QFutureWatcher Class

template <typename T> class QFutureWatcher

The QFutureWatcher class allows monitoring a QFuture using signals and slots. More...

Header:	#include <QFutureWatcher>
qmake:	QT += core
Since:	Qt 4.4
Inherits:	QObject

This class was introduced in Qt 4.4.

• List of all members, including inherited members

Note: All functions in this class are reentrant.

Public Functions

	QFutureWatcher(QObject *parent = nullptr)
virtual	~QFutureWatcher()
QFuture<T>	future() const
bool	isCanceled() const
bool	isFinished() const
bool	isPaused() const
bool	isRunning() const
bool	isStarted() const
int	progressMaximum() const
int	progressMinimum() const
QString	progressText() const
int	progressValue() const
T	result() const
T	resultAt(int index) const
void	setFuture(const QFuture<T> &future)
void	setPendingResultsLimit(int limit)
void	waitForFinished()

Public Slots

void	cancel()
void	pause()
void	resume()
void	setPaused(bool paused)
void	togglePaused()

Signals

void	canceled()
void	finished()
void	paused()
void	progressRangeChanged(int minimum, int maximum)
void	progressTextChanged(const QString &progressText)
void	progressValueChanged(int progressValue)
void	resultReadyAt(int index)
void	resultsReadyAt(int beginIndex, int endIndex)
void	resumed()
void	started()

Detailed Description

QFutureWatcher provides information and notifications about a QFuture. Use the setFuture() function to start watching a particular QFuture. The future() function returns the future set with setFuture().

For convenience, several of QFuture's functions are also available in QFutureWatcher: progressValue(), progressMinimum(), progressMaximum(), progressText(), isStarted(), isFinished(), isRunning(), isCanceled(), isPaused(), waitForFinished(), result(), and resultAt(). The cancel(), setPaused(), pause(), resume(), and togglePaused() functions are slots in QFutureWatcher.

Status changes are reported via the started(), finished(), canceled(), paused(), resumed(), resultReadyAt(), and resultsReadyAt() signals. Progress information is provided from the progressRangeChanged(), void progressValueChanged(), and progressTextChanged() signals.

Throttling control is provided by the setPendingResultsLimit() function. When the number of pending resultReadyAt() or resultsReadyAt() signals exceeds the limit, the computation represented by the future will be throttled automatically. The computation will resume once the number of pending signals drops below the limit.

Example: Starting a computation and getting a slot callback when it's finished:

// Instantiate the objects and connect to the finished signal.
MyClass myObject;
QFutureWatcher<int> watcher;
connect(&watcher, &QFutureWatcher<int>::finished, &myObject, &MyClass::handleFinished);

// Start the computation.
QFuture<int> future = QtConcurrent::run(...);
watcher.setFuture(future);

Be aware that not all running asynchronous computations can be canceled or paused. For example, the future returned by QtConcurrent::run() cannot be canceled; but the future returned by QtConcurrent::mappedReduced() can.

QFutureWatcher<void> is specialized to not contain any of the result fetching functions. Any QFuture<T> can be watched by a QFutureWatcher<void> as well. This is useful if only status or progress information is needed; not the actual result data.

See also QFuture and Qt Concurrent.

Member Function Documentation

QFutureWatcher::QFutureWatcher(QObject *parent = nullptr)

Constructs a new QFutureWatcher with the given parent. Until a future is set with setFuture(), the functions isStarted(), isCanceled(), and isFinished() return true.

[slot] void QFutureWatcher::cancel()

Cancels the asynchronous computation represented by the future(). Note that the cancelation is asynchronous. Use waitForFinished() after calling cancel() when you need synchronous cancelation.

Currently available results may still be accessed on a canceled QFuture, but new results will not become available after calling this function. Also, this QFutureWatcher will not deliver progress and result ready signals once canceled. This includes the progressValueChanged(), progressRangeChanged(), progressTextChanged(), resultReadyAt(), and resultsReadyAt() signals.

Be aware that not all running asynchronous computations can be canceled. For example, the QFuture returned by QtConcurrent::run() cannot be canceled; but the QFuture returned by QtConcurrent::mappedReduced() can.

[signal] void QFutureWatcher::canceled()

This signal is emitted if the watched future is canceled.

[signal] void QFutureWatcher::finished()

This signal is emitted when the watched future finishes.

[slot] void QFutureWatcher::pause()

Pauses the asynchronous computation represented by the future(). This is a convenience method that simply calls setPaused(true).

See also resume().

[signal] void QFutureWatcher::paused()

This signal is emitted when the watched future is paused.

See also setPaused().

[signal] void QFutureWatcher::progressRangeChanged(int minimum, int maximum)

The progress range for the watched future has changed to minimum and maximum

[signal] void QFutureWatcher::progressTextChanged(const QString &progressText)

This signal is emitted when the watched future reports textual progress information, progressText.

