class Gio::Task

• Gio::Task
• GObject::Object
• Reference
• Object

Overview

A GTask represents and manages a cancellable ‘task’.

Asynchronous operations

The most common usage of GTask 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#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#return_pointer or 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 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 Gio::AsyncResult), and you can use Gio::Task#propagate_pointer or the like to extract the return value.

Using GTask requires the thread-default 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 Gio::AsyncResult:

WARNING ⚠️ 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

GTask also tries to simplify asynchronous operations that internally chain together several smaller asynchronous operations. Gio::Task#cancellable, Gio::Task#context, and Gio::Task#priority allow you to get back the task’s Gio::Cancellable, GLib::MainContext, and I/O 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:

WARNING ⚠️ 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 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 GLib::MainContext#push_thread_default) where the GTask was created.

Running a task in a thread:

WARNING ⚠️ 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, 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#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:

WARNING ⚠️ 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 Gio::SimpleAsyncResult

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

• You can save task-specific data with Gio::Task#task_data=, and retrieve it later with Gio::Task#task_data. This replaces the abuse of Gio::SimpleAsyncResult#op_res_gpointer= for the same purpose with Gio::SimpleAsyncResult.
• In addition to the task data, GTask also keeps track of the 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#return_error_if_cancelled provides simplified handling for cancellation. In addition, cancellation overrides any other GTask return value by default, like Gio::SimpleAsyncResult does when Gio::SimpleAsyncResult#check_cancellable= is called. (You can use Gio::Task#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#return_error_if_cancelled check if you need the old behavior.
• The ‘return’ methods (eg, 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 GLib::MainContext, or delayed until the next iteration of the current GLib::MainContext.)
• The ‘finish’ functions for GTask based operations are generally much simpler than Gio::SimpleAsyncResult ones, normally consisting of only a single call to Gio::Task#propagate_pointer or the like. Since 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 Gio::SimpleAsyncResult, it was common to call 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 Gio::AsyncResult#legacy_propagate_error to do old-style Gio::SimpleAsyncResult error-returning behavior, and 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 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

Included Modules

• Gio::AsyncResult

Defined in:

Constructors

• .new

  Initialize a new Task.

• .new(*, completed : Bool | Nil = nil)
• .new(source_object : GObject::Object | Nil, cancellable : Gio::Cancellable | Nil, &callback : Gio::AsyncReadyCallback) : self

  Creates a #GTask acting on source_object, which will eventually be used to invoke callback in the current thread-default main context (see GLib::MainContext#push_thread_default).

Class Method Summary

• .g_type : UInt64

  Returns the type id (GType) registered in GLib type system.

• .is_valid(result : Gio::AsyncResult, source_object : GObject::Object | Nil) : Bool

  Checks that result is a #GTask, and that source_object is its source object (or that source_object is nil and result has no source object).

• .report_error(source_object : GObject::Object | Nil, callback : Gio::AsyncReadyCallback | Nil, callback_data : Pointer(Void) | Nil, source_tag : Pointer(Void) | Nil, error : GLib::Error) : Nil

  Creates a #GTask and then immediately calls g_task_return_error() on it.

Instance Method Summary

• #==(other : self)

  Returns true if this reference is the same as other.

• #cancellable : Gio::Cancellable | Nil

  Gets task's #GCancellable

• #check_cancellable : Bool

  Gets task's check-cancellable flag.

• #check_cancellable=(check_cancellable : Bool) : Nil

  Sets or clears task's check-cancellable flag.

• #completed : Bool

  Gets the value of #GTask:completed.

• #completed? : Bool
• #context : GLib::MainContext

  Gets the #GMainContext that task will return its result in (that is, the context that was the thread-default main context (see GLib::MainContext#push_thread_default) at the point when task was created).

• #had_error : Bool

  Tests if task resulted in an error.

• #hash(hasher)

  See Object#hash(hasher)

• #name : String | Nil

  Gets task’s name.

• #name=(name : String | Nil) : Nil

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

• #priority : Int32

  Gets task's priority

• #priority=(priority : Int32) : Nil

  Sets task's priority.

• #propagate_boolean : Bool

  Gets the result of task as a #gboolean.

• #propagate_int : Int64

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

• #propagate_pointer : Pointer(Void) | Nil

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

• #propagate_value : GObject::Value

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

• #return_boolean(result : Bool) : Nil

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

• #return_error(error : GLib::Error) : Nil

  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).

• #return_error_if_cancelled : Bool

  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).

• #return_int(result : Int64) : Nil

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

• #return_new_error_literal(domain : UInt32, code : Int32, message : String) : Nil

  Sets task’s result to a new GLib::Error created from domain, code, message and completes the task.

• #return_on_cancel : Bool

  Gets task's return-on-cancel flag.

• #return_on_cancel=(return_on_cancel : Bool) : Bool

  Sets or clears task's return-on-cancel flag.

• #return_pointer(result : Pointer(Void) | Nil, result_destroy : GLib::DestroyNotify | Nil) : Nil

  Sets task's result to result and completes the task.

• #return_value(result : _ | Nil) : Nil

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

• #run_in_thread(task_func : Gio::TaskThreadFunc) : Nil

  Runs task_func in another thread.

• #run_in_thread_sync(task_func : Gio::TaskThreadFunc) : Nil

  Runs task_func in another thread, and waits for it to return or be cancelled.

• #set_task_data(task_data : Pointer(Void) | Nil, task_data_destroy : GLib::DestroyNotify | Nil) : Nil

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

• #source_object : GObject::Object | Nil

  Gets the source object from task.

• #source_tag : Pointer(Void) | Nil

  Gets task's source tag.

• #source_tag=(source_tag : Pointer(Void) | Nil) : Nil

  Sets task's source tag.

• #static_name=(name : String | Nil) : Nil

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

• #task_data : Pointer(Void) | Nil

  Gets task's #task_data.

Constructor Detail

Class Method Detail

Instance Method Detail