Class Cancellable

java.lang.Object
org.javagi.base.ProxyInstance
org.gnome.gobject.TypeInstance
org.gnome.gobject.GObject
org.gnome.gio.Cancellable
All Implemented Interfaces:
Proxy
@Generated("org.javagi.JavaGI") public class Cancellable extends GObject

GCancellable allows operations to be cancelled.

GCancellable is a thread-safe operation cancellation stack used throughout GIO to allow for cancellation of synchronous and asynchronous operations.

  • Constructor Details

    • Cancellable

      public Cancellable(MemorySegment address)
      Create a Cancellable instance for the provided memory address.
      Parameters:
      address - the memory address of the native object

    • Cancellable

      public Cancellable()
      Create a new Cancellable.

  • Method Details

    • getType

      public static @Nullable Type getType()
      Get the GType of the Cancellable class.
      Returns:
      the GType

    • getMemoryLayout

      public static MemoryLayout getMemoryLayout()
      The memory layout of the native struct.
      Returns:
      the memory layout

    • asParent

      protected Cancellable asParent()
      Return this instance as if it were its parent type. Comparable to the Java super keyword, but ensures the parent typeclass is also used in native code.
      Overrides:
      asParent in class GObject
      Returns:
      the instance as if it were its parent type

    • getCurrent

      public static @Nullable Cancellable getCurrent()
      Gets the top cancellable from the stack.
      Returns:
      a GCancellable from the top of the stack, or null if the stack is empty.

    • cancel

      public void cancel()

      Will set this Cancellable to cancelled, and will emit the GCancellable::cancelled signal. (However, see the warning about race conditions in the documentation for that signal if you are planning to connect to it.)

      This function is thread-safe. In other words, you can safely call it from a thread other than the one running the operation that was passed the cancellable.

      If this Cancellable is null, this function returns immediately for convenience.

      The convention within GIO is that cancelling an asynchronous operation causes it to complete asynchronously. That is, if you cancel the operation from the same thread in which it is running, then the operation's GAsyncReadyCallback will not be invoked until the application returns to the main loop.

      It is safe (although useless, since it will be a no-op) to call this function from a Gio.Cancellable::cancelled signal handler.

    • connect

      public int connect(@Nullable Callback callback)

      Convenience function to connect to the GCancellable::cancelled signal. Also handles the race condition that may happen if the cancellable is cancelled right before connecting.

      callback is called exactly once each time this Cancellable is cancelled, either directly at the time of the connect if this Cancellable is already cancelled, or when this Cancellable is cancelled in some thread. In case the cancellable is reset via reset() then the callback can be called again if the this Cancellable is cancelled and if it had not been previously cancelled at the time connect(Callback) was called (e.g. if the connection actually took place, returning a non-zero value).

      dataDestroyFunc will be called when the handler is disconnected, or immediately if the cancellable is already cancelled.

      See GCancellable::cancelled for details on how to use this.

      Since GLib 2.40, the lock protecting this Cancellable is not held when callback is invoked. This lifts a restriction in place for earlier GLib versions which now makes it easier to write cleanup code that unconditionally invokes e.g. cancel(). Note that since 2.82 GLib still holds a lock during the callback but it’s designed in a way that most of the Cancellable methods can be called, including cancel() or GObject#unref.

      There are still some methods that will deadlock (by design) when called from the Gio.Cancellable::cancelled callbacks:

      Parameters:
      callback - The GCallback to connect.
      Returns:
      The id of the signal handler or 0 if this Cancellable has already been cancelled.
      Since:
      2.22

    • disconnect

      public void disconnect(int handlerId)

      Disconnects a handler from a cancellable instance similar to g_signal_handler_disconnect(). Additionally, in the event that a signal handler is currently running, this call will block until the handler has finished. Calling this function from a GCancellable::cancelled signal handler will therefore result in a deadlock.

      This avoids a race condition where a thread cancels at the same time as the cancellable operation is finished and the signal handler is removed. See GCancellable::cancelled for details on how to use this.

      If this Cancellable is null or handlerId is 0 this function does nothing.

      Parameters:
      handlerId - Handler id of the handler to be disconnected, or 0.
      Since:
      2.22

    • getFd

      public int getFd()

      Gets the file descriptor for a cancellable job. This can be used to implement cancellable operations on Unix systems. The returned fd will turn readable when this Cancellable is cancelled.

      You are not supposed to read from the fd yourself, just check for readable status. Reading to unset the readable status is done with g_cancellable_reset().

      After a successful return from this function, you should use g_cancellable_release_fd() to free up resources allocated for the returned file descriptor.

      See also g_cancellable_make_pollfd().

      Returns:
      A valid file descriptor. -1 if the file descriptor is not supported, or on errors.

    • isCancelled

      public boolean isCancelled()
      Checks if a cancellable job has been cancelled.
      Returns:
      true if this Cancellable is cancelled, FALSE if called with null or if item is not cancelled.

    • makePollfd

      public boolean makePollfd(PollFD pollfd)

      Creates a GPollFD corresponding to cancellable; this can be passed to g_poll() and used to poll for cancellation. This is useful both for unix systems without a native poll and for portability to windows.

      When this function returns true, you should use g_cancellable_release_fd() to free up resources allocated for the pollfd. After a false return, do not call g_cancellable_release_fd().

