Gio support

Most of the Gio facilities are supported natively through gobject-introspection layer. However, lgi provides Gio.Async which help in using Gio-style asynchronous I/O operations.

Asynchronous IO support

Native Gio asynchronous operations are based on traditional callback scheme, i.e. the operation is started, pushed to be performed on the background and when it finishes, it calls registered callback with operation results as arguments. While this scheme is widely used for asynchronous programming, it leads to spaghetti code full of callbacks. Lua provides coroutines, which can be used to make the code look 'synchronous' again, but still retaining the advantage of non-blocking I/O operations.

Gio-style asynchronous functions come as pair of two methods; name_async which starts operation and registers callback, and name_finish, which should be called in the context of registered callback and allows retrieveing operation results. When Gio override is loaded, lgi detects any of these pairs (in any object, not just from Gio namespace) and when found, it synthesizes async_name operations, which wraps native methods and uses Lua coroutines to convert callbacks into synchronous code. In order for async_method to work, these methods have to be called in the context of functions called through Gio.Async spawning facilities; either Gio.Async.call for synchronous calls and Gio.Async.start for starting routine on background.

Gio.Async class

This helper class implemented by lgi (not originating from introspected Gio module) contains interface for using lgi asynchronous support. This class contains only static methods and attributes, it is not possible to instantiate it.

Gio.Async.call and Gio.Async.start

local call_results = Gio.Async.call(user_function[, cancellable[, io_priority])(user_args)
local resume_results = Gio.Async.start(user_function[, cancellable[, io_priority])(user_args)

These methods accept user function to be run as argument and return function which starts execution of the user function in async-enabled context.

Any async_name methods called inside context do not accept io_priority and cancellable arguments (as their name_async original counterparts do), instead global cancellable and io_priority values given as arguments to Gio.Async.call/start are used.

Gio.Async.cancellable and Gio.Async.io_priority

Code running inside async-enabled context can query or change value of context-default cancellable and io_priority attributes by getting or setting them as attributes of Gio.Async class.

If cancellable or io_priority arguments are not provided to Gio.Async.start or Gio.Async.call, they are automatically inherited from currently running async-enabled coroutine, or default values are used (if caller is not running in async-enabled context).

Simple asynchronous I/O example

Following example reacts on the press of button, reads contents of /etc/passwd and dumps it to standard output.

local window = Gtk.Window {
  ...   Gtk.Button { id = 'button', label = 'Breach' }, ...
}

function window.child.button:on_clicked()
    local function dump_file(filename)
        local file = Gio.File.new_for_path(filename)
        local info = file:async_query_info('standard::size', 'NONE')
        local stream = file:async_read()
        local bytes = stream:async_read_bytes(info:get_size())
        print(bytes.data)
        stream:async_close()
    end
    Gio.Async.start(dump_file)('/etc/passwd')
end

Note that all reading happens running on background, on_clicked() handler finished when the operation is still running on background, so if you have a few gigabytes worth /etc/passwd file, the application will not freeze while dumping it.

samples/giostream.lua provides far more involved sample illustrating use of asynchronous operations.

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