Gio.Cancellable

g GObject.Object GObject.Object Gio.Cancellable Gio.Cancellable GObject.Object->Gio.Cancellable

Subclasses:None

Methods

Inherited:GObject.Object (37)
Structs:GObject.ObjectClass (5)
class get_current ()
class new ()
  cancel ()
  connect (callback, *data)
  disconnect (handler_id)
  get_fd ()
  is_cancelled ()
  make_pollfd (pollfd)
  pop_current ()
  push_current ()
  release_fd ()
  reset ()
  set_error_if_cancelled ()
  source_new ()

Virtual Methods

Inherited:GObject.Object (7)
  do_cancelled ()

Properties

None

Signals

Inherited:GObject.Object (1)
Name Short Description
cancelled Emitted when the operation has been cancelled.

Fields

Inherited:GObject.Object (1)
Name Type Access Description
parent_instance GObject.Object r  

Class Details

class Gio.Cancellable(**kwargs)
Bases:GObject.Object
Abstract:No
Structure:Gio.CancellableClass

Gio.Cancellable is a thread-safe operation cancellation stack used throughout GIO to allow for cancellation of synchronous and asynchronous operations.

classmethod get_current()[source]
Returns:a Gio.Cancellable from the top of the stack, or None if the stack is empty.
Return type:Gio.Cancellable or None

Gets the top cancellable from the stack.

classmethod new()[source]
Returns:a Gio.Cancellable.
Return type:Gio.Cancellable

Creates a new Gio.Cancellable object.

Applications that want to start one or more operations that should be cancellable should create a Gio.Cancellable and pass it to the operations.

One Gio.Cancellable can be used in multiple consecutive operations or in multiple concurrent operations.

cancel()[source]

Will set self to cancelled, and will emit the Gio.Cancellable ::cancelled signal. (However, see the warning about race conditions in the documentation for that signal if you are planning to connect to it.)

This function is thread-safe. In other words, you can safely call it from a thread other than the one running the operation that was passed the self.

If self is None, this function returns immediately for convenience.

The convention within GIO is that cancelling an asynchronous operation causes it to complete asynchronously. That is, if you cancel the operation from the same thread in which it is running, then the operation’s Gio.AsyncReadyCallback will not be invoked until the application returns to the main loop.

connect(callback, *data)[source]
Parameters:
Returns:

The id of the signal handler or 0 if self has already been cancelled.

Return type:

int

Convenience function to connect to the Gio.Cancellable ::cancelled signal. Also handles the race condition that may happen if the cancellable is cancelled right before connecting.

callback is called at most once, either directly at the time of the connect if self is already cancelled, or when self is cancelled in some thread.

data_destroy_func will be called when the handler is disconnected, or immediately if the cancellable is already cancelled.

See Gio.Cancellable ::cancelled for details on how to use this.

Since GLib 2.40, the lock protecting self is not held when callback is invoked. This lifts a restriction in place for earlier GLib versions which now makes it easier to write cleanup code that unconditionally invokes e.g. Gio.Cancellable.cancel().

New in version 2.22.

disconnect(handler_id)[source]
Parameters:handler_id (int) – Handler id of the handler to be disconnected, or 0.

Disconnects a handler from a cancellable instance similar to GObject.signal_handler_disconnect(). Additionally, in the event that a signal handler is currently running, this call will block until the handler has finished. Calling this function from a Gio.Cancellable ::cancelled signal handler will therefore result in a deadlock.

This avoids a race condition where a thread cancels at the same time as the cancellable operation is finished and the signal handler is removed. See Gio.Cancellable ::cancelled for details on how to use this.

If self is None or handler_id is 0 this function does nothing.

New in version 2.22.

get_fd()[source]
Returns:A valid file descriptor. -1 if the file descriptor is not supported, or on errors.
Return type:int

Gets the file descriptor for a cancellable job. This can be used to implement cancellable operations on Unix systems. The returned fd will turn readable when self is cancelled.

You are not supposed to read from the fd yourself, just check for readable status. Reading to unset the readable status is done with Gio.Cancellable.reset().

After a successful return from this function, you should use Gio.Cancellable.release_fd() to free up resources allocated for the returned file descriptor.

