A Task represents and manages a cancellable "task".

Asynchronous operations

The most common usage of Task is as a AsyncResult, to manage data during an asynchronous operation. You call taskNew in the "start" method, followed by taskSetTaskData 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 taskReturnPointer or taskReturnError, 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 Task back to the operation's finish function (as a AsyncResult), and you can use taskPropagatePointer or the like to extract the return value.

Here is an example for using GTask as a GAsyncResult:

C code

   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

Task also tries to simplify asynchronous operations that internally chain together several smaller asynchronous operations. taskGetCancellable, taskGetContext, and taskGetPriority allow you to get back the task's Cancellable, MainContext, 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 MainContext and priority).

Here is an example for chained asynchronous operations:

C code

   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 Task was created.

Running a task in a thread:

C code

   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 taskSetReturnOnCancel, 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:

C code

   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

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

• You can save task-specific data with taskSetTaskData, and retrieve it later with taskGetTaskData. This replaces the abuse of g_simple_async_result_set_op_res_gpointer() for the same purpose with SimpleAsyncResult.
• In addition to the task data, Task also keeps track of the [priority][io-priority], Cancellable, and 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.
• taskReturnErrorIfCancelled provides simplified handling for cancellation. In addition, cancellation overrides any other Task return value by default, like SimpleAsyncResult does when simpleAsyncResultSetCheckCancellable is called. (You can use taskSetCheckCancellable 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 Cancellable is already cancelled before the task gets a chance to run; you can start your task_func with a taskReturnErrorIfCancelled check if you need the old behavior.
• The "return" methods (eg, taskReturnPointer) automatically cause the task to be "completed" as well, and there is no need to worry about the "complete" vs "complete in idle" distinction. (tTask automatically figures out whether the task's callback can be invoked directly, or if it needs to be sent to another MainContext, or delayed until the next iteration of the current MainContext.)
• The "finish" functions for Task based operations are generally much simpler than SimpleAsyncResult ones, normally consisting of only a single call to taskPropagatePointer or the like. Since taskPropagatePointer "steals" the return value from the Task, it is not necessary to juggle pointers around to prevent it from being freed twice.
• With SimpleAsyncResult, it was common to call simpleAsyncResultPropagateError 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 asyncResultLegacyPropagateError to do old-style SimpleAsyncResult error-returning behavior, and asyncResultIsTagged 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 inputStreamReadAsync).

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

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

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

Since: 2.44