      If this function returns false, either no this Cancellable was given or resource limits prevent this function from allocating the necessary structures for polling. (On Linux, you will likely have reached the maximum number of file descriptors.) The suggested way to handle these cases is to ignore the cancellable.

      You are not supposed to read from the fd yourself, just check for readable status. Reading to unset the readable status is done with g_cancellable_reset().

      Note that in the event that a Gio.Cancellable::cancelled signal handler is currently running, this call will block until the handler has finished. Calling this function from a signal handler will therefore result in a deadlock.

      Parameters:
      pollfd - a pointer to a GPollFD
      Returns:
      true if pollfd was successfully initialized, false on failure to prepare the cancellable.
      Since:
      2.22

    • popCurrent

      public void popCurrent()
      Pops this Cancellable off the cancellable stack (verifying that this Cancellable is on the top of the stack).

    • pushCurrent

      public void pushCurrent()

      Pushes this Cancellable onto the cancellable stack. The current cancellable can then be received using g_cancellable_get_current().

      This is useful when implementing cancellable operations in code that does not allow you to pass down the cancellable object.

      This is typically called automatically by e.g. GFile operations, so you rarely have to call this yourself.

    • releaseFd

      public void releaseFd()

      Releases a resources previously allocated by g_cancellable_get_fd() or g_cancellable_make_pollfd().

      For compatibility reasons with older releases, calling this function is not strictly required, the resources will be automatically freed when the this Cancellable is finalized. However, the this Cancellable will block scarce file descriptors until it is finalized if this function is not called. This can cause the application to run out of file descriptors when many GCancellables are used at the same time.

      Note that in the event that a Gio.Cancellable::cancelled signal handler is currently running, this call will block until the handler has finished. Calling this function from a signal handler will therefore result in a deadlock.

      Since:
      2.22

    • reset

      public void reset()

      Resets this Cancellable to its uncancelled state.

      If cancellable is currently in use by any cancellable operation then the behavior of this function is undefined.

      Note that it is generally not a good idea to reuse an existing cancellable for more operations after it has been cancelled once, as this function might tempt you to do. The recommended practice is to drop the reference to a cancellable after cancelling it, and let it die with the outstanding async operations. You should create a fresh cancellable for further async operations.

      In the event that a Gio.Cancellable::cancelled signal handler is currently running, this call will block until the handler has finished. Calling this function from a signal handler will therefore result in a deadlock.

    • setErrorIfCancelled

      public boolean setErrorIfCancelled() throws GErrorException
      If the this Cancellable is cancelled, sets the error to notify that the operation was cancelled.
      Returns:
      true if this Cancellable was cancelled, false if it was not
      Throws:
      GErrorException - see GError

    • sourceNew

      public Source sourceNew()

      Creates a source that triggers if this Cancellable is cancelled and calls its callback of type GCancellableSourceFunc. This is primarily useful for attaching to another (non-cancellable) source with g_source_add_child_source() to add cancellability to it.

      For convenience, you can call this with a null GCancellable, in which case the source will never trigger.

      The new GSource will hold a reference to the GCancellable.

      Returns:
      the new GSource.
      Since:
      2.28

    • cancelled

      protected void cancelled()

    • onCancelled

      public SignalConnection<Cancellable.CancelledCallback> onCancelled(Cancellable.CancelledCallback handler)

      Emitted when the operation has been cancelled.

      Can be used by implementations of cancellable operations. If the operation is cancelled from another thread, the signal will be emitted in the thread that cancelled the operation, not the thread that is running the operation.

      Note that disconnecting from this signal (or any signal) in a multi-threaded program is prone to race conditions. For instance it is possible that a signal handler may be invoked even after a call to g_signal_handler_disconnect() for that handler has already returned.

      There is also a problem when cancellation happens right before connecting to the signal. If this happens the signal will unexpectedly not be emitted, and checking before connecting to the signal leaves a race condition where this is still happening.

      In order to make it safe and easy to connect handlers there are two helper functions: g_cancellable_connect() and g_cancellable_disconnect() which protect against problems like this.

      An example of how to us this:

          // Make sure we don't do unnecessary work if already cancelled
    if (g_cancellable_set_error_if_cancelled (cancellable, error))
      return;

    // Set up all the data needed to be able to handle cancellation
    // of the operation
    my_data = my_data_new (...);

    id = 0;
    if (cancellable)
      id = g_cancellable_connect (cancellable,
    			      G_CALLBACK (cancelled_handler)
    			      data, NULL);

    // cancellable operation here...

    g_cancellable_disconnect (cancellable, id);

    // cancelled_handler is never called after this, it is now safe
    // to free the data
    my_data_free (my_data);

      Note that the cancelled signal is emitted in the thread that the user cancelled from, which may be the main thread. So, the cancellable signal should not do something that can block.

      Parameters:
      handler - the signal handler
      Returns:
      a signal handler ID to keep track of the signal connection
      See Also:

    • emitCancelled

      public void emitCancelled()
      Emits the "cancelled" signal. See onCancelled(Cancellable.CancelledCallback).

    • builder

      public static Cancellable.Builder<? extends Cancellable.Builder> builder()
      A Cancellable.Builder object constructs a Cancellable with the specified properties. Use the various set...() methods to set properties, and finish construction with Cancellable.Builder.build().
      Returns:
      the builder object