See also Gio.Cancellable.make_pollfd().

is_cancelled()[source]
Returns:True if self is cancelled, False if called with None or if item is not cancelled.
Return type:bool

Checks if a cancellable job has been cancelled.

make_pollfd(pollfd)[source]
Parameters:pollfd (GLib.PollFD) – a pointer to a GLib.PollFD
Returns:True if pollfd was successfully initialized, False on failure to prepare the cancellable.
Return type:bool

Creates a GLib.PollFD corresponding to self; this can be passed to GLib.poll() and used to poll for cancellation. This is useful both for unix systems without a native poll and for portability to windows.

When this function returns True, you should use Gio.Cancellable.release_fd() to free up resources allocated for the pollfd. After a False return, do not call Gio.Cancellable.release_fd().

If this function returns False, either no self was given or resource limits prevent this function from allocating the necessary structures for polling. (On Linux, you will likely have reached the maximum number of file descriptors.) The suggested way to handle these cases is to ignore the self.

You are not supposed to read from the fd yourself, just check for readable status. Reading to unset the readable status is done with Gio.Cancellable.reset().

New in version 2.22.

pop_current()[source]

Pops self off the cancellable stack (verifying that self is on the top of the stack).

push_current()[source]

Pushes self onto the cancellable stack. The current cancellable can then be received using Gio.Cancellable.get_current().

This is useful when implementing cancellable operations in code that does not allow you to pass down the cancellable object.

This is typically called automatically by e.g. Gio.File operations, so you rarely have to call this yourself.

release_fd()[source]

Releases a resources previously allocated by Gio.Cancellable.get_fd() or Gio.Cancellable.make_pollfd().

For compatibility reasons with older releases, calling this function is not strictly required, the resources will be automatically freed when the self is finalized. However, the self will block scarce file descriptors until it is finalized if this function is not called. This can cause the application to run out of file descriptors when many Gio.Cancellables are used at the same time.

New in version 2.22.

reset()[source]

Resets self to its uncancelled state.

If cancellable is currently in use by any cancellable operation then the behavior of this function is undefined.

Note that it is generally not a good idea to reuse an existing cancellable for more operations after it has been cancelled once, as this function might tempt you to do. The recommended practice is to drop the reference to a cancellable after cancelling it, and let it die with the outstanding async operations. You should create a fresh cancellable for further async operations.

set_error_if_cancelled()[source]
Raises:GLib.Error
Returns:True if self was cancelled, False if it was not
Return type:bool

If the self is cancelled, sets the error to notify that the operation was cancelled.

source_new()[source]
Returns:the new GLib.Source.
Return type:GLib.Source

Creates a source that triggers if self is cancelled and calls its callback of type Gio.CancellableSourceFunc. This is primarily useful for attaching to another (non-cancellable) source with GLib.Source.add_child_source() to add cancellability to it.

For convenience, you can call this with a None Gio.Cancellable, in which case the source will never trigger.

The new GLib.Source will hold a reference to the Gio.Cancellable.

New in version 2.28.

do_cancelled() virtual

Signal Details

Gio.Cancellable.signals.cancelled(cancellable)
Signal Name:cancelled
Flags:RUN_LAST
Parameters:cancellable (Gio.Cancellable) – The object which received the signal

Emitted when the operation has been cancelled.

Can be used by implementations of cancellable operations. If the operation is cancelled from another thread, the signal will be emitted in the thread that cancelled the operation, not the thread that is running the operation.

Note that disconnecting from this signal (or any signal) in a multi-threaded program is prone to race conditions. For instance it is possible that a signal handler may be invoked even after a call to GObject.signal_handler_disconnect() for that handler has already returned.

There is also a problem when cancellation happens right before connecting to the signal. If this happens the signal will unexpectedly not be emitted, and checking before connecting to the signal leaves a race condition where this is still happening.

In order to make it safe and easy to connect handlers there are two helper functions: Gio.Cancellable.connect() and Gio.Cancellable.disconnect() which protect against problems like this.

An example of how to us this:

// Make sure we don't do unnecessary work if already cancelled
if (g_cancellable_set_error_if_cancelled (cancellable, error))
  return;

// Set up all the data needed to be able to handle cancellation
// of the operation
my_data = my_data_new (...);

id = 0;
if (cancellable)
  id = g_cancellable_connect (cancellable,
                              G_CALLBACK (cancelled_handler)
                              data, NULL);

// cancellable operation here...

g_cancellable_disconnect (cancellable, id);

// cancelled_handler is never called after this, it is now safe
// to free the data
my_data_free (my_data);

Note that the cancelled signal is emitted in the thread that the user cancelled from, which may be the main thread. So, the cancellable signal should not do something that can block.