Struct Task

pub struct Task<V: ValueType + Send> { /* private fields */ }

Task provides idiomatic access to gio’s GTask API, for instance by being generic over their value type, while not completely departing from the underlying C API. Task is Send and Sync and requires its value to also be Send and Sync, thus is useful to to implement gio style asynchronous tasks that run in threads. If you need to only run tasks in glib main loop see the LocalTask type.

The constructors of LocalTask and Task is marked as unsafe because this API does not allow to automatically enforce all the invariants required to be a completely safe abstraction. The caller is responsible to ensure the following requirements are satisfied

• You should not create a LocalTask, upcast it to a glib::Object and then downcast it to a Task, as this will bypass the thread safety requirements
• You should ensure that the return_result, return_error_if_cancelled and propagate() methods are only called once. 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 new() in the ‘start’ method, followed by [set_task_data()][Self::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 [return_pointer()][Self::return_pointer()] or [return_error()][Self::return_error()], which will save the value you give it and then invoke the task’s callback function in the thread-default main context (see [glib::MainContext::push_thread_default()][crate::glib::MainContext::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 AsyncResult), and you can use [propagate_pointer()][Self::propagate_pointer()] 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: ⚠️ The following code is in c ⚠️

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. cancellable(), context(), and priority() 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. [attach_source()][Self::attach_source()] 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: ⚠️ The following code is in c ⚠️

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 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 (see [glib::MainContext::push_thread_default()][crate::glib::MainContext::push_thread_default()]) where the GTask was created.

Running a task in a thread: ⚠️ The following code is in c ⚠️

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, run_in_thread() and [run_in_thread_sync()][Self::run_in_thread_sync()] can be used to turn an uncancellable operation into a cancellable one. If you call set_return_on_cancel(), 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: ⚠️ The following code is in c ⚠️

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 Gio::SimpleAsyncResult

GTask’s API attempts to be simpler than Gio::SimpleAsyncResult’s in several ways:

• You can save task-specific data with [set_task_data()][Self::set_task_data()], and retrieve it later with [task_data()][Self::task_data()]. This replaces the abuse of Gio::SimpleAsyncResult::set_op_res_gpointer() for the same purpose with Gio::SimpleAsyncResult.
• In addition to the task data, GTask also keeps track of the priority, Cancellable, and glib::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.
• return_error_if_cancelled() provides simplified handling for cancellation. In addition, cancellation overrides any other GTask return value by default, like Gio::SimpleAsyncResult does when Gio::SimpleAsyncResult::set_check_cancellable() is called. (You can use set_check_cancellable() to turn off that behavior.) On the other hand, 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 return_error_if_cancelled() check if you need the old behavior.
• The ‘return’ methods (eg, [return_pointer()][Self::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 glib::MainContext, or delayed until the next iteration of the current glib::MainContext.)
• The ‘finish’ functions for GTask based operations are generally much simpler than Gio::SimpleAsyncResult ones, normally consisting of only a single call to [propagate_pointer()][Self::propagate_pointer()] or the like. Since [propagate_pointer()][Self::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 Gio::SimpleAsyncResult, it was common to call Gio::SimpleAsyncResult::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 AsyncResultExt::legacy_propagate_error() to do old-style Gio::SimpleAsyncResult error-returning behavior, and Gio::AsyncResult::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 InputStreamExtManual::read_async()).

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

Properties

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. 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 #GTask. Callers must do that tracking themselves, typically by linking the lifetime of the #GTask to the control flow of their code.

This property is guaranteed to change from false to true exactly once.

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

Readable

Implements

[trait@glib::ObjectExt], AsyncResultExt

GLib type: GObject with reference counted clone semantics.

Implementations

impl<V: Into<Value> + ValueType + Send> Task<V>

pub unsafe fn new<S, P, Q>( source_object: Option<&S>, cancellable: Option<&P>, callback: Q, ) -> Self

where S: IsA<Object> + Send, P: IsA<Cancellable>, Q: FnOnce(Task<V>, Option<&S>) + Send + 'static,

pub fn cancellable(&self) -> Option<Cancellable>

pub fn is_check_cancellable(&self) -> bool

pub fn set_check_cancellable(&self, check_cancellable: bool)

pub fn set_name(&self, name: Option<&str>)

Available on crate feature v2_60 only.

pub fn set_return_on_cancel(&self, return_on_cancel: bool) -> bool

pub fn is_valid( result: &impl IsA<AsyncResult>, source_object: Option<&impl IsA<Object>>, ) -> bool

pub fn priority(&self) -> Priority

pub fn set_priority(&self, priority: Priority)

pub fn is_completed(&self) -> bool

pub fn context(&self) -> MainContext

pub fn name(&self) -> Option<GString>

Available on crate feature v2_60 only.

pub fn is_return_on_cancel(&self) -> bool

pub fn had_error(&self) -> bool

pub fn connect_completed_notify<F>(&self, f: F) -> SignalHandlerId

where F: Fn(&Task<V>) + Send + 'static,

pub unsafe fn return_error_if_cancelled(&self) -> bool

pub unsafe fn return_result(self, result: Result<V, Error>)

pub unsafe fn propagate(self) -> Result<V, Error>

impl<V: ValueType + Send> Task<V>

pub fn run_in_thread<S, Q>(&self, task_func: Q)

where S: IsA<Object> + Send, Q: FnOnce(Self, Option<&S>, Option<&Cancellable>) + Send + 'static,

Trait Implementations

impl<V: ValueType + Send> Clone for Task<V>

fn clone(&self) -> Self

Makes a clone of this shared reference.

This increments the strong reference count of the object. Dropping the object will decrement it again.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<V: ValueType + Send> Debug for Task<V>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<V: ValueType + Send> HasParamSpec for Task<V>

type ParamSpec = ParamSpecObject

type SetValue = Task<V>

Preferred value to be used as setter for the associated ParamSpec.

type BuilderFn = fn(_: &str) -> ParamSpecObjectBuilder<'_, Task<V>>

fn param_spec_builder() -> Self::BuilderFn

impl<V: ValueType + Send> Hash for Task<V>

fn hash<H>(&self, state: &mut H)

where H: Hasher,

Hashes the memory address of this object.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<V: ValueType + Send> Ord for Task<V>

fn cmp(&self, other: &Self) -> Ordering

Comparison for two GObjects.

Compares the memory addresses of the provided objects.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl<V: ValueType + Send> ParentClassIs for Task<V>

type Parent = Object

impl<OT: ObjectType, V: ValueType + Send> PartialEq<OT> for Task<V>

fn eq(&self, other: &OT) -> bool

Equality for two GObjects.

Two GObjects are equal if their memory addresses are equal.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<OT: ObjectType, V: ValueType + Send> PartialOrd<OT> for Task<V>

fn partial_cmp(&self, other: &OT) -> Option<Ordering>

Partial comparison for two GObjects.

Compares the memory addresses of the provided objects.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl<V: ValueType + Send> StaticType for Task<V>

fn static_type() -> Type

Returns the type identifier of Self.

impl<V: ValueType + Send> Eq for Task<V>

impl<V: ValueType + Send> IsA<AsyncResult> for Task<V>

impl<V: ValueType + Send> Send for Task<V>

impl<V: ValueType + Send> Sync for Task<V>