gio:async-result, gobject:object, common-lisp:standard-object, common-lisp:t

None

The g:task class represents and manages a cancellable "task".

Asynchronous operations

The most common usage of the g:task object is as the g:async-result object, to manage data during an asynchronous operation. You call the g:task-new function in the "start" method, followed by the g:task-set-task-data function 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 the g:task-return-pointer or the g:task-return-error function, 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 g:task object back to the operation's finish function (as a g:async-result object), and you can use the g:task-propagate-pointer function or the like to extract the return value.

Here is an example for using the g:task object as a g:async-result object:

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

The g:task object also tries to simplify asynchronous operations that internally chain together several smaller asynchronous operations. The g:task-get-cancellable, g:task-get-context, and g:task-get-priority functions allow you to get back the task's g:cancellable object, g:main-context instance, and I/O priority when starting a new subtask, so you do not have to keep track of them yourself. The g:task-attach-source function simplifies the case of waiting for a source to fire (automatically using the correct the g:main-context instance 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 the g:task-run-in-thread function 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 g:task object 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, the g:task-run-in-thread and g:task-run-in-thread-sync functions can be used to turn an uncancellable operation into a cancellable one. If you call the g:task-return-on-cancel function, passing true, then if the task's g:cancellable object 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

The g:task API attempts to be simpler than the g:simple-async-result API in several ways:

• You can save task-specific data with the g:task-set-task-data function, and retrieve it later with the g;task-get-task-data function. This replaces the abuse of the g:simple-async-result-set-op-res-gpointer function for the same purpose with the g:simple-async-result object.
• In addition to the task data, the g:task object also keeps track of the priority, the g:cancellable object, and the g:main-context instance 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.
• The g:task-return-error-if-cancelled function provides simplified handling for cancellation. In addition, cancellation overrides any other g:task return value by default, like the g:simple-async-result function does when the g:simple-async-result-set-check-cancellable function is called. (You can use the g:task-set-check-cancellable function to turn off that behavior.) On the other hand, the g:task-run-in-thread function guarantees that it will always run your task_func, even if the task's g:cancellable object 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, for example, the g:task-return-pointer function, automatically cause the task to be "completed" as well, and there is no need to worry about the "complete" vs "complete in idle" distinction. (the g:task object automatically figures out whether the task's callback can be invoked directly, or if it needs to be sent to another g:main-context instance, or delayed until the next iteration of the current g:main-context instance.)
• The "finish" functions for the g:task object based operations are generally much simpler than the g:simple-async-result object ones, normally consisting of only a single call to the g:task-propagate-pointer function or the like. Since the g:task-propagate-pointer function "steals" the return value from the g:task object, it is not necessary to juggle pointers around to prevent it from being freed twice.
• With the g:simple-async-result object, it was common to call the g:simple-async-result-propagate-error function 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 the g:async-result-legacy-propagate-error function to do old-style g:simple-async-result error-returning behavior, and the g:async-result-is-tagged function 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 the g:input-stream-read-async function).