Gio.Task

Subclasses:

None

Methods

Inherited:

GObject.Object (37), Gio.AsyncResult (4)

Structs:

GObject.ObjectClass (5)

class	is_valid (result, source_object)
class	new (source_object, cancellable, callback, *callback_data)
class	report_error (source_object, callback, callback_data, source_tag, error)
	get_cancellable ()
	get_check_cancellable ()
	get_completed ()
	get_context ()
	get_name ()
	get_priority ()
	get_return_on_cancel ()
	get_source_object ()
	get_source_tag ()
	get_task_data ()
	had_error ()
	propagate_boolean ()
	propagate_int ()
	propagate_pointer ()
	propagate_value ()
	return_boolean (result)
	return_error (error)
	return_error_if_cancelled ()
	return_int (result)
	return_pointer (result, result_destroy)
	return_value (result)
	run_in_thread (task_func)
	run_in_thread_sync (task_func)
	set_check_cancellable (check_cancellable)
	set_name (name)
	set_priority (priority)
	set_return_on_cancel (return_on_cancel)
	set_source_tag (source_tag)
	set_static_name (name)
	set_task_data (task_data, task_data_destroy)

Virtual Methods

Inherited:

GObject.Object (7), Gio.AsyncResult (3)

Properties

Name	Type	Flags	Short Description
completed	bool	r	Whether the task has completed yet

Signals

Inherited:

GObject.Object (1)

Fields

Inherited:

GObject.Object (1)

Class Details

class Gio.Task(**kwargs)

Bases:

GObject.Object, Gio.AsyncResult

Abstract:

No

Structure:

Gio.TaskClass

A Gio.Task represents and manages a cancellable “task”.

Asynchronous operations

The most common usage of Gio.Task is as a Gio.AsyncResult, to manage data during an asynchronous operation. You call Gio.Task.new() in the “start” method, followed by Gio.Task.set_task_data() and the like if you need to keep some additional data associated with the task, and then pass the task object around through your asynchronous operation. Eventually, you will call a method such as Gio.Task.return_pointer() or Gio.Task.return_error(), which will save the value you give it and then invoke the task’s callback function in the thread-default main context where it was created (waiting until the next iteration of the main loop first, if necessary). The caller will pass the Gio.Task back to the operation’s finish function (as a Gio.AsyncResult), and you can use Gio.Task.propagate_pointer() or the like to extract the return value.

Using Gio.Task requires the thread-default GLib.MainContext from when the Gio.Task was constructed to be running at least until the task has completed and its data has been freed.

If a Gio.Task has been constructed and its callback set, it is an error to not call g_task_return_*() on it. GLib will warn at runtime if this happens (since 2.76).

Here is an example for using Gio.Task as a Gio.AsyncResult:

typedef struct {
  CakeFrostingType frosting;
  char *message;
} DecorationData;

static void
decoration_data_free (DecorationData *decoration)
{
  g_free (decoration->message);
  g_slice_free (DecorationData, decoration);
}

static void
baked_cb (Cake     *cake,
          gpointer  user_data)
{
  GTask *task = user_data;
  DecorationData *decoration = g_task_get_task_data (task);
  GError *error = NULL;

  if (cake == NULL)
    {
      g_task_return_new_error (task, BAKER_ERROR, BAKER_ERROR_NO_FLOUR,
                               "Go to the supermarket");
      g_object_unref (task);
      return;
    }

  if (!cake_decorate (cake, decoration->frosting, decoration->message, &error))
    {
      g_object_unref (cake);
      // g_task_return_error() takes ownership of error
      g_task_return_error (task, error);
      g_object_unref (task);
      return;
    }

  g_task_return_pointer (task, cake, g_object_unref);
  g_object_unref (task);
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  GTask *task;
  DecorationData *decoration;
  Cake  *cake;

  task = g_task_new (self, cancellable, callback, user_data);
  if (radius < 3)
    {
      g_task_return_new_error (task, BAKER_ERROR, BAKER_ERROR_TOO_SMALL,
                               "%ucm radius cakes are silly",
                               radius);
      g_object_unref (task);
      return;
    }

  cake = _baker_get_cached_cake (self, radius, flavor, frosting, message);
  if (cake != NULL)
    {
      // _baker_get_cached_cake() returns a reffed cake
      g_task_return_pointer (task, cake, g_object_unref);
      g_object_unref (task);
      return;
    }

  decoration = g_slice_new (DecorationData);
  decoration->frosting = frosting;
  decoration->message = g_strdup (message);
  g_task_set_task_data (task, decoration, (GDestroyNotify) decoration_data_free);

