GI.Gio.Objects.Task

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 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 (see mainContextPushThreadDefault) 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 taskPropagatePointer or the like to extract the return value.

Using GTask requires the thread-default 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:

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

GTask 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 when starting a new subtask, so you don’t have to keep track of them yourself. 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 taskRunInThread 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 mainContextPushThreadDefault) where the GTask 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, taskRunInThread and taskRunInThreadSync 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 SimpleAsyncResult

GTask’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 SimpleAsyncResult.set_op_res_gpointer() for the same purpose with SimpleAsyncResult.
• In addition to the task data, GTask also keeps track of the 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 GTask return value by default, like SimpleAsyncResult does when simpleAsyncResultSetCheckCancellable is called. (You can use taskSetCheckCancellable to turn off that behavior.) On the other hand, taskRunInThread 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. (GTask 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 GTask 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 GTask, 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).

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

Exported types

newtype Task

Memory-managed wrapper type.

Constructors

Task (ManagedPtr Task)

Instances

Eq Task

Defined in GI.Gio.Objects.Task

Methods

(==) :: Task -> Task -> Bool

(/=) :: Task -> Task -> Bool

GObject Task

Defined in GI.Gio.Objects.Task

ManagedPtrNewtype Task

Defined in GI.Gio.Objects.Task

Methods

toManagedPtr :: Task -> ManagedPtr Task

TypedObject Task

Defined in GI.Gio.Objects.Task

Methods

glibType :: IO GType

HasParentTypes Task

Defined in GI.Gio.Objects.Task

IsGValue (Maybe Task)

Convert Task to and from GValue. See toGValue and fromGValue.

Defined in GI.Gio.Objects.Task

Methods

gvalueGType_ :: IO GType

gvalueSet_ :: Ptr GValue -> Maybe Task -> IO ()

gvalueGet_ :: Ptr GValue -> IO (Maybe Task)

type ParentTypes Task

Defined in GI.Gio.Objects.Task

type ParentTypes Task = '[Object, AsyncResult]

class (GObject o, IsDescendantOf Task o) => IsTask o

Type class for types which can be safely cast to Task, for instance with toTask.

Instances

(GObject o, IsDescendantOf Task o) => IsTask o

Defined in GI.Gio.Objects.Task

toTask :: (MonadIO m, IsTask o) => o -> m Task

Cast to Task, for types for which this is known to be safe. For general casts, use castTo.

Methods

Click to display all available methods, including inherited ones

Methods

bindProperty, bindPropertyFull, forceFloating, freezeNotify, getv, hadError, isFloating, isTagged, legacyPropagateError, notify, notifyByPspec, propagateBoolean, propagateInt, propagatePointer, propagateValue, ref, refSink, returnBoolean, returnError, returnErrorIfCancelled, returnInt, returnNewErrorLiteral, returnPointer, returnValue, runDispose, runInThread, runInThreadSync, stealData, stealQdata, thawNotify, unref, watchClosure.

Getters

getCancellable, getCheckCancellable, getCompleted, getContext, getData, getName, getPriority, getProperty, getQdata, getReturnOnCancel, getSourceObject, getSourceTag, getTaskData, getUserData.

Setters

setCheckCancellable, setData, setDataFull, setName, setPriority, setProperty, setReturnOnCancel, setSourceTag, setStaticName, setTaskData.

getCancellable

taskGetCancellable

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m (Maybe Cancellable)

Returns: task's Cancellable

Gets task's Cancellable

Since: 2.36

getCheckCancellable

taskGetCheckCancellable

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: the Task

-> m Bool

Gets task's check-cancellable flag. See taskSetCheckCancellable for more details.

Since: 2.36

getCompleted

taskGetCompleted

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task.

-> m Bool

Returns: True if the task has completed, False otherwise.

Gets the value of Task: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

getContext

taskGetContext

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m MainContext

Returns: task's MainContext

Gets the MainContext 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).

This will always return a non-Nothing value, even if the task's context is the default MainContext.

