Class

GioTask

Description

final class Gio.Task : GObject.Object {
  /* No available fields */
}

A GTask represents and manages a cancellable ‘task’.

Asynchronous operations

The most common usage of GTask is as a GAsyncResult, to manage data during an asynchronous operation. You call g_task_new() in the ‘start’ method, followed by g_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 g_task_return_pointer() or g_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 g_main_context_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 GAsyncResult), and you can use g_task_propagate_pointer() or the like to extract the return value.

Using GTask requires the thread-default GMainContext 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 GAsyncResult:

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. g_task_get_cancellable(), g_task_get_context(), and g_task_get_priority() allow you to get back the task’s GCancellable, GMainContext, 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 GMainContext 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 g_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 g_main_context_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, g_task_run_in_thread() and g_task_run_in_thread_sync() can be used to turn an uncancellable operation into a cancellable one. If you call g_task_set_return_on_cancel(), passing TRUE, then if the task’s GCancellable 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 `GSimpleAsyncResult`

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

You can save task-specific data with g_task_set_task_data(), and retrieve it later with g_task_get_task_data(). This replaces the abuse of g_simple_async_result_set_op_res_gpointer() for the same purpose with GSimpleAsyncResult.
In addition to the task data, GTask also keeps track of the priority, GCancellable, and GMainContext 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.
g_task_return_error_if_cancelled() provides simplified handling for cancellation. In addition, cancellation overrides any other GTask return value by default, like GSimpleAsyncResult does when g_simple_async_result_set_check_cancellable() is called. (You can use g_task_set_check_cancellable() to turn off that behavior.) On the other hand, g_task_run_in_thread() guarantees that it will always run your task_func, even if the task’s GCancellable is already cancelled before the task gets a chance to run; you can start your task_func with a g_task_return_error_if_cancelled() check if you need the old behavior.
The ‘return’ methods (eg, g_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 GMainContext, or delayed until the next iteration of the current GMainContext.)
The ‘finish’ functions for GTask based operations are generally much simpler than GSimpleAsyncResult ones, normally consisting of only a single call to g_task_propagate_pointer() or the like. Since g_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 GSimpleAsyncResult, it was common to call g_simple_async_result_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 g_async_result_legacy_propagate_error() to do old-style GSimpleAsyncResult error-returning behavior, and g_async_result_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 g_input_stream_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.

Ancestors

GObject

Implements

GAsyncResult

Constructors

g_task_new

Creates a GTask acting on source_object, which will eventually be used to invoke callback in the current [thread-default main context][g-main-context-push-thread-default].

since: 2.36

Functions

g_task_is_valid

Checks that result is a GTask, and that source_object is its source object (or that source_object is NULL and result has no source object). This can be used in g_return_if_fail() checks.

since: 2.36

g_task_report_error

Creates a GTask and then immediately calls g_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 g_async_result_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.

since: 2.36

g_task_report_new_error

Creates a GTask and then immediately calls g_task_return_new_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 g_async_result_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.

since: 2.36

Instance methods

g_task_attach_source

A utility function for dealing with async operations where you need to wait for a GSource to trigger. Attaches source to task‘s GMainContext with task‘s priority, and sets source‘s callback to callback, with task as the callback’s user_data.

since: 2.36

g_task_get_cancellable

Gets task‘s GCancellable.

since: 2.36

g_task_get_check_cancellable

Gets task‘s check-cancellable flag. See g_task_set_check_cancellable() for more details.

since: 2.36

g_task_get_completed

Gets the value of GTask:completed. This changes from FALSE to TRUE after the task’s callback is invoked, and will return FALSE if called from inside the callback.

since: 2.44

g_task_get_context

Gets the GMainContext that task will return its result in (that is, the context that was the [thread-default main context][g-main-context-push-thread-default] at the point when task was created).

since: 2.36

g_task_get_name

Gets task’s name. See g_task_set_name().

since: 2.60

g_task_get_priority

Gets task‘s priority.

since: 2.36

g_task_get_return_on_cancel

Gets task‘s return-on-cancel flag. See g_task_set_return_on_cancel() for more details.

