Class

GioTask

Description [src]

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][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.

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][io-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][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

GTasks API attempts to be simpler than GSimpleAsyncResults 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][io-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()).

Hierarchy

hierarchy this GTask implements_0 GAsyncResult this--implements_0 ancestor_0 GObject ancestor_0--this

Ancestors

Implements

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].

Available 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.

Available 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.

Available 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.

Available 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 tasks GMainContext with tasks [priority][io-priority], and sets sources callback to callback, with task as the callback’s user_data.

Available since: 2.36

g_task_get_cancellable

Gets tasks GCancellable.

Available since: 2.36

g_task_get_check_cancellable

Gets tasks check-cancellable flag. See g_task_set_check_cancellable() for more details.

Available 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.

Available 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).

Available since: 2.36

g_task_get_name

Gets task’s name. See g_task_set_name().

Available since: 2.60

g_task_get_priority

Gets tasks priority.

Available since: 2.36

g_task_get_return_on_cancel

Gets tasks return-on-cancel flag. See g_task_set_return_on_cancel() for more details.

Available 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.

Available since: 2.36

g_task_get_source_tag

Gets tasks source tag. See g_task_set_source_tag().

Available since: 2.36

g_task_get_task_data

Gets tasks task_data.

Available since: 2.36

g_task_had_error

Tests if task resulted in an error.

Available since: 2.36

g_task_propagate_boolean

Gets the result of task as a #gboolean.

Available since: 2.36

g_task_propagate_int

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

Available since: 2.36

g_task_propagate_pointer

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

Available 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.

Available since: 2.64

g_task_return_boolean

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

Available since: 2.36

g_task_return_error

Sets tasks 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).

Available since: 2.36

g_task_return_error_if_cancelled

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

Available since: 2.36

g_task_return_int

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

Available since: 2.36

g_task_return_new_error

Sets tasks 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).

Available since: 2.36

g_task_return_pointer

Sets tasks 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().

Available since: 2.36

g_task_return_value

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

Available since: 2.64

g_task_run_in_thread

Runs task_func in another thread. When task_func returns, tasks GAsyncReadyCallback will be invoked in tasks GMainContext.

Available 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.

Available since: 2.36

g_task_set_check_cancellable

Sets or clears tasks 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.

Available since: 2.36

g_task_set_name

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

Available since: 2.60

g_task_set_priority

Sets tasks priority. If you do not call this, it will default to G_PRIORITY_DEFAULT.

Available since: 2.36

g_task_set_return_on_cancel

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

Available since: 2.36

g_task_set_source_tag

Sets tasks 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 g_task_get_source_tag() (or g_async_result_is_tagged()) in the task’s “finish” function, to figure out if the response came from a particular place.

Available since: 2.36

g_task_set_task_data

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

Available since: 2.36

Methods inherited from GObject (43)
Methods inherited from GAsyncResult (4)
g_async_result_get_source_object

Gets the source object from a GAsyncResult.

g_async_result_get_user_data

Gets the user data from a GAsyncResult.

g_async_result_is_tagged

Checks if res has the given source_tag (generally a function pointer indicating the function res was created by).

Available since: 2.34

g_async_result_legacy_propagate_error

If res is a GSimpleAsyncResult, this is equivalent to g_simple_async_result_propagate_error(). Otherwise it returns FALSE.

Available since: 2.34

Properties

Gio.Task:completed

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

Available since: 2.44

Signals

Signals inherited from GObject (1)

Class structure

struct GioTaskClass {
  /* no available fields */
}