Package org.gnome.gio

Class Task

java.lang.Object
org.javagi.base.ProxyInstance
org.gnome.gobject.TypeInstance
org.gnome.gobject.GObject
org.gnome.gio.Task
All Implemented Interfaces:
AsyncResult, Proxy
@Generated("io.github.jwharm.JavaGI") public class Task extends GObject implements 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() in the ‘start’ method, followed by setTaskData(java.lang.foreign.MemorySegment) 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 returnPointer(java.lang.foreign.MemorySegment) or returnError(org.gnome.glib.GError), which will save the value you give it and then invoke the task’s callback function in the thread-default main context (see MainContext.pushThreadDefault()) 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 propagatePointer() 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. getCancellable(), getContext(), and getPriority() 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. attachSource(org.gnome.glib.Source, org.gnome.glib.SourceFunc) 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 runInThread(org.gnome.gio.TaskThreadFunc) 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 MainContext.pushThreadDefault()) 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, runInThread(org.gnome.gio.TaskThreadFunc) and runInThreadSync(org.gnome.gio.TaskThreadFunc) can be used to turn an uncancellable operation into a cancellable one. If you call setReturnOnCancel(boolean), 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 [class@Gio.SimpleAsyncResult]
GTask’s API attempts to be simpler than SimpleAsyncResult’s in several ways:

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