Gio.Task

class — extends GObject.Object, AsyncResult

A GTask represents and manages a cancellable ‘task’.

Asynchronous operations

The most common usage of GTask is as a AsyncResult, to manage data during an asynchronous operation. You call Task.new in the ‘start’ method, followed by 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 Task.return_pointer or 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 (see GLib.MainContext.push_thread_default) where it was created (waiting until the next iteration of the main loop first, if necessary). The caller will pass the GTask back to the operation’s finish function (as a AsyncResult), and you can use Task.propagate_pointer or the like to extract the return value.

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

If a GTask 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 GTask as a 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

GTask also tries to simplify asynchronous operations that internally chain together several smaller asynchronous operations. Task.get_cancellable, Task.get_context, and Task.get_priority allow you to get back the task’s Cancellable, GLib.MainContext, and I/O priority when starting a new subtask, so you don’t have to keep track of them yourself. 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 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 (see GLib.MainContext.push_thread_default) where the GTask 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, Task.run_in_thread and Task.run_in_thread_sync can be used to turn an uncancellable operation into a cancellable one. If you call Task.set_return_on_cancel, passing TRUE, then if the task’s 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 SimpleAsyncResult

GTask’s API attempts to be simpler than SimpleAsyncResult’s in several ways:

• You can save task-specific data with Task.set_task_data, and retrieve it later with Task.get_task_data. This replaces the abuse of SimpleAsyncResult.set_op_res_gpointer for the same purpose with SimpleAsyncResult.
• In addition to the task data, GTask also keeps track of the priority, 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.
• Task.return_error_if_cancelled provides simplified handling for cancellation. In addition, cancellation overrides any other GTask return value by default, like SimpleAsyncResult does when SimpleAsyncResult.set_check_cancellable is called. (You can use Task.set_check_cancellable to turn off that behavior.) On the other hand, Task.run_in_thread guarantees that it will always run your task_func, even if the task’s Cancellable is already cancelled before the task gets a chance to run; you can start your task_func with a Task.return_error_if_cancelled check if you need the old behavior.
• The ‘return’ methods (eg, 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. (GTask 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 GTask based operations are generally much simpler than SimpleAsyncResult ones, normally consisting of only a single call to Task.propagate_pointer or the like. Since Task.propagate_pointer ‘steals’ the return value from the GTask, it is not necessary to juggle pointers around to prevent it from being freed twice.
• With SimpleAsyncResult, it was common to call 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 AsyncResult.legacy_propagate_error to do old-style SimpleAsyncResult error-returning behavior, and 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 InputStream.read_async).

Thread-safety considerations

Due to some infelicities in the API design, there is a thread-safety concern that users of GTask 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

Constructors

new

@classmethod
def new(cls, source_object: GObject.Object | None = ..., cancellable: Cancellable | None = ..., callback: Callable[[Task | None, AsyncResult], None] | None = ...) -> Task

Creates a Task acting on source_object, which will eventually be used to invoke callback in the current thread-default main context (see GLib.MainContext.push_thread_default).

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

By default, if cancellable is cancelled, then the return value of the task will always be 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 Task.set_check_cancellable to change it.

Parameters:

• source_object — the GObject.Object that owns this task, or None.
• cancellable — optional Cancellable object, None to ignore.
• callback — a GAsyncReadyCallback.

Methods

get_cancellable

def get_cancellable(self) -> Cancellable | None

Gets task's Cancellable

get_check_cancellable

def get_check_cancellable(self) -> bool

Gets task's check-cancellable flag. See Task.set_check_cancellable for more details.

get_completed

def get_completed(self) -> bool

Gets the value of 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.

get_context

def get_context(self) -> GLib.MainContext

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

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

get_name

def get_name(self) -> str | None

Gets task’s name. See Task.set_name.

get_priority

def get_priority(self) -> int

Gets task's priority

get_return_on_cancel

def get_return_on_cancel(self) -> bool

Gets task's return-on-cancel flag. See Task.set_return_on_cancel for more details.

get_source_object

def get_source_object(self) -> GObject.Object | None

Gets the source object from task. Like AsyncResult.get_source_object, but does not ref the object.

get_source_tag

def get_source_tag(self) -> int | None

Gets task's source tag. See Task.set_source_tag.

get_task_data

def get_task_data(self) -> int | None

Gets task's task_data.

had_error

def had_error(self) -> bool

Tests if task resulted in an error.

propagate_boolean

def propagate_boolean(self) -> bool

Gets the result of task as a #gboolean.

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.

propagate_int

def propagate_int(self) -> int

Gets the result of task 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.

propagate_pointer

def propagate_pointer(self) -> int | None

Gets the result of task 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.

propagate_value

def propagate_value(self) -> tuple[bool, GObject.Value]

Gets the result of task as a GObject.Value, and transfers ownership of that value to the caller. As with Task.return_value, this is a generic low-level method; 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.

return_boolean

def return_boolean(self, result: bool) -> None

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

Parameters:

• result — the #gboolean result of a task function.

return_error

def return_error(self, error: GLib.Error) -> None

Sets task's result to error (which task assumes ownership of) and completes the task (see 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 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 Task.return_new_error, Task.return_new_error_literal.

