Struct gio::Task

pub struct Task(_);

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 new() in the “start” method, followed by g_task_set_task_data() and the like if you need to keep some additional data associated with the task, and then pass the task object around through your asynchronous operation. Eventually, you will call a method such as g_task_return_pointer() or return_error(), which will save the value you give it and then invoke the task’s callback function in the [thread-default main context][g-main-context-push-thread-default] where it was created (waiting until the next iteration of the main loop first, if necessary). The caller will pass the Task back to the operation’s finish function (as a AsyncResult), and you can use g_task_propagate_pointer() or the like to extract the return value.

Here is an example for using GTask as a GAsyncResult:

⚠️ 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

Task 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][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 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 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:

⚠️ 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, 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 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 GSimpleAsyncResult

Task’s API attempts to be simpler than GSimpleAsyncResult’s in several ways:

• You can save task-specific data with g_task_set_task_data(), and retrieve it later with g_task_get_task_data(). This replaces the abuse of g_simple_async_result_set_op_res_gpointer() for the same purpose with GSimpleAsyncResult.
• In addition to the task data, Task also keeps track of the [priority][io-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 Task return value by default, like GSimpleAsyncResult does when g_simple_async_result_set_check_cancellable() is called. (You can use set_check_cancellable() to turn off that behavior.) On the other hand, g_task_run_in_thread() guarantees that it will always run your task_func, even if the task’s 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, g_task_return_pointer()) automatically cause the task to be “completed” as well, and there is no need to worry about the “complete” vs “complete in idle” distinction. (Task 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 Task based operations are generally much simpler than GSimpleAsyncResult ones, normally consisting of only a single call to g_task_propagate_pointer() or the like. Since g_task_propagate_pointer() “steals” the return value from the Task, it is not necessary to juggle pointers around to prevent it from being freed twice.
• With GSimpleAsyncResult, it was common to call g_simple_async_result_propagate_error() from the _finish() wrapper function, and have virtual method implementations only deal with successful returns. This behavior is deprecated, because it makes it difficult for a subclass to chain to a parent class’s async methods. Instead, the wrapper function should just be a simple wrapper, and the virtual method should call an appropriate g_task_propagate_ function. Note that wrapper methods can now use AsyncResultExt::legacy_propagate_error() to do old-style GSimpleAsyncResult error-returning behavior, and g_async_result_is_tagged() to check if a result is tagged as having come from the _async() wrapper function (for “short-circuit” results, such as when passing 0 to InputStreamExtManual::read_async()).

Implements

glib::ObjectExt, AsyncResultExt

Implementations

impl Task

pub fn cancellable(&self) -> Cancellable

Gets self’s Cancellable

Returns

self’s Cancellable

pub fn is_check_cancellable(&self) -> bool

Gets self’s check-cancellable flag. See set_check_cancellable() for more details.

pub fn is_completed(&self) -> bool

Gets the value of property::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.

Returns

true if the task has completed, false otherwise.

pub fn context(&self) -> MainContext

Gets the glib::MainContext that self 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 self was created).

This will always return a non-None value, even if the task’s context is the default glib::MainContext.

Returns

self’s glib::MainContext

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

This is supported on crate feature v2_60 only.

Gets self’s name. See set_name().

Returns

self’s name, or None

pub fn is_return_on_cancel(&self) -> bool

Gets self’s return-on-cancel flag. See set_return_on_cancel() for more details.

pub fn had_error(&self) -> bool

Tests if self resulted in an error.

Returns

true if the task resulted in an error, false otherwise.

pub fn return_error_if_cancelled(&self) -> bool

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

Returns

true if self has been cancelled, false if not

pub fn set_check_cancellable(&self, check_cancellable: bool)

Sets or clears self’s check-cancellable flag. If this is true (the default), then g_task_propagate_pointer(), etc, and had_error() 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 (IOErrorEnum::Cancelled), regardless of any other error or return value the task may have had.

If check_cancellable is false, then the Task will not check the cancellable itself, and it is up to self’s owner to do this (eg, via return_error_if_cancelled()).

If you are using set_return_on_cancel() as well, then you must leave check-cancellable set true.

check_cancellable

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

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

This is supported on crate feature v2_60 only.

Sets self’s name, used in debugging and profiling. The name defaults to None.