since: 2.36

g_task_get_source_object

Gets the source object from task. Like g_async_result_get_source_object(), but does not ref the object.

since: 2.36

g_task_get_source_tag

Gets task‘s source tag. See g_task_set_source_tag().

since: 2.36

g_task_get_task_data

Gets task‘s task_data.

since: 2.36

g_task_had_error

Tests if task resulted in an error.

since: 2.36

g_task_propagate_boolean

Gets the result of task as a #gboolean.

since: 2.36

g_task_propagate_int

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

since: 2.36

g_task_propagate_pointer

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

since: 2.36

g_task_propagate_value

Gets the result of task as a GValue, and transfers ownership of that value to the caller. As with g_task_return_value(), this is a generic low-level method; g_task_propagate_pointer() and the like will usually be more useful for C code.

since: 2.64

g_task_return_boolean

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

since: 2.36

g_task_return_error

Sets task‘s result to error (which task assumes ownership of) and completes the task (see g_task_return_pointer() for more discussion of exactly what this means).

since: 2.36

g_task_return_error_if_cancelled

Checks if task‘s GCancellable has been cancelled, and if so, sets task‘s error accordingly and completes the task (see g_task_return_pointer() for more discussion of exactly what this means).

since: 2.36

g_task_return_int

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

since: 2.36

g_task_return_new_error

Sets task‘s result to a new GError created from domain, code, format, and the remaining arguments, and completes the task (see g_task_return_pointer() for more discussion of exactly what this means).

since: 2.36

g_task_return_new_error_literal

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

since: 2.80

g_task_return_pointer

Sets task‘s result to result and completes the task. If result is not NULL, then result_destroy will be used to free result if the caller does not take ownership of it with g_task_propagate_pointer().

since: 2.36

g_task_return_prefixed_error

Sets task‘s result to error (which task assumes ownership of), with the message prefixed according to format, and completes the task (see g_task_return_pointer() for more discussion of exactly what this means).

since: 2.80

g_task_return_value

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

since: 2.64

g_task_run_in_thread

Runs task_func in another thread. When task_func returns, task‘s GAsyncReadyCallback will be invoked in task‘s GMainContext.

since: 2.36

g_task_run_in_thread_sync

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

since: 2.36

g_task_set_check_cancellable

Sets or clears task‘s check-cancellable flag. If this is TRUE (the default), then g_task_propagate_pointer(), etc, and g_task_had_error() will check the task’s GCancellable first, and if it has been cancelled, then they will consider the task to have returned an “Operation was cancelled” error (G_IO_ERROR_CANCELLED), regardless of any other error or return value the task may have had.

since: 2.36

g_task_set_name

Sets task’s name, used in debugging and profiling. The name defaults to NULL.

since: 2.60

g_task_set_priority

Sets task‘s priority. If you do not call this, it will default to G_PRIORITY_DEFAULT.

since: 2.36

g_task_set_return_on_cancel

Sets or clears task‘s return-on-cancel flag. This is only meaningful for tasks run via g_task_run_in_thread() or g_task_run_in_thread_sync().

since: 2.36

g_task_set_source_tag

Sets task‘s source tag.

since: 2.36

g_task_set_static_name

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

since: 2.76

g_task_set_task_data

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

since: 2.36

Methods inherited from GObject (43)

Please see GObject for a full list of methods.

Properties

Gio.Task:completed

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

since: 2.44

Signals

Signals inherited from GObject (1)

GObject::notify

The notify signal is emitted on an object when one of its properties has its value set through g_object_set_property(), g_object_set(), et al.

Class structure

struct GioTaskClass {
  /* no available fields */
}

No description available.

Description

Asynchronous operations

Chained asynchronous operations

Asynchronous operations from synchronous ones

Adding cancellability to uncancellable tasks

Porting from GSimpleAsyncResult

Thread-safety considerations

Ancestors

Implements

Constructors

Functions

Instance methods

Methods inherited from GObject (43)

Methods inherited from GAsyncResult (4)

Properties

Signals

Signals inherited from GObject (1)

Class structure

Porting from `GSimpleAsyncResult`