Gio.Task
Description: Reference for Gio.Task

### Class

# GioTask

#### Description [src]

    final class Gio.Task : GObject.Object
      implements Gio.AsyncResult {
      /* No available fields */
    }

A `GTask` represents and manages a cancellable ‘task’.

## Asynchronous operations

The most common usage of `GTask` is as a `GAsyncResult`, to manage data during an asynchronous operation. You call `g_task_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 `g_task_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 `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 `GTask` back to the operation’s finish function (as a `GAsyncResult`), and you can use `g_task_propagate_pointer()` or the like to extract the return value.

Using `GTask` requires the thread-default `GMainContext` 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 `GAsyncResult`:

    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. `g_task_get_cancellable()`, `g_task_get_context()`, and `g_task_get_priority()` allow you to get back the task’s `GCancellable`, `GMainContext`, and I/O 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 `GMainContext` 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 `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 (see `g_main_context_push_thread_default()`) 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, `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 `g_task_set_return_on_cancel()`, passing `TRUE`, then if the task’s `GCancellable` 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 `GSimpleAsyncResult`

`GTask`’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, `GTask` also keeps track of the priority, `GCancellable`, and `GMainContext` 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.
- `g_task_return_error_if_cancelled()` provides simplified handling for cancellation. In addition, cancellation overrides any other `GTask` return value by default, like `GSimpleAsyncResult` does when `g_simple_async_result_set_check_cancellable()` is called. (You can use `g_task_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 `GCancellable` is already cancelled before the task gets a chance to run; you can start your `task_func` with a `g_task_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. (`GTask` automatically figures out whether the task’s callback can be invoked directly, or if it needs to be sent to another `GMainContext`, or delayed until the next iteration of the current `GMainContext`.)
- The ‘finish’ functions for `GTask` 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 `GTask`, 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 `g_async_result_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 `g_input_stream_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.

#### Ancestors

- GObject

#### Implements

- GAsyncResult

#### Constructors

###### g_task_new

Creates a `GTask` acting on `source_object`, which will eventually be used to invoke `callback` in the current thread-default main context (see `g_main_context_push_thread_default()`).

since: 2.36

#### Functions

###### g_task_is_valid

Checks that `result` is a `GTask`, and that `source_object` is its source object (or that `source_object` is `NULL` and `result` has no source object). This can be used in `g_return_if_fail()` checks.

since: 2.36

###### g_task_report_error

Creates a `GTask` and then immediately calls `g_task_return_error()` 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 `g_async_result_is_tagged()` 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.

since: 2.36

###### g_task_report_new_error

Creates a `GTask` and then immediately calls `g_task_return_new_error()` 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 `g_async_result_is_tagged()` 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.

since: 2.36

#### Instance methods

###### g_task_attach_source

A utility function for dealing with async operations where you need to wait for a `GSource` to trigger. Attaches `source` to `task`‘s `GMainContext` with `task`‘s priority, and sets `source`‘s callback to `callback`, with `task` as the callback’s `user_data`.

since: 2.36

###### g_task_get_cancellable

Gets `task`‘s `GCancellable`.

since: 2.36

###### g_task_get_check_cancellable

Gets `task`‘s check-cancellable flag. See `g_task_set_check_cancellable()` for more details.

since: 2.36

###### g_task_get_completed

Gets the value of `GTask: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

###### g_task_get_context

Gets the `GMainContext` that `task` will return its result in (that is, the context that was the thread-default main context (see `g_main_context_push_thread_default()`) at the point when `task` was created).

since: 2.36

###### g_task_get_name

Gets `task`’s name. See g_task_set_name().

since: 2.60

###### g_task_get_priority

Gets `task`‘s priority.

since: 2.36

###### g_task_get_return_on_cancel

Gets `task`‘s return-on-cancel flag. See `g_task_set_return_on_cancel()` for more details.

since: 2.36

###### g_task_get_source_object

Gets the source object from `task`. Like g_async_result_get_source_object(), but does not ref the object.

since: 2.36

###### g_task_get_source_tag

Gets `task`‘s source tag. See g_task_set_source_tag().

since: 2.36

###### g_task_get_task_data

Gets `task`‘s `task_data`.

