Enable Concepts

This document contains notes from a brain dump session by Peter Wang with Janet Swisher on March 19, 2008. It is notes rather than full explanations, which we hope to add later.

Enable Component

Component is the most important object in Enable, the center of everything. It represents a visual component. It both draws a screen object, and receives input for it (keyboard, mouse, and multitouch events).

Basic traits of Component include:

• visible: Whether it’s visible
• invisible_layout: Whether it uses space even when not visible (by default, invisible objects don’t take up space in layout)

Padding

Layout in Enable uses padding, similar to CSS. In Chaco, it’s used for things around the edges of plot, like labels and tick marks that extend outside the main plot area.

• fill_padding: Whether the background color fills the padding area as well as the main area of the component.
• padding_left
• padding_right
• padding_top
• padding_bottom
• padding: Sets or gets all 4 padding size traits at once
• hpadding: Read-only convenience property for the total amount of horizontal padding
• vpadding: Read-only convenience property for the total amount of vertical padding
• padding_accepts_focus: Whether the component responds to mouse events over the padding area

Parent Classes

Component subclasses both CoordinateBox (for drawing) and Interactor (for input). CoordinateBox has position and bounds traits, and some secondary attributes for convenience: x, y, x2, y2, width, height. Interactor mixes in responses for event types. You can subclass one of these classes if you want only its capabilities. For example, if you want something that doesn’t draw but does respond to events, subclass Interactor (e.g., a tool).

Interactor defines common traits for screen interaction, including:

• pointer: The cursor shape when the interactor is active
• event_state: The object’s event state, used for event dispatch

Containers

All components have a container. They can only have a single container. One component can’t be contained by two objects.

Whenever you request a component to redraw itself, it actually requests its container to redraw it, and a whole chain goes all the up to the top-level window.

Top-level Window

A component also has a reference to the top-level window. This window serves as a bridge between the OS and GUI toolkit. The window trait delegates all the way up the containment chain to the top-level component, which has an actual reference to the actual window.

The reference to the window is useful because Chaco doesn’t make calls directly to the GUI toolkit. Rather, it asks the window to do things for it, such as creating a context menu.

Event Dispatch

The key methods of Interactor are dispatch() and _dispatch_stateful_event(). There’s a complex method resolution that occurs beween Interactor, Component, Container (which is a subclass of Component), and the Chaco-based subclasses of Enable Component and Container.

When a component gets an event, it tries to handle it in a standard way, which is to dispatch to:

1. its active tool
2. its overlays
3. itself, so that any event handler methods on itself get called
4. its underlays
5. its listener tools

That logic is in Component, in the _new_dispatch() method, which is called from Component.dispatch() (_old_dispatch() will be removed in 3.0). If any of these handlers sets event.handled to True, event propagation stops. If an event gets as far as the listener tools, then all of them get the event.

(The notion of an active tool is not used in current code, just older client code. Experience has shown that the notion of a tool promoting itself to be the “active” tool isn’t really useful, because usually the tools need to interact with each other. For newer tools, such as Pan, Zoom, or !DragZoom, when the user starts interacting with a tool, that tool calls capture_mouse() at the window level, and then all mouse events go to that tool, circumventing the entire dispatch() mechanism.)

The event handlers that Component dispatches to are of the form :samp:{event_state}{event_suffix}`, where event_suffix corresponds to the actual kind of event that happened, e.g., left_down, left_up, left_dclick, etc. Most objects default to having just a single event state, which is the “normal” event state. To make an Enable component that handled a left-click, you could subclass Component, and implement normal_left_down() or normal_left_up(). The signature for handler methods is just one parameter, which is an event object that is an instance of (a subclass of) BasicEvent. Some subclasses of BasicEvent include MouseEvent, DragEvent, KeyEvent, and BlobEvent (for multitouch). It’s fairly easy to extend this event system with new kinds of events and new suffixes (as was done for multitouch). A disadvantage is that you don’t necessarily get feedback when you misspell an event handler method name in its definition.

(This scheme is difficult to implement when the number of states and events gets large. There’s nothing to tell you if you’ve forgotten to implement one of the possible combinations.)

If an interactor transforms an event, then it has to return the full transformation that it applies to the event.