Since: 2.36

getName

taskGetName

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m (Maybe Text)

Returns: task’s name, or Nothing

Gets task’s name. See taskSetName.

Since: 2.60

getPriority

taskGetPriority

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m Int32

Returns: task's priority

Gets task's priority

Since: 2.36

getReturnOnCancel

taskGetReturnOnCancel

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: the Task

-> m Bool

Gets task's return-on-cancel flag. See taskSetReturnOnCancel for more details.

Since: 2.36

getSourceObject

taskGetSourceObject

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m (Maybe Object)

Returns: task's source object, or Nothing

Gets the source object from task. Like asyncResultGetSourceObject, but does not ref the object.

Since: 2.36

getSourceTag

taskGetSourceTag

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m (Ptr ())

Returns: task's source tag

Gets task's source tag. See taskSetSourceTag.

Since: 2.36

getTaskData

taskGetTaskData

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m (Ptr ())

Returns: task's task_data.

Gets task's task_data.

Since: 2.36

hadError

taskHadError

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task.

-> m Bool

Returns: True if the task resulted in an error, False otherwise.

Tests if task resulted in an error.

Since: 2.36

isValid

taskIsValid

Arguments

:: (HasCallStack, MonadIO m, IsAsyncResult a, IsObject b)

=> a

result: A AsyncResult

-> Maybe b

sourceObject: the source object expected to be associated with the task

-> m Bool

Returns: True if result and sourceObject are valid, False if not

Checks that result is a Task, and that sourceObject is its source object (or that sourceObject is Nothing and result has no source object). This can be used in g_return_if_fail() checks.

Since: 2.36

new

taskNew

Arguments

:: (HasCallStack, MonadIO m, IsObject a, IsCancellable b)

=> Maybe a

sourceObject: the Object that owns this task, or Nothing.

-> Maybe b

cancellable: optional Cancellable object, Nothing to ignore.

-> Maybe AsyncReadyCallback

callback: a AsyncReadyCallback.

-> m Task

Returns: a Task.

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

Call this in the "start" method of your asynchronous method, and pass the Task around throughout the asynchronous operation. You can use taskSetTaskData to attach task-specific data to the object, which you can retrieve later via taskGetTaskData.

By default, if cancellable is cancelled, then the return value of the task will always be IOErrorEnumCancelled, even if the task had already completed before the cancellation. This allows for simplified handling in cases where cancellation may imply that other objects that the task depends on have been destroyed. If you do not want this behavior, you can use taskSetCheckCancellable to change it.

Since: 2.36

propagateBoolean

taskPropagateBoolean

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task.

-> m ()

(Can throw GError)

Gets the result of task as a Bool.

If the task resulted in an error, or was cancelled, then this will instead return False and set error.

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

Since: 2.36

propagateInt

taskPropagateInt

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task.

-> m Int64

Returns: the task result, or -1 on error (Can throw GError)

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

If the task resulted in an error, or was cancelled, then this will instead return -1 and set error.

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

Since: 2.36

propagatePointer

taskPropagatePointer

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m (Ptr ())

Returns: the task result, or Nothing on error (Can throw GError)

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

If the task resulted in an error, or was cancelled, then this will instead return Nothing and set error.

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

Since: 2.36

propagateValue

taskPropagateValue

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m GValue

(Can throw GError)

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

If the task resulted in an error, or was cancelled, then this will instead set error and return False.

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

Since: 2.64

reportError

taskReportError

Arguments

:: (HasCallStack, MonadIO m, IsObject a)

=> Maybe a

sourceObject: the Object that owns this task, or Nothing.

-> Maybe AsyncReadyCallback

callback: a AsyncReadyCallback.

-> Ptr ()

sourceTag: an opaque pointer indicating the source of this task

-> GError

error: error to report

-> m ()

Creates a Task and then immediately calls taskReturnError 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 asyncResultIsTagged 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.

See also g_task_report_new_error().

Since: 2.36

returnBoolean

taskReturnBoolean

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task.

-> Bool

result: the Bool result of a task function.

-> m ()

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

Since: 2.36

returnError

taskReturnError

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task.

