Task

open class Task(pointer: <Error class: unknown class><<Error class: unknown class>>) : AsyncResult

A GTask represents and manages a cancellable ‘task’.

Asynchronous operations

The most common usage of GTask is as a iface@Gio.AsyncResult, to manage data during an asynchronous operation. You call ctor@Gio.Task.new in the ‘start’ method, followed by method@Gio.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 method@Gio.Task.return_pointer or method@Gio.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 method@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 iface@Gio.AsyncResult), and you can use method@Gio.Task.propagate_pointer or the like to extract the return value.

Using GTask requires the thread-default struct@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 iface@Gio.AsyncResult:

typedef struct {
  CakeFrostingType frosting;
  char *message;
} DecorationData;

static void
decoration_data_free (DecorationData *decoration)
{
  g_free (decoration->message);
  g_slice_free (DecorationData, decoration);
}

static void
baked_cb (Cake     *cake,
          gpointer  user_data)
{
  GTask *task = user_data;
  DecorationData *decoration = g_task_get_task_data (task);
  GError *error = NULL;

  if (cake == NULL)
    {
      g_task_return_new_error (task, BAKER_ERROR, BAKER_ERROR_NO_FLOUR,
                               "Go to the supermarket");
      g_object_unref (task);
      return;
    }

  if (!cake_decorate (cake, decoration->frosting, decoration->message, &error))
    {
      g_object_unref (cake);
      // g_task_return_error() takes ownership of error
      g_task_return_error (task, error);
      g_object_unref (task);
      return;
    }

  g_task_return_pointer (task, cake, g_object_unref);
  g_object_unref (task);
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  GTask *task;
  DecorationData *decoration;
  Cake  *cake;

  task = g_task_new (self, cancellable, callback, user_data);
  if (radius < 3)
    {
      g_task_return_new_error (task, BAKER_ERROR, BAKER_ERROR_TOO_SMALL,
                               "%ucm radius cakes are silly",
                               radius);
      g_object_unref (task);
      return;
    }

  cake = _baker_get_cached_cake (self, radius, flavor, frosting, message);
  if (cake != NULL)
    {
      // _baker_get_cached_cake() returns a reffed cake
      g_task_return_pointer (task, cake, g_object_unref);
      g_object_unref (task);
      return;
    }

  decoration = g_slice_new (DecorationData);
  decoration->frosting = frosting;
  decoration->message = g_strdup (message);
  g_task_set_task_data (task, decoration, (GDestroyNotify) decoration_data_free);

  _baker_begin_cake (self, radius, flavor, cancellable, baked_cb, task);
}

Cake *
baker_bake_cake_finish (Baker         *self,
                        GAsyncResult  *result,
                        GError       **error)
{
  g_return_val_if_fail (g_task_is_valid (result, self), NULL);

  return g_task_propagate_pointer (G_TASK (result), error);
}

Chained asynchronous operations

GTask also tries to simplify asynchronous operations that internally chain together several smaller asynchronous operations. method@Gio.Task.get_cancellable, method@Gio.Task.get_context, and method@Gio.Task.get_priority allow you to get back the task’s class@Gio.Cancellable, struct@GLib.MainContext, and iface.AsyncResult.html#io-priority when starting a new subtask, so you don’t have to keep track of them yourself. method@Gio.Task.attach_source simplifies the case of waiting for a source to fire (automatically using the correct struct@GLib.MainContext and priority).

Here is an example for chained asynchronous operations:

typedef struct {
  Cake *cake;
  CakeFrostingType frosting;
  char *message;
} BakingData;

static void
decoration_data_free (BakingData *bd)
{
  if (bd->cake)
    g_object_unref (bd->cake);
  g_free (bd->message);
  g_slice_free (BakingData, bd);
}

static void
decorated_cb (Cake         *cake,
              GAsyncResult *result,
              gpointer      user_data)
{
  GTask *task = user_data;
  GError *error = NULL;

  if (!cake_decorate_finish (cake, result, &error))
    {
      g_object_unref (cake);
      g_task_return_error (task, error);
      g_object_unref (task);
      return;
    }

  // baking_data_free() will drop its ref on the cake, so we have to
  // take another here to give to the caller.
  g_task_return_pointer (task, g_object_ref (cake), g_object_unref);
  g_object_unref (task);
}

static gboolean
decorator_ready (gpointer user_data)
{
  GTask *task = user_data;
  BakingData *bd = g_task_get_task_data (task);

  cake_decorate_async (bd->cake, bd->frosting, bd->message,
                       g_task_get_cancellable (task),
                       decorated_cb, task);

  return G_SOURCE_REMOVE;
}

static void
baked_cb (Cake     *cake,
          gpointer  user_data)
{
  GTask *task = user_data;
  BakingData *bd = g_task_get_task_data (task);
  GError *error = NULL;

  if (cake == NULL)
    {
      g_task_return_new_error (task, BAKER_ERROR, BAKER_ERROR_NO_FLOUR,
                               "Go to the supermarket");
      g_object_unref (task);
      return;
    }

  bd->cake = cake;