  _baker_begin_cake (self, radius, flavor, cancellable, baked_cb, task);
}

Cake *
baker_bake_cake_finish (Baker         *self,
                        GAsyncResult  *result,
                        GError       **error)
{
  g_return_val_if_fail (g_task_is_valid (result, self), NULL);

  return g_task_propagate_pointer (G_TASK (result), error);
}

Chained asynchronous operations

Gio.Task also tries to simplify asynchronous operations that internally chain together several smaller asynchronous operations. Gio.Task.get_cancellable(), Gio.Task.get_context(), and Gio.Task.get_priority() allow you to get back the task’s Gio.Cancellable, GLib.MainContext, and I/O priority when starting a new subtask, so you don’t have to keep track of them yourself. g_task_attach_source() simplifies the case of waiting for a source to fire (automatically using the correct GLib.MainContext and priority).

Here is an example for chained asynchronous operations:

typedef struct {
  Cake *cake;
  CakeFrostingType frosting;
  char *message;
} BakingData;

static void
decoration_data_free (BakingData *bd)
{
  if (bd->cake)
    g_object_unref (bd->cake);
  g_free (bd->message);
  g_slice_free (BakingData, bd);
}

static void
decorated_cb (Cake         *cake,
              GAsyncResult *result,
              gpointer      user_data)
{
  GTask *task = user_data;
  GError *error = NULL;

  if (!cake_decorate_finish (cake, result, &error))
    {
      g_object_unref (cake);
      g_task_return_error (task, error);
      g_object_unref (task);
      return;
    }

  // baking_data_free() will drop its ref on the cake, so we have to
  // take another here to give to the caller.
  g_task_return_pointer (task, g_object_ref (cake), g_object_unref);
  g_object_unref (task);
}

static gboolean
decorator_ready (gpointer user_data)
{
  GTask *task = user_data;
  BakingData *bd = g_task_get_task_data (task);

  cake_decorate_async (bd->cake, bd->frosting, bd->message,
                       g_task_get_cancellable (task),
                       decorated_cb, task);

  return G_SOURCE_REMOVE;
}

static void
baked_cb (Cake     *cake,
          gpointer  user_data)
{
  GTask *task = user_data;
  BakingData *bd = g_task_get_task_data (task);
  GError *error = NULL;

  if (cake == NULL)
    {
      g_task_return_new_error (task, BAKER_ERROR, BAKER_ERROR_NO_FLOUR,
                               "Go to the supermarket");
      g_object_unref (task);
      return;
    }

  bd->cake = cake;

  // Bail out now if the user has already cancelled
  if (g_task_return_error_if_cancelled (task))
    {
      g_object_unref (task);
      return;
    }

  if (cake_decorator_available (cake))
    decorator_ready (task);
  else
    {
      GSource *source;

      source = cake_decorator_wait_source_new (cake);
      // Attach @source to @task's GMainContext and have it call
      // decorator_ready() when it is ready.
      g_task_attach_source (task, source, decorator_ready);
      g_source_unref (source);
    }
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       gint                 priority,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  GTask *task;
  BakingData *bd;

  task = g_task_new (self, cancellable, callback, user_data);
  g_task_set_priority (task, priority);

  bd = g_slice_new0 (BakingData);
  bd->frosting = frosting;
  bd->message = g_strdup (message);
  g_task_set_task_data (task, bd, (GDestroyNotify) baking_data_free);

  _baker_begin_cake (self, radius, flavor, cancellable, baked_cb, task);
}

Cake *
baker_bake_cake_finish (Baker         *self,
                        GAsyncResult  *result,
                        GError       **error)
{
  g_return_val_if_fail (g_task_is_valid (result, self), NULL);

  return g_task_propagate_pointer (G_TASK (result), error);
}

Asynchronous operations from synchronous ones

You can use Gio.Task.run_in_thread() to turn a synchronous operation into an asynchronous one, by running it in a thread. When it completes, the result will be dispatched to the thread-default main context where the Gio.Task was created.