since: 2.36

###### g_task_had_error

Tests if `task` resulted in an error.

since: 2.36

###### g_task_propagate_boolean

Gets the result of `task` as a #gboolean.

since: 2.36

###### g_task_propagate_int

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

since: 2.36

###### g_task_propagate_pointer

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

since: 2.36

###### g_task_propagate_value

Gets the result of `task` as a `GValue`, and transfers ownership of that value to the caller. As with g_task_return_value(), this is a generic low-level method; `g_task_propagate_pointer()` and the like will usually be more useful for C code.

since: 2.64

###### g_task_return_boolean

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

since: 2.36

###### g_task_return_error

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

since: 2.36

###### g_task_return_error_if_cancelled

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

since: 2.36

###### g_task_return_int

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

since: 2.36

###### g_task_return_new_error

Sets `task`‘s result to a new `GError` created from `domain`, `code`, `format`, and the remaining arguments, and completes the task (see `g_task_return_pointer()` for more discussion of exactly what this means).

since: 2.36

###### g_task_return_new_error_literal

Sets `task`’s result to a new `GError` created from `domain`, `code`, `message` and completes the task.

since: 2.80

###### g_task_return_pointer

Sets `task`‘s result to `result` and completes the task. If `result` is not `NULL`, then `result_destroy` will be used to free `result` if the caller does not take ownership of it with g_task_propagate_pointer().

since: 2.36

###### g_task_return_prefixed_error

Sets `task`‘s result to `error` (which `task` assumes ownership of), with the message prefixed according to `format`, and completes the task (see `g_task_return_pointer()` for more discussion of exactly what this means).

since: 2.80

###### g_task_return_value

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

since: 2.64

###### g_task_run_in_thread

Runs `task_func` in another thread. When `task_func` returns, `task`‘s `GAsyncReadyCallback` will be invoked in `task`‘s `GMainContext`.

since: 2.36

###### g_task_run_in_thread_sync

Runs `task_func` in another thread, and waits for it to return or be cancelled. You can use g_task_propagate_pointer(), etc, afterward to get the result of `task_func`.

since: 2.36

###### g_task_set_check_cancellable

Sets or clears `task`‘s check-cancellable flag. If this is `TRUE` (the default), then g_task_propagate_pointer(), etc, and `g_task_had_error()` will check the task’s `GCancellable` first, and if it has been cancelled, then they will consider the task to have returned an “Operation was cancelled” error (`G_IO_ERROR_CANCELLED`), regardless of any other error or return value the task may have had.

since: 2.36

###### g_task_set_name

Sets `task`’s name, used in debugging and profiling. The name defaults to `NULL`.

since: 2.60

###### g_task_set_priority

Sets `task`‘s priority. If you do not call this, it will default to `G_PRIORITY_DEFAULT`.

since: 2.36

###### g_task_set_return_on_cancel

Sets or clears `task`‘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().

since: 2.36

###### g_task_set_source_tag

Sets `task`‘s source tag.

since: 2.36

###### g_task_set_static_name

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

since: 2.76

###### g_task_set_task_data

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

since: 2.36

##### Methods inherited from GObject (43)

Please see GObject for a full list of methods.

##### Methods inherited from GAsyncResult (4)

###### g_async_result_get_source_object

Gets the source object from a `GAsyncResult`.

###### g_async_result_get_user_data

Gets the user data from a `GAsyncResult`.

###### g_async_result_is_tagged

Checks if `res` has the given `source_tag` (generally a function pointer indicating the function `res` was created by).

since: 2.34

###### g_async_result_legacy_propagate_error

If `res` is a `GSimpleAsyncResult`, this is equivalent to `g_simple_async_result_propagate_error()`. Otherwise it returns `FALSE`.

since: 2.34

#### Properties

###### Gio.Task:completed

Whether the task has completed, meaning its callback (if set) has been invoked.

since: 2.44

#### Signals

##### Signals inherited from GObject (1)

###### GObject::notify

The notify signal is emitted on an object when one of its properties has its value set through g_object_set_property(), g_object_set(), et al.

#### Class structure

    struct GioTaskClass {
      /* no available fields */
    }

No description available.