  // Bail out now if the user has already cancelled
  if (g_task_return_error_if_cancelled (task))
    {
      g_object_unref (task);
      return;
    }

  if (cake_decorator_available (cake))
    decorator_ready (task);
  else
    {
      GSource *source;

      source = cake_decorator_wait_source_new (cake);
      // Attach @source to @task’s GMainContext and have it call
      // decorator_ready() when it is ready.
      g_task_attach_source (task, source, decorator_ready);
      g_source_unref (source);
    }
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       gint                 priority,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  GTask *task;
  BakingData *bd;

  task = g_task_new (self, cancellable, callback, user_data);
  g_task_set_priority (task, priority);

  bd = g_slice_new0 (BakingData);
  bd->frosting = frosting;
  bd->message = g_strdup (message);
  g_task_set_task_data (task, bd, (GDestroyNotify) baking_data_free);

  _baker_begin_cake (self, radius, flavor, cancellable, baked_cb, task);
}

Cake *
baker_bake_cake_finish (Baker         *self,
                        GAsyncResult  *result,
                        GError       **error)
{
  g_return_val_if_fail (g_task_is_valid (result, self), NULL);

  return g_task_propagate_pointer (G_TASK (result), error);
}

Asynchronous operations from synchronous ones

You can use method@Gio.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 method@GLib.MainContext.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, method@Gio.Task.run_in_thread and method@Gio.Task.run_in_thread_sync can be used to turn an uncancellable operation into a cancellable one. If you call method@Gio.Task.set_return_on_cancel, passing TRUE, then if the task’s class@Gio.Cancellable is cancelled, it will return control back to the caller immediately, while allowing the task thread to continue running in the background (and simply discarding its result when it finally does finish). Provided that the task thread is careful about how it uses locks and other externally-visible resources, this allows you to make ‘GLib-friendly’ asynchronous and cancellable synchronous variants of blocking APIs.

Cancelling a task:

static void
bake_cake_thread (GTask         *task,
                  gpointer       source_object,
                  gpointer       task_data,
                  GCancellable  *cancellable)
{
  Baker *self = source_object;
  CakeData *cake_data = task_data;
  Cake *cake;
  GError *error = NULL;

  cake = bake_cake (baker, cake_data->radius, cake_data->flavor,
                    cake_data->frosting, cake_data->message,
                    &error);
  if (error)
    {
      g_task_return_error (task, error);
      return;
    }

  // If the task has already been cancelled, then we don’t want to add
  // the cake to the cake cache. Likewise, we don’t  want to have the
  // task get cancelled in the middle of updating the cache.
  // g_task_set_return_on_cancel() will return true here if it managed
  // to disable return-on-cancel, or false if the task was cancelled
  // before it could.
  if (g_task_set_return_on_cancel (task, FALSE))
    {
      // If the caller cancels at this point, their
      // GAsyncReadyCallback won’t be invoked until we return,
      // so we don’t have to worry that this code will run at
      // the same time as that code does. But if there were
      // other functions that might look at the cake cache,
      // then we’d probably need a GMutex here as well.
      baker_add_cake_to_cache (baker, cake);
      g_task_return_pointer (task, cake, g_object_unref);
    }
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  CakeData *cake_data;
  GTask *task;

  cake_data = g_slice_new (CakeData);

  ...

  task = g_task_new (self, cancellable, callback, user_data);
  g_task_set_task_data (task, cake_data, (GDestroyNotify) cake_data_free);
  g_task_set_return_on_cancel (task, TRUE);
  g_task_run_in_thread (task, bake_cake_thread);
}

Cake *
baker_bake_cake_sync (Baker               *self,
                      guint                radius,
                      CakeFlavor           flavor,
                      CakeFrostingType     frosting,
                      const char          *message,
                      GCancellable        *cancellable,
                      GError             **error)
{
  CakeData *cake_data;
  GTask *task;
  Cake *cake;

  cake_data = g_slice_new (CakeData);

  ...

  task = g_task_new (self, cancellable, NULL, NULL);
  g_task_set_task_data (task, cake_data, (GDestroyNotify) cake_data_free);
  g_task_set_return_on_cancel (task, TRUE);
  g_task_run_in_thread_sync (task, bake_cake_thread);

  cake = g_task_propagate_pointer (task, error);
  g_object_unref (task);
  return cake;
}