Running a task in a thread:

typedef struct {
  guint radius;
  CakeFlavor flavor;
  CakeFrostingType frosting;
  char *message;
} CakeData;

static void
cake_data_free (CakeData *cake_data)
{
  g_free (cake_data->message);
  g_slice_free (CakeData, cake_data);
}

static void
bake_cake_thread (GTask         *task,
                  gpointer       source_object,
                  gpointer       task_data,
                  GCancellable  *cancellable)
{
  Baker *self = source_object;
  CakeData *cake_data = task_data;
  Cake *cake;
  GError *error = NULL;

  cake = bake_cake (baker, cake_data->radius, cake_data->flavor,
                    cake_data->frosting, cake_data->message,
                    cancellable, &error);
  if (cake)
    g_task_return_pointer (task, cake, g_object_unref);
  else
    g_task_return_error (task, error);
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  CakeData *cake_data;
  GTask *task;

  cake_data = g_slice_new (CakeData);
  cake_data->radius = radius;
  cake_data->flavor = flavor;
  cake_data->frosting = frosting;
  cake_data->message = g_strdup (message);
  task = g_task_new (self, cancellable, callback, user_data);
  g_task_set_task_data (task, cake_data, (GDestroyNotify) cake_data_free);
  g_task_run_in_thread (task, bake_cake_thread);
  g_object_unref (task);
}

Cake *
baker_bake_cake_finish (Baker         *self,
                        GAsyncResult  *result,
                        GError       **error)
{
  g_return_val_if_fail (g_task_is_valid (result, self), NULL);

  return g_task_propagate_pointer (G_TASK (result), error);
}

Adding cancellability to uncancellable tasks

Finally, Gio.Task.run_in_thread() and Gio.Task.run_in_thread_sync() can be used to turn an uncancellable operation into a cancellable one. If you call Gio.Task.set_return_on_cancel(), passing True, then if the task’s Gio.Cancellable is cancelled, it will return control back to the caller immediately, while allowing the task thread to continue running in the background (and simply discarding its result when it finally does finish). Provided that the task thread is careful about how it uses locks and other externally-visible resources, this allows you to make “GLib-friendly” asynchronous and cancellable synchronous variants of blocking APIs.

Cancelling a task:

static void
bake_cake_thread (GTask         *task,
                  gpointer       source_object,
                  gpointer       task_data,
                  GCancellable  *cancellable)
{
  Baker *self = source_object;
  CakeData *cake_data = task_data;
  Cake *cake;
  GError *error = NULL;

  cake = bake_cake (baker, cake_data->radius, cake_data->flavor,
                    cake_data->frosting, cake_data->message,
                    &error);
  if (error)
    {
      g_task_return_error (task, error);
      return;
    }

  // If the task has already been cancelled, then we don't want to add
  // the cake to the cake cache. Likewise, we don't  want to have the
  // task get cancelled in the middle of updating the cache.
  // g_task_set_return_on_cancel() will return %TRUE here if it managed
  // to disable return-on-cancel, or %FALSE if the task was cancelled
  // before it could.
  if (g_task_set_return_on_cancel (task, FALSE))
    {
      // If the caller cancels at this point, their
      // GAsyncReadyCallback won't be invoked until we return,
      // so we don't have to worry that this code will run at
      // the same time as that code does. But if there were
      // other functions that might look at the cake cache,
      // then we'd probably need a GMutex here as well.
      baker_add_cake_to_cache (baker, cake);
      g_task_return_pointer (task, cake, g_object_unref);
    }
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  CakeData *cake_data;
  GTask *task;

  cake_data = g_slice_new (CakeData);

  ...

  task = g_task_new (self, cancellable, callback, user_data);
  g_task_set_task_data (task, cake_data, (GDestroyNotify) cake_data_free);
  g_task_set_return_on_cancel (task, TRUE);
  g_task_run_in_thread (task, bake_cake_thread);
}

Cake *
baker_bake_cake_sync (Baker               *self,
                      guint                radius,
                      CakeFlavor           flavor,
                      CakeFrostingType     frosting,
                      const char          *message,
                      GCancellable        *cancellable,
                      GError             **error)
{
  CakeData *cake_data;
  GTask *task;
  Cake *cake;

  cake_data = g_slice_new (CakeData);

  ...

  task = g_task_new (self, cancellable, NULL, NULL);
  g_task_set_task_data (task, cake_data, (GDestroyNotify) cake_data_free);
  g_task_set_return_on_cancel (task, TRUE);
  g_task_run_in_thread_sync (task, bake_cake_thread);

  cake = g_task_propagate_pointer (task, error);
  g_object_unref (task);
  return cake;
}

Porting from Gio.SimpleAsyncResult

Gio.Task's API attempts to be simpler than Gio.SimpleAsyncResult's in several ways:

• You can save task-specific data with Gio.Task.set_task_data(), and retrieve it later with Gio.Task.get_task_data(). This replaces the abuse of g_simple_async_result_set_op_res_gpointer() for the same purpose with Gio.SimpleAsyncResult.