When an event comes in, it has a reference to the GUI toolkit window that the event came from. Lots of code calls methods on event.window to get the window to do things, such as set a tooltip or create a context menu. That is the correct thing to do, because it’s possible for there to be two windows showing the same underlying component, so responses to events in a window should only happen in that window. When the user generates an event, that event propagates down the containment stack and things happen in response; a draw or update doesn’t actually happen until the next paint(). By that time, the component no longer has a reference to the event or the event’s window; instead it uses its own reference to the window, self.window.

Coordinate Systems

Every component has x and y traits from CoordinateBox. These are positions relative to the component’s parent container. When a container dispatches events, or loops over its children to draw, it transforms the coordinate system, so that as far as its children are concerned, the events are relative to the lower-left corner of the parent container. Objects don’t have to be bounded, but they do have to have an origin. The component is going to give coordinates to the GraphicsContext in its own coordinate system, and the container is responsible for offsetting the GC, and setting up the transform correctly. Likewise, when a component gets an event, it expects that event to be in the coordinate system of its parent container.

(This introduces some complexity in trying to handle mouse event capture. If a tool or component captures the mouse, the top-level window has no idea what the coordinate system of that object is. It has to be able to ask an event, “give me your total transformation up to this point”, and then apply that transformation to all subsequent events. Programmers using Chaco or Enable don’t usually have to think about this, but the interactor does have to be able to do it. Containers implement this, so if you’re just writing a standard component, you don’t have to worry about it.)

Viewports

A component can have a list of viewports, which are views onto the component. Currently, this is used for the canvas, and for geophysical plotting. You could use it for something like a magnifying-glass view of a portion of a component or plot without duplicating it.

Layout

Containers are the sizers that do layout. Components within containers can declare that they are resizable, for example, but that doesn’t matter if the container they are in doesn’t do layout.

The basic traits on Component for layout are resizable, aspect_ratio, auto_center. For the resizable trait, you can specify which directions the component is resizable in. Components also have lists of overlays and underlays.

You can get access to the actual bounds of the component, including its padding with the outer_name attributes. Those also take into account the thickness of any border around the component.

Rendering

Every component can have several layers:

• background
• image (Chaco only, not Enable)
• underlay
• main layer (the actual component)
• overlay

These are defined by DEFAULT_DRAWING_ORDER, and stored in the drawing_order trait.

Complexity arises when you have multiple components in a container: How do their layers affect each other? Do you want the “overlay” layer of a component to draw on top of all components? Do you want the “background” elements to be behind everything else?

This is resolved by the unified_draw trait. If it is False (the default), the corresponding layers of all components are drawn in sequence. The container is responsible for calling the components to draw their layers in the correct sequence. If it is True, then all layers of the component are drawn in strict sequence. The point is the overall sequence at which a component with unified_draw==True is drawn is determined by its draw_layer trait, which by default is ‘mainlayer’.

For example, if you want a plot to act as an overlay, you could set unified_draw==True and draw_layer=='overlay'. These values tell the container to render the component when it gets to the ‘overlay’ layer.

Set overlay_border to True if you want the border to draw as part of the overlay; otherwise it draws as part of the background. By default, the border is drawn just inside the plot area; set inset_border to False to draw it just outside the plot area.

Backbuffer

A backbuffer provides the ability to render into an offscreen buffer, which is blitted on every draw, until it is invalidated. Various traits such as use_backbuffer and backbuffer_padding control the behavior of the backbuffer. A backbuffer is used for non-OpenGL backends, such as agg and on OS X. If use_backbuffer is False, a backbuffer is never used, even if a backbuffer is referenced by a component.

Users typically subclass Chaco PlotComponent, but may need features from Enable Component.

Enable Container

Container is a subclass of Enable Component. Containers can be nested. Containers are responsible for event dispatch, draw dispatch, and layout. Containers override a lot of Component methods, so that they behave more like containers than plain components do.

Examples

BasicDraw

Just draws a box. Subclasses Component, and implements _draw_mainlayer(). Uses DemoFrame and demo_main() boilerplate code to simplify the example code.

BasicMove

Adds interaction. Has a more complex _draw_mainlayer(); defines an event handler.