# Introduction

## Scope of this package

GtkObservables is a package building on the functionality of Gtk.jl and Observables.jl. Its main purpose is to simplify the handling of interactions among components of a graphical user interface (GUI).

Creating a GUI generally involves some or all of the following:

1. creating the controls
2. arranging the controls (layout) in one or more windows
3. specifying the interactions among components of the GUI
4. (for graphical applications) canvas drawing
5. (for graphical applications) canvas interaction (mouse clicks, drags, etc.)

GtkObservables is targeted primarily at items 1, 3, and 5. Layout is handled by Gtk.jl, and drawing (with a couple of exceptions) is handled by plotting packages or at a lower level by Cairo.

GtkObservables is suitable for:

• "quick and dirty" applications which you might create from the command line
• more sophisticated GUIs where layout is specified using tools like Glade

For usage with Glade, the Input widgets and Output widgets defined by this package allow you to supply a preexisting widget (which you might load with GtkBuilder) rather than creating one from scratch. Users interested in using GtkObservables with Glade are encouraged to see how the player widget is constructed (see src/extrawidgets.jl).

At present, GtkObservables supports only a small subset of the widgets provided by Gtk. It is fairly straightforward to add new ones, and pull requests would be welcome.

## Concepts

The central concept of Observables.jl is the Observable, a type that allows updating with new values that then triggers actions that may update other Observables or execute functions. Your GUI ends up being represented as a "graph" of Observables that collectively propagate the state of your GUI. GtkObservables couples Observables to Gtk.jl's widgets. In essence, Observables.jl allows ordinary Julia objects to become the triggers for callback actions; the primary advantage of using Julia objects, rather than Gtk widgets, as the "application logic" triggers is that it simplifies reasoning about the GUI and seems to reduce the number of times ones needs to consult the Gtk documentation.

Changes to the UI can only be performed safely if the code is run in the same thread that the Gtk main loop is running on. Observable handlers may run on a different thread. If a UI update operation occurs on a different thread, the process (Julia) can crash. The solution is to wrap the offending code block with Gtk.@idle_add, which requests Gtk to run the code block on the UI thread when the CPU is idle. However, this also means that code inside the block will not run synchronously with code outside it.
on(myobs) do val
end