• In addition to the task data, Gio.Task also keeps track of the priority, Gio.Cancellable, and GLib.MainContext associated with the task, so tasks that consist of a chain of simpler asynchronous operations will have easy access to those values when starting each sub-task.

• Gio.Task.return_error_if_cancelled() provides simplified handling for cancellation. In addition, cancellation overrides any other Gio.Task return value by default, like Gio.SimpleAsyncResult does when Gio.SimpleAsyncResult.set_check_cancellable() is called. (You can use Gio.Task.set_check_cancellable() to turn off that behavior.) On the other hand, Gio.Task.run_in_thread() guarantees that it will always run your task_func, even if the task’s Gio.Cancellable is already cancelled before the task gets a chance to run; you can start your task_func with a Gio.Task.return_error_if_cancelled() check if you need the old behavior.

• The “return” methods (eg, Gio.Task.return_pointer()) automatically cause the task to be “completed” as well, and there is no need to worry about the “complete” vs “complete in idle” distinction. (Gio.Task automatically figures out whether the task’s callback can be invoked directly, or if it needs to be sent to another GLib.MainContext, or delayed until the next iteration of the current GLib.MainContext.)

• The “finish” functions for Gio.Task based operations are generally much simpler than Gio.SimpleAsyncResult ones, normally consisting of only a single call to Gio.Task.propagate_pointer() or the like. “steals” the return value from the Gio.Task, it is not necessary to juggle pointers around to prevent it from being freed twice.

• With Gio.SimpleAsyncResult, it was common to call Gio.SimpleAsyncResult.propagate_error() from the _finish() wrapper function, and have virtual method implementations only deal with successful returns. This behavior is deprecated, because it makes it difficult for a subclass to chain to a parent class’s async methods. Instead, the wrapper function should just be a simple wrapper, and the virtual method should call an appropriate g_task_propagate_ function. Note that wrapper methods can now use Gio.AsyncResult.legacy_propagate_error() to do old-style Gio.SimpleAsyncResult error-returning behavior, and Gio.AsyncResult.is_tagged() to check if a result is tagged as having come from the _async() wrapper function (for “short-circuit” results, such as when passing 0 to Gio.InputStream.read_async()).

Thread-safety considerations

Due to some infelicities in the API design, there is a thread-safety concern that users of Gio.Task have to be aware of:

If the main thread drops its last reference to the source object or the task data before the task is finalized, then the finalizers of these objects may be called on the worker thread.

This is a problem if the finalizers use non-threadsafe API, and can lead to hard-to-debug crashes. Possible workarounds include:

• Clear task data in a signal handler for notify::completed

• Keep iterating a main context in the main thread and defer dropping the reference to the source object to that main context when the task is finalized

New in version g_task_propagate_pointer(.

classmethod is_valid(result, source_object)

Parameters:

• result (Gio.AsyncResult) – A Gio.AsyncResult

• source_object (GObject.Object or None) – the source object expected to be associated with the task

Returns:

True if result and source_object are valid, False if not

Return type:

bool

Checks that result is a Gio.Task, and that source_object is its source object (or that source_object is None and result has no source object). This can be used in g_return_if_fail() checks.

New in version 2.36.

classmethod new(source_object, cancellable, callback, *callback_data)

Parameters:

• source_object (GObject.Object or None) – the GObject.Object that owns this task, or None.

• cancellable (Gio.Cancellable or None) – optional Gio.Cancellable object, None to ignore.

• callback (Gio.AsyncReadyCallback or None) – a Gio.AsyncReadyCallback.

• callback_data (object or None) – user data passed to callback.

Returns:

a Gio.Task.

Return type:

Gio.Task

Creates a Gio.Task acting on source_object, which will eventually be used to invoke callback in the current thread-default main context.

Call this in the “start” method of your asynchronous method, and pass the Gio.Task around throughout the asynchronous operation. You can use Gio.Task.set_task_data() to attach task-specific data to the object, which you can retrieve later via Gio.Task.get_task_data().

