Gio.Task

const Gio = imports.gi.Gio;

let task = new Gio.Task();
  

A Gio.Task represents and manages a cancellable "task".

## Asynchronous operations

The most common usage of Gio.Task is as a Gio.AsyncResult, to manage data during an asynchronous operation. You call Gio.Task.new in the "start" method, followed by Gio.Task.prototype.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 Gio.Task.prototype.return_pointer or Gio.Task.prototype.return_error, which will save the value you give it and then invoke the task's callback function (waiting until the next iteration of the main loop first, if necessary). The caller will pass the Gio.Task back to the operation's finish function (as a Gio.AsyncResult), and you can use Gio.Task.prototype.propagate_pointer or the like to extract the return value.

Here is an example for using GTask as a GAsyncResult: |[<!-- language="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); // Gio.Task.prototype.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

Gio.Task also tries to simplify asynchronous operations that internally chain together several smaller asynchronous operations. Gio.Task.prototype.get_cancellable, Gio.Task.prototype.get_context, and Gio.Task.prototype.get_priority allow you to get back the task's Gio.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. Gio.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: |[<!-- language="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 (result, g_object_ref (cake), g_object_unref); g_object_unref (task); }

static void 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); }

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, G_CALLBACK (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 Gio.Task.run_in_thread to turn a synchronous operation into an asynchronous one, by running it in a thread which will then dispatch the result back to the caller's GLib.MainContext when it completes.

Running a task in a thread: |[<!-- language="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); }

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, Gio.Task.run_in_thread and Gio.Task.run_in_thread_sync can be used to turn an uncancellable operation into a cancellable one. If you call Gio.Task.prototype.set_return_on_cancel, passing true, then if the task's 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: |[<!-- language="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. // Gio.Task.prototype.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

Gio.Task's API attempts to be simpler than Gio.SimpleAsyncResult's in several ways: - You can save task-specific data with Gio.Task.prototype.set_task_data, and retrieve it later with Gio.Task.prototype.get_task_data. This replaces the abuse of Gio.SimpleAsyncResult.set_op_res_gpointer for the same purpose with Gio.SimpleAsyncResult. - In addition to the task data, Gio.Task also keeps track of the [priority][io-priority], Gio.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. - Gio.Task.prototype.return_error_if_cancelled provides simplified handling for cancellation. In addition, cancellation overrides any other Gio.Task return value by default, like Gio.SimpleAsyncResult does when Gio.SimpleAsyncResult.prototype.set_check_cancellable is called. (You can use Gio.Task.prototype.set_check_cancellable to turn off that behavior.) On the other hand, Gio.Task.run_in_thread guarantees that it will always run your `task_func`, even if the task's Gio.Cancellable is already cancelled before the task gets a chance to run; you can start your `task_func` with a Gio.Task.prototype.return_error_if_cancelled check if you need the old behavior. - The "return" methods (eg, Gio.Task.prototype.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. (Gio.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 Gio.Task-based operations are generally much simpler than Gio.SimpleAsyncResult ones, normally consisting of only a single call to Gio.Task.prototype.propagate_pointer or the like. Since Gio.Task.prototype.propagate_pointer "steals" the return value from the Gio.Task, it is not necessary to juggle pointers around to prevent it from being freed twice. - With Gio.SimpleAsyncResult, it was common to call Gio.SimpleAsyncResult.prototype.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 Gio.AsyncResult.prototype.legacy_propagate_error to do old-style Gio.SimpleAsyncResult error-returning behavior, and Gio.AsyncResult.prototype.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 Gio.InputStream.prototype.read_async).