('#my_method: a section of code')

#mark_and_print is also available, if you want to print to the console when you mark the section.

Example output:

You should see some output like this when running the World example scene. The first tick which renders the tile layer actually takes a long time.

Zif::TickTraceService: Slow tick.  59.609ms elapsed >  20.000ms threshold, longest step 'Zif::LayeredTileMap: #refresh: Rerendering tiles at [1800, 1200, 1280, 720]'  29.817ms:
       mark     delta label
    0.010ms   0.010ms ZifExample: #standard_tick: begin
    0.059ms   0.049ms ZifExample: #standard_tick: Scene #perform_tick complete
   29.876ms  29.817ms Zif::LayeredTileMap: #refresh: Rerendering tiles at [1800, 1200, 1280, 720]
   59.079ms  29.203ms Zif::LayeredTileMap: #refresh: Rerendered tiles
   59.098ms   0.019ms Zif::LayeredTileMap: #refresh: Rerendering stuff at [1800, 1200, 1280, 720]
   59.484ms   0.386ms Zif::LayeredTileMap: #refresh: Rerendered stuff
   59.496ms   0.012ms Zif::LayeredTileMap: #refresh: Rerendering avatar at [1800, 1200, 1280, 720]
   59.517ms   0.021ms Zif::LayeredTileMap: #refresh: Rerendered avatar
   59.528ms   0.011ms Zif::LayeredTileMap: #refresh: Rerendering top_effects at [1800, 1200, 1280, 720]
   59.539ms   0.011ms Zif::LayeredTileMap: #refresh: Rerendered top_effects
   59.545ms   0.006ms Zif::LayeredTileMap: #refresh: Rerendered all layers
   59.582ms   0.037ms ZifExample: #standard_tick: Scene switching handled
   59.609ms   0.027ms ZifExample: #standard_tick: end