Package org.gnome.gio

Class Cancellable

java.lang.Object

io.github.jwharm.javagi.base.ProxyInstance

org.gnome.gobject.TypeInstance

org.gnome.gobject.GObject

org.gnome.gio.Cancellable

All Implemented Interfaces:

Proxy

@Generated("io.github.jwharm.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.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static class	Cancellable.Builder<B extends Cancellable.Builder<B>>	Inner class implementing a builder pattern to construct a GObject with properties.
static class	Cancellable.CancellableClass
static interface	Cancellable.CancelledCallback	Functional interface declaration of the CancelledCallback callback.

Nested classes/interfaces inherited from class org.gnome.gobject.GObject

GObject.NotifyCallback, GObject.ObjectClass

Constructor Summary

Constructors

Constructor	Description
Cancellable()	Creates a new GCancellable object.
Cancellable(MemorySegment address)	Create a Cancellable proxy instance for the provided memory address.

Method Summary

Modifier and Type	Method	Description
protected Cancellable	asParent()	Returns this instance as if it were its parent type.
static Cancellable.Builder<? extends Cancellable.Builder>	builder()	A Cancellable.Builder object constructs a Cancellable with the specified properties.
void	cancel()	Will set this Cancellable to cancelled, and will emit the GCancellable::cancelled signal.
protected void	cancelled()
int	connect(Callback callback)	Convenience function to connect to the GCancellable::cancelled signal.
void	disconnect(int handlerId)	Disconnects a handler from a cancellable instance similar to g_signal_handler_disconnect().
void	emitCancelled()	Emits the "cancelled" signal.
static Cancellable	getCurrent()	Gets the top cancellable from the stack.
int	getFd()	Gets the file descriptor for a cancellable job.
static MemoryLayout	getMemoryLayout()	The memory layout of the native struct.
static Type	getType()	Get the GType of the Cancellable class
boolean	isCancelled()	Checks if a cancellable job has been cancelled.
boolean	makePollfd(PollFD pollfd)	Creates a GPollFD corresponding to this Cancellable; this can be passed to g_poll() and used to poll for cancellation.
SignalConnection<Cancellable.CancelledCallback>	onCancelled(Cancellable.CancelledCallback handler)	Emitted when the operation has been cancelled.
void	popCurrent()	Pops this Cancellable off the cancellable stack (verifying that this Cancellable is on the top of the stack).
void	pushCurrent()	Pushes this Cancellable onto the cancellable stack.
void	releaseFd()	Releases a resources previously allocated by g_cancellable_get_fd() or g_cancellable_make_pollfd().
void	reset()	Resets this Cancellable to its uncancelled state.
boolean	setErrorIfCancelled()	If the this Cancellable is cancelled, sets the error to notify that the operation was cancelled.
Source	sourceNew()	Creates a source that triggers if this Cancellable is cancelled and calls its callback of type GCancellableSourceFunc.

Methods inherited from class org.gnome.gobject.GObject

addToggleRef, addWeakPointer, bindProperty, bindProperty, bindProperty, bindPropertyFull, bindPropertyFull, bindPropertyWithClosures, bindPropertyWithClosures, compatControl, connect, connect, connect, constructed, disconnect, dispatchPropertiesChanged, dispose, dupData, dupQdata, emit, emitNotify, finalize_, forceFloating, freezeNotify, get, getData, getProperty, getProperty, getProperty, getQdata, getv, interfaceFindProperty, interfaceInstallProperty, interfaceListProperties, isFloating, newInstance, newInstance, newv, notify, notify, notifyByPspec, onNotify, ref, refSink, removeToggleRef, removeWeakPointer, replaceData, replaceQdata, runDispose, set, setData, setDataFull, setProperty, setProperty, setProperty, setQdata, setQdataFull, setv, stealData, stealQdata, takeRef, thawNotify, unref, watchClosure, weakRef, weakUnref, withProperties

Methods inherited from class org.gnome.gobject.TypeInstance

callParent, callParent, getPrivate, readGClass, writeGClass

Methods inherited from class io.github.jwharm.javagi.base.ProxyInstance

equals, handle, hashCode

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait

Constructor Details

Cancellable

public Cancellable(MemorySegment address)

Create a Cancellable proxy instance for the provided memory address.

Parameters:

address - the memory address of the native object

Cancellable

public Cancellable()

Creates a new GCancellable object.

Applications that want to start one or more operations that should be cancellable should create a GCancellable and pass it to the operations.

One GCancellable can be used in multiple consecutive operations or in multiple concurrent operations.

Method Details

getType

public static 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()

Returns this instance as if it were its parent type. This is mostly synonymous to the Java super keyword, but will set the native typeclass function pointers to the parent type. When overriding a native virtual method in Java, "chaining up" with super.methodName() doesn't work, because it invokes the overridden function pointer again. To chain up, call asParent().methodName(). This will call the native function pointer of this virtual method in the typeclass of the parent type.

Overrides:

asParent in class GObject

getCurrent

public static 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 this 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.

connect

public int connect(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 at most once, either directly at the time of the connect if this Cancellable is already cancelled, or when this Cancellable is cancelled in some thread.

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. g_cancellable_cancel().

Parameters:

callback - The GCallback to connect.

Returns:

The id of the signal handler or 0 if this Cancellable has already been cancelled.

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.

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 this 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 this 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().

Parameters:

pollfd - a pointer to a GPollFD

Returns:

true if pollfd was successfully initialized, false on failure to prepare the cancellable.

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.

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.

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.

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:

• Cancellable.CancelledCallback.run()

emitCancelled

public void emitCancelled()

Emits the "cancelled" signal. See onCancelled(org.gnome.gio.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().