[signal] void QFutureWatcher::progressValueChanged(int progressValue)

This signal is emitted when the watched future reports progress, progressValue gives the current progress. In order to avoid overloading the GUI event loop, QFutureWatcher limits the progress signal emission rate. This means that listeners connected to this slot might not get all progress reports the future makes. The last progress update (where progressValue equals the maximum value) will always be delivered.

[signal] void QFutureWatcher::resultReadyAt(int index)

This signal is emitted when the watched future reports a ready result at index. If the future reports multiple results, the index will indicate which one it is. Results can be reported out-of-order. To get the result, call resultAt(index);

[signal] void QFutureWatcher::resultsReadyAt(int beginIndex, int endIndex)

This signal is emitted when the watched future reports ready results. The results are indexed from beginIndex to endIndex.

[slot] void QFutureWatcher::resume()

Resumes the asynchronous computation represented by the future(). This is a convenience method that simply calls setPaused(false).

See also pause().

[signal] void QFutureWatcher::resumed()

This signal is emitted when the watched future is resumed.

[slot] void QFutureWatcher::setPaused(bool paused)

If paused is true, this function pauses the asynchronous computation represented by the future(). If the computation is already paused, this function does nothing. This QFutureWatcher will stop delivering progress and result ready signals while the future is paused. Signal delivery will continue once the computation is resumed.

If paused is false, this function resumes the asynchronous computation. If the computation was not previously paused, this function does nothing.

Be aware that not all computations can be paused. For example, the QFuture returned by QtConcurrent::run() cannot be paused; but the QFuture returned by QtConcurrent::mappedReduced() can.

See also paused(), pause(), resume(), and togglePaused().

[signal] void QFutureWatcher::started()

This signal is emitted when this QFutureWatcher starts watching the future set with setFuture().

[slot] void QFutureWatcher::togglePaused()

Toggles the paused state of the asynchronous computation. In other words, if the computation is currently paused, calling this function resumes it; if the computation is running, it becomes paused. This is a convenience method for calling setPaused(!isPaused()).

See also setPaused(), pause(), and resume().

[virtual] QFutureWatcher::~QFutureWatcher()

Destroys the QFutureWatcher.

QFuture<T> QFutureWatcher::future() const

Returns the watched future.

See also setFuture().

bool QFutureWatcher::isCanceled() const

Returns true if the asynchronous computation has been canceled with the cancel() function, or if no future has been set; otherwise returns false.

Be aware that the computation may still be running even though this function returns true. See cancel() for more details.

bool QFutureWatcher::isFinished() const

Returns true if the asynchronous computation represented by the future() has finished, or if no future has been set; otherwise returns false.

bool QFutureWatcher::isPaused() const

Returns true if the asynchronous computation has been paused with the pause() function; otherwise returns false.

Be aware that the computation may still be running even though this function returns true. See setPaused() for more details.

See also setPaused() and togglePaused().

bool QFutureWatcher::isRunning() const

Returns true if the asynchronous computation represented by the future() is currently running; otherwise returns false.

bool QFutureWatcher::isStarted() const

Returns true if the asynchronous computation represented by the future() has been started, or if no future has been set; otherwise returns false.

int QFutureWatcher::progressMaximum() const

Returns the maximum progressValue().

See also progressValue() and progressMinimum().

int QFutureWatcher::progressMinimum() const

Returns the minimum progressValue().

See also progressValue() and progressMaximum().

QString QFutureWatcher::progressText() const

Returns the (optional) textual representation of the progress as reported by the asynchronous computation.

Be aware that not all computations provide a textual representation of the progress, and as such, this function may return an empty string.

int QFutureWatcher::progressValue() const

Returns the current progress value, which is between the progressMinimum() and progressMaximum().

See also progressMinimum() and progressMaximum().

T QFutureWatcher::result() const

Returns the first result in the future(). If the result is not immediately available, this function will block and wait for the result to become available. This is a convenience method for calling resultAt(0).

See also resultAt().

T QFutureWatcher::resultAt(int index) const

Returns the result at index in the future(). If the result is not immediately available, this function will block and wait for the result to become available.

See also result().

void QFutureWatcher::setFuture(const QFuture<T> &future)

Starts watching the given future.

One of the signals might be emitted for the current state of the future. For example, if the future is already stopped, the finished signal will be emitted.

To avoid a race condition, it is important to call this function after doing the connections.

See also future().

void QFutureWatcher::setPendingResultsLimit(int limit)

The setPendingResultsLimit() provides throttling control. When the number of pending resultReadyAt() or resultsReadyAt() signals exceeds the limit, the computation represented by the future will be throttled automatically. The computation will resume once the number of pending signals drops below the limit.

void QFutureWatcher::waitForFinished()

Waits for the asynchronous computation to finish (including cancel()ed computations).