GLib.Task – gio-2.0 Reference Manual

Task

Object Hierarchy:

Description:

[ CCode ( type_id = "g_task_get_type ()" ) ]
public class Task : Object, AsyncResult

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 Task in the "start" method, followed by 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 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 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 propagate_pointer or the like to extract the return value.

Using Task requires the thread-default MainContext from when the Task was constructed to be running at least until the task has completed and its data has been freed.

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

Task also tries to simplify asynchronous operations that internally chain together several smaller asynchronous operations. get_cancellable, get_context, and get_priority allow you to get back the task's Cancellable, MainContext , and I/O priority when starting a new subtask, so you don't have to keep track of them yourself. attach_source simplifies the case of waiting for a source to fire (automatically using the correct MainContext and priority).

Here is an example for chained asynchronous operations:

		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 where the Task 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, run_in_thread and 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:

		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 SimpleAsyncResult 's in several ways: - You can save task-specific data with set_task_data, and retrieve it later with get_task_data. This replaces the abuse of set_op_res_gpointer for the same purpose with SimpleAsyncResult. - In addition to the task data, Task also keeps track of the priority, Cancellable, and MainContext associated with the task, so tasks that consist of a chain of simpler asynchronous operations will have easy access to those values when starting each sub-task. - return_error_if_cancelled provides simplified handling for cancellation. In addition, cancellation overrides any other Task return value by default, like SimpleAsyncResult does when 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) 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 MainContext, or delayed until the next iteration of the current MainContext.) - The "finish" functions for Task based operations are generally much simpler than SimpleAsyncResult ones, normally consisting of only a single call to propagate_pointer or the like. Since 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 SimpleAsyncResult, it was common to call 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 legacy_propagate_error to do old-style SimpleAsyncResult error-returning behavior, and 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 read_async).

Namespace: GLib

Package: gio-2.0

Content:

Properties:

public bool completed { get; }
Whether the task has completed, meaning its callback (if set) has been invoked.

Static methods:

public static bool is_valid (AsyncResult result, Object? source_object)
Checks that result is a Task, and that source_object is its source object (or that source_object is null and result has no source object).
public static void report_error (Object? source_object, TaskReadyCallback callback, void* source_tag, owned Error error)
Creates a Task and then immediately calls return_error on it.
public static void report_new_error (Object? source_object, TaskReadyCallback callback, void* source_tag, Quark domain, int code, string format, ...)
Creates a Task and then immediately calls return_new_error on it.

Creation methods:

public Task (Object? source_object, Cancellable? cancellable, TaskReadyCallback callback)
Creates a Task acting on source_object, which will eventually be used to invoke callback in the current thread-default main context .

Methods:

public void attach_source (Source source, TaskSourceFunc callback)
A utility function for dealing with async operations where you need to wait for a SocketSource to trigger.
public unowned Cancellable get_cancellable ()
Gets this's Cancellable
public bool get_check_cancellable ()
Gets this's check-cancellable flag.
public bool get_completed ()
Gets the value of completed.
public unowned MainContext get_context ()
Gets the MainContext that this will return its result in (that is, the context that was the thread-default main context at the point when this was created).
public unowned string? get_name ()
Gets this’s name.
public int get_priority ()
Gets this's priority
public bool get_return_on_cancel ()
Gets this's return-on-cancel flag.
public void* get_source_tag ()
Gets this's source tag.
public void* get_task_data ()
Gets this's `task_data`.
public unowned Object? get_unowned_source_object ()
Gets the source object from this.
public bool had_error ()
Tests if this resulted in an error.
public bool propagate_boolean () throws Error
Gets the result of this as a bool.
public ssize_t propagate_int () throws Error
Gets the result of this as an integer (ssize_t).
public void* propagate_pointer () throws Error
Gets the result of this as a pointer, and transfers ownership of that value to the caller.
public bool propagate_value (out Value value) throws Error
Gets the result of this as a Value, and transfers ownership of that value to the caller.
public void return_boolean (bool result)
Sets this's result to result and completes the task (see return_pointer for more discussion of exactly what this means).
public void return_error (owned Error error)
Sets this's result to error (which this assumes ownership of) and completes the task (see return_pointer for more discussion of exactly what this means).
public bool return_error_if_cancelled ()
Checks if this's Cancellable has been cancelled, and if so, sets this 's error accordingly and completes the task (see return_pointer for more discussion of exactly what this means).
public void return_int (ssize_t result)
Sets this's result to result and completes the task (see return_pointer for more discussion of exactly what this means).
public void return_new_error (Quark domain, int code, string format, ...)
Sets this's result to a new Error created from domain, code, format, and the remaining arguments, and completes the task (see return_pointer for more discussion of exactly what this means).
public void return_pointer (void* result, DestroyNotify? result_destroy)
Sets this's result to result and completes the task.
public void return_value (Value? result)
Sets this's result to result (by copying it) and completes the task.
public void run_in_thread (TaskThreadFunc task_func)
Runs task_func in another thread.
public void run_in_thread_sync (TaskThreadFunc task_func)
Runs task_func in another thread, and waits for it to return or be cancelled.
public void set_check_cancellable (bool check_cancellable)
Sets or clears this's check-cancellable flag.
public void set_name (string? name)
Sets this’s name, used in debugging and profiling.
public void set_priority (int priority)
Sets this's priority.
public bool set_return_on_cancel (bool return_on_cancel)
Sets or clears this's return-on-cancel flag.
public void set_source_tag (void* source_tag)
Sets this's source tag.
public void set_task_data (void* task_data, DestroyNotify? task_data_destroy)
Sets this's task data (freeing the existing task data, if any).

Inherited Members:

All known members inherited from class GLib.Object

@get
@new
@ref
@set
add_toggle_ref
add_weak_pointer
bind_property
connect
constructed
disconnect
dispose
dup_data
dup_qdata
force_floating
freeze_notify
get_class
get_data

get_property
get_qdata
get_type
getv
interface_find_property
interface_install_property
interface_list_properties
is_floating
new_valist
new_with_properties
newv
notify
notify_property
ref_count
ref_sink
remove_toggle_ref
remove_weak_pointer

replace_data
replace_qdata
set_data
set_data_full
set_property
set_qdata
set_qdata_full
set_valist
setv
steal_data
steal_qdata
thaw_notify
unref
watch_closure
weak_ref
weak_unref

All known members inherited from interface GLib.AsyncResult

is_tagged

legacy_propagate_error

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