By default, if cancellable is cancelled, then the return value of the task will always be Gio.IOErrorEnum.CANCELLED, even if the task had already completed before the cancellation. This allows for simplified handling in cases where cancellation may imply that other objects that the task depends on have been destroyed. If you do not want this behavior, you can use Gio.Task.set_check_cancellable() to change it.

New in version 2.36.

classmethod report_error(source_object, callback, callback_data, source_tag, error)

Parameters:

• source_object (GObject.Object or None) – the GObject.Object that owns this task, or None.

• callback (Gio.AsyncReadyCallback or None) – a Gio.AsyncReadyCallback.

• callback_data (object or None) – user data passed to callback.

• source_tag (object or None) – an opaque pointer indicating the source of this task

• error (GLib.Error) – error to report

Creates a Gio.Task and then immediately calls Gio.Task.return_error() on it. Use this in the wrapper function of an asynchronous method when you want to avoid even calling the virtual method. You can then use Gio.AsyncResult.is_tagged() in the finish method wrapper to check if the result there is tagged as having been created by the wrapper method, and deal with it appropriately if so.

See also g_task_report_new_error().

New in version 2.36.

get_cancellable()

Returns:

self's Gio.Cancellable

Return type:

Gio.Cancellable or None

Gets self's Gio.Cancellable

New in version 2.36.

get_check_cancellable()

Return type:

bool

Gets self's check-cancellable flag. See Gio.Task.set_check_cancellable() for more details.

New in version 2.36.

get_completed()

Returns:

True if the task has completed, False otherwise.

Return type:

bool

Gets the value of Gio.Task :completed. This changes from False to True after the task’s callback is invoked, and will return False if called from inside the callback.

New in version 2.44.

get_context()

Returns:

self's GLib.MainContext

Return type:

GLib.MainContext

Gets the GLib.MainContext that self will return its result in (that is, the context that was the thread-default main context at the point when self was created).

This will always return a non-None value, even if the task’s context is the default GLib.MainContext.

New in version 2.36.

get_name()

Returns:

self’s name, or None

Return type:

str or None

Gets self’s name. See Gio.Task.set_name().

New in version 2.60.

get_priority()

Returns:

self's priority

Return type:

int

Gets self's priority

New in version 2.36.

get_return_on_cancel()

Return type:

bool

Gets self's return-on-cancel flag. See Gio.Task.set_return_on_cancel() for more details.

New in version 2.36.

get_source_object()

Returns:

self's source object, or None

Return type:

GObject.Object or None

Gets the source object from self. Like Gio.AsyncResult.get_source_object(), but does not ref the object.

New in version 2.36.

get_source_tag()

Returns:

self's source tag

Return type:

object or None

Gets self's source tag. See Gio.Task.set_source_tag().

New in version 2.36.

get_task_data()

Returns:

self's task_data.

Return type:

object or None

Gets self's task_data.

New in version 2.36.

had_error()

Returns:

True if the task resulted in an error, False otherwise.

Return type:

bool

Tests if self resulted in an error.

New in version 2.36.

propagate_boolean()

Raises:

GLib.Error

Returns:

the task result, or False on error

Return type:

bool

Gets the result of self as a bool.

If the task resulted in an error, or was cancelled, then this will instead return False and set error.

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

New in version 2.36.

propagate_int()

Raises:

GLib.Error

Returns:

the task result, or -1 on error

Return type:

int

Gets the result of self as an integer (#gssize).

If the task resulted in an error, or was cancelled, then this will instead return -1 and set error.

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

New in version 2.36.

propagate_pointer()

Raises:

GLib.Error

Returns:

the task result, or None on error

Return type:

object or None

Gets the result of self as a pointer, and transfers ownership of that value to the caller.

If the task resulted in an error, or was cancelled, then this will instead return None and set error.

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

New in version 2.36.

propagate_value()

Raises:

GLib.Error

Returns:

True if self succeeded, False on error.

value:

return location for the GObject.Value

Return type:

(bool, value: GObject.Value)

Gets the result of self as a GObject.Value, and transfers ownership of that value to the caller. As with Gio.Task.return_value(), this is a generic low-level method; Gio.Task.propagate_pointer() and the like will usually be more useful for C code.

If the task resulted in an error, or was cancelled, then this will instead set error and return False.

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

New in version 2.64.

return_boolean(result)

Parameters:

result (bool) – the bool result of a task function.

Sets self's result to result and completes the task (see Gio.Task.return_pointer() for more discussion of exactly what this means).