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 glib::Source used for idle completion of the task.

This function may only be called before the self is first used in a thread other than the one it was constructed in.

name

a human readable name for the task, or None to unset it

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

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

If return_on_cancel is true, then cancelling self’s Cancellable will immediately cause it to return, as though the task’s GTaskThreadFunc had called return_error_if_cancelled() and then returned.

This allows you to create a cancellable wrapper around an uninterruptible function. The GTaskThreadFunc 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 set_return_on_cancel() 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, set_return_on_cancel() 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 set_return_on_cancel() 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 g_task_run_in_thread()/g_task_run_in_thread_sync(), then the GTaskThreadFunc will still be run (for consistency), but the task will also be completed right away.

return_on_cancel

whether the task returns automatically when it is cancelled.

Returns

true if self’s return-on-cancel flag was changed to match return_on_cancel. false if self has already been cancelled.

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

Checks that result is a Task, and that source_object is its source object (or that source_object is None and result has no source object). This can be used in g_return_if_fail() checks.

result

A AsyncResult

source_object

the source object expected to be associated with the task

Returns

true if result and source_object are valid, false if not

pub fn connect_completed_notify<F: Fn(&Self) + 'static>(
    &self,
    f: F
) -> SignalHandlerId

impl Task

pub fn new<P: IsA<Cancellable>, Q: FnOnce(&AsyncResult, Option<&Object>) + 'static>(
    source_object: Option<&Object>,
    cancellable: Option<&P>,
    callback: Q
) -> Task

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

Call this in the “start” method of your asynchronous method, and pass the Task around throughout the asynchronous operation. You can use g_task_set_task_data() to attach task-specific data to the object, which you can retrieve later via g_task_get_task_data().

By default, if cancellable is cancelled, then the return value of the task will always be IOErrorEnum::Cancelled, 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 set_check_cancellable() to change it.

source_object

the glib::Object that owns this task, or None.

cancellable

optional Cancellable object, None to ignore.

callback

a GAsyncReadyCallback.

callback_data

user data passed to callback.

Returns

a Task.

pub fn return_error(&self, error: Error)

Sets self’s result to error (which self assumes ownership of) and completes the task (see g_task_return_pointer() 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 return_error(), you cannot assume that error is still valid after calling this. Call g_error_copy() on the error if you need to keep a local copy as well.

See also g_task_return_new_error().

error

the glib::Error result of a task function.

pub fn priority(&self) -> Priority

Gets self’s priority

Returns

self’s priority

pub fn set_priority(&self, priority: Priority)

Sets self’s priority. If you do not call this, it will default to G_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 priority().

priority

the [priority][io-priority] of the request

pub fn return_value(&self, result: &Value)

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

If result is None then a glib::Value of type G_TYPE_POINTER with a value of None will be used for the result.

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

result

the glib::Value result of a task function

pub fn propagate_value(&self) -> Result<Value, Error>

Gets the result of self as a glib::Value, and transfers ownership of that value to the caller. As with return_value(), this is a generic low-level method; g_task_propagate_pointer() 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.

Returns

true if self succeeded, false on error.

value

return location for the glib::Value

Trait Implementations

impl Clone for Task

fn clone(&self) -> Task

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for Task

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

Formats the value using the given formatter.

impl Display for Task

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

Formats the value using the given formatter.

impl Hash for Task

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

Feeds this value into the given Hasher.

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

Feeds a slice of this type into the given Hasher.

impl Ord for Task

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

This method returns an Ordering between self and other.

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

Compares and returns the maximum of two values.

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

Compares and returns the minimum of two values.

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

Restrict a value to a certain interval.

impl ParentClassIs for Task

type Parent = Object

impl<T: ObjectType> PartialEq<T> for Task

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

This method tests for self and other values to be equal, and is used by ==.

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

This method tests for !=.

impl<T: ObjectType> PartialOrd<T> for Task

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

This method returns an ordering between self and other values if one exists.

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

This method tests less than (for self and other) and is used by the < operator.

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

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

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

This method tests greater than (for self and other) and is used by the > operator.

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

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

impl StaticType for Task

fn static_type() -> Type

Returns the type identifier of Self.

impl Eq for Task

impl IsA<AsyncResult> for Task

impl StructuralEq for Task