I started patching the focus subsystem of the EFL widget toolkit quite some time ago. During this time, people have started to assign me everything that somehow looks like an issue with focus, it sometimes only takes the existence of the word “focus” somewhere in a backtrace for this to happen. I’ve discovered that most people mix up these different types, so in this blog post I hope to provide some clarity about them.
How EFL Gets Focused
There are 3 different places focus happens in EFL:
- ecore-evas – the window manager abstraction
- evas – the canvas library
- elementary – the widget toolkit.
First of all, I should point out what focus itself is, I think a good example is to consider your typical smartphone interaction. While interacting with your smartphone, your complete attention is given to its screen and all interactions are with the interface of the device; you will also likely cease interactions with the outside environment entirely. In the same way, each abstraction in EFL has its own interaction partners:
- The focused canvas object gets the attention from the keyboard
- The focused widget is highlighted visually, so the user can see where his attention should go
- And the window manager in the end focuses an application, which is probably an EFL Application.
These differences are often the source of people’s confusion when it comes to focus in EFL. For example, loosing the toolkit focus on a window object does not mean that the window lost the input from the user; instead, it means that another widget got the toolkit focus and the window manager still has focus on this window.
For another example, consider a toolkit widget that’s built out of two objects: an image and a text field below the image. In this example, the widget receives the toolkit’s focus, and the focus of the canvas moves to the image. Then, the user presses some key bindings to change the name of the image and the canvas focus moves to the text field. In this case, the canvas focus moves, creating update events on the canvas focus. However, the widget’s focus stayed the same, and the user is meant to have their attention on that widget, meaning there was no change to it.
Some Tips for Understanding EFL Focus
The focus property can only be true on one single object in an entity, in practice this means:
- One window focused per user window manager session
- One object focused per user canvas
- One widget focused per widget tree in the toolkit
- Canvas focus is only used for injecting keyboard inputs from the input system of the display technology.
- Widget focus is used for navigation, in the case of focus movement initiated by key bindings, this is the position from where the next upper/down/right/left element is calculated.
If you ever use change events for some kind of a focus property, here’s what you need to know about focus in EFL:
- It’s window manager focus if the user has their attention on your application.
- It’s canvas focus if you need to know where the keyboard inputs are coming from.
- It’s a widget focus if the user has moved their attention to a subset of canvas objects that are bound together as a widget implementation.
If you have any questions about any of this content, head to the comments section!