-> GError

error: the GError result of a task function.

-> m ()

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

Note that since the task takes ownership of error, and since the task may be completed before returning from taskReturnError, you cannot assume that error is still valid after calling this. Call errorCopy on the error if you need to keep a local copy as well.

See also Task.return_new_error(), taskReturnNewErrorLiteral.

Since: 2.36

returnErrorIfCancelled

taskReturnErrorIfCancelled

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> m Bool

Returns: True if task has been cancelled, False if not

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

Since: 2.36

returnInt

taskReturnInt

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task.

-> Int64

result: the integer (gssize) result of a task function.

-> m ()

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

Since: 2.36

returnNewErrorLiteral

taskReturnNewErrorLiteral

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task.

-> Word32

domain: a GQuark.

-> Int32

code: an error code.

-> Text

message: an error message

-> m ()

Sets task’s result to a new [typegLib.Error] created from domain, code, message and completes the task.

See taskReturnPointer for more discussion of exactly what ‘completing the task’ means.

See also Task.return_new_error().

Since: 2.80

returnPointer

taskReturnPointer

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> Ptr ()

result: the pointer result of a task function

-> Maybe DestroyNotify

resultDestroy: a DestroyNotify function.

-> m ()

Sets task's result to result and completes the task. If result is not Nothing, then resultDestroy will be used to free result if the caller does not take ownership of it with taskPropagatePointer.

"Completes the task" means that for an ordinary asynchronous task it will either invoke the task's callback, or else queue that callback to be invoked in the proper MainContext, or in the next iteration of the current MainContext. For a task run via taskRunInThread or taskRunInThreadSync, calling this method will save result to be returned to the caller later, but the task will not actually be completed until the TaskThreadFunc exits.

Note that since the task may be completed before returning from taskReturnPointer, you cannot assume that result is still valid after calling this, unless you are still holding another reference on it.

Since: 2.36

returnValue

taskReturnValue

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> Maybe GValue

result: the Value result of a task function

-> m ()

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

If result is Nothing then a Value of type G_TYPE_POINTER with a value of Nothing will be used for the result.

This is a very generic low-level method intended primarily for use by language bindings; for C code, taskReturnPointer and the like will normally be much easier to use.

Since: 2.64

runInThread

taskRunInThread

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> TaskThreadFunc

taskFunc: a TaskThreadFunc

-> m ()

Runs taskFunc in another thread. When taskFunc returns, task's AsyncReadyCallback will be invoked in task's MainContext.

This takes a ref on task until the task completes.

See TaskThreadFunc for more details about how taskFunc is handled.

Although GLib currently rate-limits the tasks queued via taskRunInThread, you should not assume that it will always do this. If you have a very large number of tasks to run (several tens of tasks), but don't want them to all run at once, you should only queue a limited number of them (around ten) at a time.

Be aware that if your task depends on other tasks to complete, use of this function could lead to a livelock if the other tasks also use this function and enough of them (around 10) execute in a dependency chain, as that will exhaust the thread pool. If this situation is possible, consider using a separate worker thread or thread pool explicitly, rather than using taskRunInThread.

Since: 2.36

runInThreadSync

taskRunInThreadSync

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> TaskThreadFunc

taskFunc: a TaskThreadFunc

-> m ()

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

See TaskThreadFunc for more details about how taskFunc is handled.

Normally this is used with tasks created with a Nothing callback, but note that even if the task does have a callback, it will not be invoked when taskFunc returns. Task:completed will be set to True just before this function returns.

Although GLib currently rate-limits the tasks queued via taskRunInThreadSync, you should not assume that it will always do this. If you have a very large number of tasks to run, but don't want them to all run at once, you should only queue a limited number of them at a time.

Since: 2.36

setCheckCancellable

taskSetCheckCancellable

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: the Task

-> Bool

checkCancellable: whether Task will check the state of its Cancellable for you.