New in version 2.36.

return_error(error)

Parameters:

error (GLib.Error) – the GLib.Error result of a task function.

Sets self's result to error (which self assumes ownership of) and completes the task (see Gio.Task.return_pointer() for more discussion of exactly what this means).

Note that since the task takes ownership of error, and since the task may be completed before returning from Gio.Task.return_error(), you cannot assume that error is still valid after calling this. Call GLib.Error.copy() on the error if you need to keep a local copy as well.

See also g_task_return_new_error().

New in version 2.36.

return_error_if_cancelled()

Returns:

True if self has been cancelled, False if not

Return type:

bool

Checks if self's Gio.Cancellable has been cancelled, and if so, sets self's error accordingly and completes the task (see Gio.Task.return_pointer() for more discussion of exactly what this means).

New in version 2.36.

return_int(result)

Parameters:

result (int) – the integer (#gssize) result of a task function.

Sets self's result to result and completes the task (see Gio.Task.return_pointer() for more discussion of exactly what this means).

New in version 2.36.

return_pointer(result, result_destroy)

Parameters:

• result (object or None) – the pointer result of a task function

• result_destroy (GLib.DestroyNotify or None) – a GLib.DestroyNotify function.

Sets self's result to result and completes the task. If result is not None, then result_destroy will be used to free result if the caller does not take ownership of it with Gio.Task.propagate_pointer().

“Completes the task” means that for an ordinary asynchronous task it will either invoke the task’s callback, or else queue that callback to be invoked in the proper GLib.MainContext, or in the next iteration of the current GLib.MainContext. For a task run via Gio.Task.run_in_thread() or Gio.Task.run_in_thread_sync(), calling this method will save result to be returned to the caller later, but the task will not actually be completed until the Gio.TaskThreadFunc exits.

Note that since the task may be completed before returning from Gio.Task.return_pointer(), you cannot assume that result is still valid after calling this, unless you are still holding another reference on it.

New in version 2.36.

return_value(result)

Parameters:

result (GObject.Value or None) – the GObject.Value result of a task function

Sets self's result to result (by copying it) and completes the task.

If result is None then a GObject.Value of type GObject.TYPE_POINTER with a value of None will be used for the result.

This is a very generic low-level method intended primarily for use by language bindings; for C code, Gio.Task.return_pointer() and the like will normally be much easier to use.

New in version 2.64.

run_in_thread(task_func)

Parameters:

task_func (Gio.TaskThreadFunc) – a Gio.TaskThreadFunc

Runs task_func in another thread. When task_func returns, self's Gio.AsyncReadyCallback will be invoked in self's GLib.MainContext.

This takes a ref on self until the task completes.

See Gio.TaskThreadFunc for more details about how task_func is handled.

Although GLib currently rate-limits the tasks queued via Gio.Task.run_in_thread(), you should not assume that it will always do this. If you have a very large number of tasks to run (several tens of tasks), but don’t want them to all run at once, you should only queue a limited number of them (around ten) at a time.

Be aware that if your task depends on other tasks to complete, use of this function could lead to a livelock if the other tasks also use this function and enough of them (around 10) execute in a dependency chain, as that will exhaust the thread pool. If this situation is possible, consider using a separate worker thread or thread pool explicitly, rather than using Gio.Task.run_in_thread().

New in version 2.36.

run_in_thread_sync(task_func)

Parameters:

task_func (Gio.TaskThreadFunc) – a Gio.TaskThreadFunc

Runs task_func in another thread, and waits for it to return or be cancelled. You can use Gio.Task.propagate_pointer(), etc, afterward to get the result of task_func.

See Gio.TaskThreadFunc for more details about how task_func is handled.

Normally this is used with tasks created with a None callback, but note that even if the task does have a callback, it will not be invoked when task_func returns. Gio.Task :completed will be set to True just before this function returns.

Although GLib currently rate-limits the tasks queued via Gio.Task.run_in_thread_sync(), you should not assume that it will always do this. If you have a very large number of tasks to run, but don’t want them to all run at once, you should only queue a limited number of them at a time.

New in version 2.36.

set_check_cancellable(check_cancellable)

Parameters:

check_cancellable (bool) – whether Gio.Task will check the state of its Gio.Cancellable for you.