Parameters:

• error — the GLib.Error result of a task function.

return_error_if_cancelled

def return_error_if_cancelled(self) -> bool

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

return_int

def return_int(self, result: int) -> None

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

Parameters:

• result — the integer (#gssize) result of a task function.

return_new_error_literal

def return_new_error_literal(self, domain: GLib.Quark, code: int, message: str) -> None

Sets task’s result to a new GLib.Error created from domain, code, message and completes the task.

See Task.return_pointer for more discussion of exactly what ‘completing the task’ means.

See also Task.return_new_error.

Parameters:

• domain — a GQuark.
• code — an error code.
• message — an error message

return_pointer

def return_pointer(self, result: int | None = ..., result_destroy: GLib.DestroyNotify | None = ...) -> None

Sets task'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 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 Task.run_in_thread or 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 GTaskThreadFunc exits.

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

Parameters:

• result — the pointer result of a task function
• result_destroy — a GDestroyNotify function.

return_value

def return_value(self, result: GObject.Value | None = ...) -> None

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

If result is None then a GObject.Value of type G_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, Task.return_pointer and the like will normally be much easier to use.

Parameters:

• result — the GObject.Value result of a task function

run_in_thread

def run_in_thread(self, task_func: TaskThreadFunc) -> None

Runs task_func in another thread. When task_func returns, task's GAsyncReadyCallback will be invoked in task's GLib.MainContext.

This takes a ref on task until the task completes.

See GTaskThreadFunc for more details about how task_func is handled.

Although GLib currently rate-limits the tasks queued via 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 Task.run_in_thread.

Parameters:

• task_func — a GTaskThreadFunc

run_in_thread_sync

def run_in_thread_sync(self, task_func: TaskThreadFunc) -> None

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

See GTaskThreadFunc 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. Task:completed will be set to True just before this function returns.

Although GLib currently rate-limits the tasks queued via 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.

Parameters:

• task_func — a GTaskThreadFunc

set_check_cancellable

def set_check_cancellable(self, check_cancellable: bool) -> None

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

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

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

Parameters:

• check_cancellable — whether Task will check the state of its Cancellable for you.

set_name

def set_name(self, name: str | None = ...) -> None

Sets task’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 task is first used in a thread other than the one it was constructed in.

Parameters:

• name — a human readable name for the task, or None to unset it

set_priority

def set_priority(self, priority: int) -> None

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

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

Parameters:

• priority — the priority of the request

set_return_on_cancel

def set_return_on_cancel(self, return_on_cancel: bool) -> bool

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

If return_on_cancel is True, then cancelling task's Cancellable will immediately cause it to return, as though the task's GTaskThreadFunc had called Task.return_error_if_cancelled and then returned.

This allows you to create a cancellable wrapper around an uninterruptible function. The GTaskThreadFunc 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 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, 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 Cancellable is cancelled while return-on-cancel is False, then calling Task.set_return_on_cancel to set it True again will cause the task to be cancelled at that point.

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

Parameters:

• return_on_cancel — whether the task returns automatically when it is cancelled.

set_source_tag

def set_source_tag(self, source_tag: int | None = ...) -> None

Sets task'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 Task.get_source_tag (or 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.

Parameters:

• source_tag — an opaque pointer indicating the source of this task

set_static_name

def set_static_name(self, name: str | None = ...) -> None

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

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

This function is called automatically by Task.set_source_tag unless a name is set.

Parameters:

• name — a human readable name for the task. Must be a string literal

set_task_data

def set_task_data(self, task_data: int | None = ..., task_data_destroy: GLib.DestroyNotify | None = ...) -> None

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

Parameters:

• task_data — task-specific data
• task_data_destroy — GDestroyNotify for task_data

Static functions

is_valid

@staticmethod
def is_valid(result: AsyncResult, source_object: GObject.Object | None = ...) -> bool

Checks that result is a 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.

Parameters:

• result — A AsyncResult
• source_object — the source object expected to be associated with the task

report_error

@staticmethod
def report_error(source_object: GObject.Object | None, callback: AsyncReadyCallback | None, source_tag: int | None, error: GLib.Error) -> None

Creates a Task and then immediately calls 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 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().

Parameters:

• source_object — the GObject.Object that owns this task, or None.
• callback — a GAsyncReadyCallback.
• source_tag — an opaque pointer indicating the source of this task
• error — error to report

Properties

completed

completed: bool  # read-only

Whether the task has completed, meaning its callback (if set) has been invoked.

This can only happen after Task.return_pointer, Task.return_error or one of the other return functions have been called on the task. However, it is not guaranteed to happen immediately after those functions are called, as the task’s callback may need to be scheduled to run in a different thread.

That means it is not safe to use this property to track whether a return function has been called on the Task. Callers must do that tracking themselves, typically by linking the lifetime of the Task to the control flow of their code.

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.