Porting from class@Gio.SimpleAsyncResult

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

You can save task-specific data with method@Gio.Task.set_task_data, and retrieve it later with method@Gio.Task.get_task_data. This replaces the abuse of method@Gio.SimpleAsyncResult.set_op_res_gpointer for the same purpose with class@Gio.SimpleAsyncResult.
In addition to the task data, GTask also keeps track of the iface.AsyncResult.html#io-priority, class@Gio.Cancellable, and struct@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.
method@Gio.Task.return_error_if_cancelled provides simplified handling for cancellation. In addition, cancellation overrides any other GTask return value by default, like class@Gio.SimpleAsyncResult does when method@Gio.SimpleAsyncResult.set_check_cancellable is called. (You can use method@Gio.Task.set_check_cancellable to turn off that behavior.) On the other hand, method@Gio.Task.run_in_thread guarantees that it will always run your task_func, even if the task’s class@Gio.Cancellable is already cancelled before the task gets a chance to run; you can start your task_func with a method@Gio.Task.return_error_if_cancelled check if you need the old behavior.
The ‘return’ methods (eg, method@Gio.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 struct@GLib.MainContext, or delayed until the next iteration of the current struct@GLib.MainContext.)
The ‘finish’ functions for GTask based operations are generally much simpler than class@Gio.SimpleAsyncResult ones, normally consisting of only a single call to method@Gio.Task.propagate_pointer or the like. Since method@Gio.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 class@Gio.SimpleAsyncResult, it was common to call method@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 method@Gio.AsyncResult.legacy_propagate_error to do old-style class@Gio.SimpleAsyncResult error-returning behavior, and method@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 method@Gio.InputStream.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

Skipped during bindings generation

method get_source_tag: Return type gpointer is unsupported
method get_task_data: Return type gpointer is unsupported
method propagate_pointer: Return type gpointer is unsupported
parameter result: gpointer
parameter task_func: TaskThreadFunc
parameter task_func: TaskThreadFunc
parameter source_tag: gpointer
parameter task_data: gpointer
parameter source_tag: gpointer

Constructors

Task

constructor(sourceObject: <Error class: unknown class>? = null, cancellable: Cancellable? = null, callback: AsyncReadyCallback)

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

constructor(pointer: <Error class: unknown class><<Error class: unknown class>>)

Types

Companion

object Companion

Properties

completed

open val completed: Boolean

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

gioAsyncResultPointer

open override val gioAsyncResultPointer: <Error class: unknown class><<Error class: unknown class>>

gioTaskPointer

val gioTaskPointer: <Error class: unknown class><<Error class: unknown class>>

Functions

getCancellable

open fun getCancellable(): Cancellable?

Gets @task's #GCancellable

getCheckCancellable

open fun getCheckCancellable(): Boolean

Gets @task's check-cancellable flag. See g_task_set_check_cancellable() for more details.

getCompleted

open fun getCompleted(): Boolean

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.

getContext

open fun getContext(): <Error class: unknown class>

Gets the #GMainContext that @task will return its result in (that is, the context that was the g-main-context-push-thread-default at the point when @task was created).

getName

open fun getName(): String?

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

getPriority

open fun getPriority(): Int

Gets @task's priority

getReturnOnCancel

open fun getReturnOnCancel(): Boolean

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

getSourceObject

open fun getSourceObject(): <Error class: unknown class>?

Gets the source object from a iface@Gio.AsyncResult.

getSourceObject_

open fun getSourceObject_(): <Error class: unknown class>?

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

hadError

open fun hadError(): Boolean

Tests if @task resulted in an error.

legacyPropagateError

open fun legacyPropagateError(): <Error class: unknown class><Boolean>

If @res is a class@Gio.SimpleAsyncResult, this is equivalent to method@Gio.SimpleAsyncResult.propagate_error. Otherwise it returns FALSE.

propagateBoolean

open fun propagateBoolean(): <Error class: unknown class><Boolean>

Gets the result of @task as a #gboolean.

propagateInt

open fun propagateInt(): <Error class: unknown class><Long>

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

propagateValue

open fun propagateValue(value: <Error class: unknown class>): <Error class: unknown class><Boolean>

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.

returnBoolean

open fun returnBoolean(result: Boolean)

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

returnError

open fun returnError(error: <Error class: unknown class>)

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).

returnErrorIfCancelled

open fun returnErrorIfCancelled(): Boolean

Checks if @task's #GCancellable has been cancelled, and if so, sets

returnInt

open fun returnInt(result: Long)

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

returnNewErrorLiteral

open fun returnNewErrorLiteral(domain: <Error class: unknown class>, code: Int, message: String)

Sets @task’s result to a new type@GLib.Error created from @domain, @code,

returnValue

open fun returnValue(result: <Error class: unknown class>? = null)

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

setCheckCancellable

open fun setCheckCancellable(checkCancellable: Boolean)

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.

setName

open fun setName(name: String? = null)

Sets @task’s name, used in debugging and profiling. The name defaults to null.

setPriority

open fun setPriority(priority: Int)

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

setReturnOnCancel

open fun setReturnOnCancel(returnOnCancel: Boolean): Boolean

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().

setStaticName

open fun setStaticName(name: String? = null)

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