Sets or clears self's check-cancellable flag. If this is True (the default), then Gio.Task.propagate_pointer(), etc, and Gio.Task.had_error() will check the task’s Gio.Cancellable first, and if it has been cancelled, then they will consider the task to have returned an “Operation was cancelled” error (Gio.IOErrorEnum.CANCELLED), regardless of any other error or return value the task may have had.

If check_cancellable is False, then the Gio.Task will not check the cancellable itself, and it is up to self's owner to do this (eg, via Gio.Task.return_error_if_cancelled()).

If you are using Gio.Task.set_return_on_cancel() as well, then you must leave check-cancellable set True.

New in version 2.36.

set_name(name)

Parameters:

name (str or None) – a human readable name for the task, or None to unset it

Sets self’s name, used in debugging and profiling. The name defaults to None.

The task name should describe in a human readable way what the task does. For example, ‘Open file’ or ‘Connect to network host’. It is used to set the name of the GLib.Source used for idle completion of the task.

This function may only be called before the self is first used in a thread other than the one it was constructed in. It is called automatically by Gio.Task.set_source_tag() if not called already.

New in version 2.60.

set_priority(priority)

Parameters:

priority (int) – the priority of the request

Sets self's priority. If you do not call this, it will default to GLib.PRIORITY_DEFAULT.

This will affect the priority of GLib.Sources created with g_task_attach_source() and the scheduling of tasks run in threads, and can also be explicitly retrieved later via Gio.Task.get_priority().

New in version 2.36.

set_return_on_cancel(return_on_cancel)

Parameters:

return_on_cancel (bool) – whether the task returns automatically when it is cancelled.

Returns:

True if self's return-on-cancel flag was changed to match return_on_cancel. False if self has already been cancelled.

Return type:

bool

Sets or clears self's return-on-cancel flag. This is only meaningful for tasks run via Gio.Task.run_in_thread() or Gio.Task.run_in_thread_sync().

If return_on_cancel is True, then cancelling self's Gio.Cancellable will immediately cause it to return, as though the task’s Gio.TaskThreadFunc had called Gio.Task.return_error_if_cancelled() and then returned.

This allows you to create a cancellable wrapper around an uninterruptible function. The Gio.TaskThreadFunc just needs to be careful that it does not modify any externally-visible state after it has been cancelled. To do that, the thread should call Gio.Task.set_return_on_cancel() again to (atomically) set return-on-cancel False before making externally-visible changes; if the task gets cancelled before the return-on-cancel flag could be changed, Gio.Task.set_return_on_cancel() will indicate this by returning False.

You can disable and re-enable this flag multiple times if you wish. If the task’s Gio.Cancellable is cancelled while return-on-cancel is False, then calling Gio.Task.set_return_on_cancel() to set it True again will cause the task to be cancelled at that point.

If the task’s Gio.Cancellable is already cancelled before you call Gio.Task.run_in_thread()/Gio.Task.run_in_thread_sync(), then the Gio.TaskThreadFunc will still be run (for consistency), but the task will also be completed right away.

New in version 2.36.

set_source_tag(source_tag)

Parameters:

source_tag (object or None) – an opaque pointer indicating the source of this task

Sets self's source tag.

You can use this to tag a task return value with a particular pointer (usually a pointer to the function doing the tagging) and then later check it using Gio.Task.get_source_tag() (or Gio.AsyncResult.is_tagged()) in the task’s “finish” function, to figure out if the response came from a particular place.

A macro wrapper around this function will automatically set the task’s name to the string form of source_tag if it’s not already set, for convenience.

New in version 2.36.

set_static_name(name)

Parameters:

name (str or None) – a human readable name for the task. Must be a string literal

Sets self’s name, used in debugging and profiling.

This is a variant of Gio.Task.set_name() that avoids copying name.

New in version 2.76.

set_task_data(task_data, task_data_destroy)

Parameters:

• task_data (object or None) – task-specific data

• task_data_destroy (GLib.DestroyNotify or None) – GLib.DestroyNotify for task_data

Sets self's task data (freeing the existing task data, if any).

New in version 2.36.

Property Details

Gio.Task.props.completed

Name:

completed

Type:

bool

Default Value:

False

Flags:

READABLE

Whether the task has completed, meaning its callback (if set) has been invoked. This can only happen after Gio.Task.return_pointer(), Gio.Task.return_error() or one of the other return functions have been called on the task.

This property is guaranteed to change from False to True exactly once.

The GObject.Object ::notify signal for this change is emitted in the same main context as the task’s callback, immediately after that callback is invoked.

New in version 2.44.