-> m ()

Sets or clears task's check-cancellable flag. If this is True (the default), then taskPropagatePointer, etc, and taskHadError will check the task's Cancellable first, and if it has been cancelled, then they will consider the task to have returned an "Operation was cancelled" error (IOErrorEnumCancelled), regardless of any other error or return value the task may have had.

If checkCancellable is False, then the Task will not check the cancellable itself, and it is up to task's owner to do this (eg, via taskReturnErrorIfCancelled).

If you are using taskSetReturnOnCancel as well, then you must leave check-cancellable set True.

Since: 2.36

setName

taskSetName

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> Maybe Text

name: a human readable name for the task, or Nothing to unset it

-> m ()

Sets task’s name, used in debugging and profiling. The name defaults to Nothing.

The task name should describe in a human readable way what the task does. For example, ‘Open file’ or ‘Connect to network host’. It is used to set the name of the Source used for idle completion of the task.

This function may only be called before the task is first used in a thread other than the one it was constructed in. It is called automatically by taskSetSourceTag if not called already.

Since: 2.60

setPriority

taskSetPriority

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: the Task

-> Int32

priority: the priority of the request

-> m ()

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

This will affect the priority of GSources created with g_task_attach_source() and the scheduling of tasks run in threads, and can also be explicitly retrieved later via taskGetPriority.

Since: 2.36

setReturnOnCancel

taskSetReturnOnCancel

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: the Task

-> Bool

returnOnCancel: whether the task returns automatically when it is cancelled.

-> m Bool

Returns: True if task's return-on-cancel flag was changed to match returnOnCancel. False if task has already been cancelled.

Sets or clears task's return-on-cancel flag. This is only meaningful for tasks run via taskRunInThread or taskRunInThreadSync.

If returnOnCancel is True, then cancelling task's Cancellable will immediately cause it to return, as though the task's TaskThreadFunc had called taskReturnErrorIfCancelled and then returned.

This allows you to create a cancellable wrapper around an uninterruptible function. The TaskThreadFunc just needs to be careful that it does not modify any externally-visible state after it has been cancelled. To do that, the thread should call taskSetReturnOnCancel again to (atomically) set return-on-cancel False before making externally-visible changes; if the task gets cancelled before the return-on-cancel flag could be changed, taskSetReturnOnCancel will indicate this by returning False.

You can disable and re-enable this flag multiple times if you wish. If the task's Cancellable is cancelled while return-on-cancel is False, then calling taskSetReturnOnCancel to set it True again will cause the task to be cancelled at that point.

If the task's Cancellable is already cancelled before you call taskRunInThread/taskRunInThreadSync, then the TaskThreadFunc will still be run (for consistency), but the task will also be completed right away.

Since: 2.36

setSourceTag

taskSetSourceTag

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: the Task

-> Ptr ()

sourceTag: an opaque pointer indicating the source of this task

-> m ()

Sets task's 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 taskGetSourceTag (or asyncResultIsTagged) in the task's "finish" function, to figure out if the response came from a particular place.

A macro wrapper around this function will automatically set the task’s name to the string form of sourceTag if it’s not already set, for convenience.

Since: 2.36

setStaticName

taskSetStaticName

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: a Task

-> Maybe Text

name: a human readable name for the task. Must be a string literal

-> m ()

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

This is a variant of taskSetName that avoids copying name.

Since: 2.76

setTaskData

taskSetTaskData

Arguments

:: (HasCallStack, MonadIO m, IsTask a)

=> a

task: the Task

-> Ptr ()

taskData: task-specific data

-> Maybe DestroyNotify

taskDataDestroy: DestroyNotify for taskData

-> m ()

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

Since: 2.36

Properties

completed

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. However, it is not guaranteed to happen immediately after those functions are called, as the task’s callback may need to be scheduled to run in a different thread.

That means it is not safe to use this property to track whether a return function has been called on the Task. Callers must do that tracking themselves, typically by linking the lifetime of the Task to the control flow of their code.

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

The Object::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

getTaskCompleted :: (MonadIO m, IsTask o) => o -> m Bool

Get the value of the “completed” property. When overloading is enabled, this is equivalent to

get task #completed