QFutureWatcher Class

 // 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);

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 cancellation is asynchronous. Use waitForFinished() after calling cancel() when you need synchronous cancellation.

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.

If future has already started, the watcher will initially emit signals that bring their listeners up to date about the future's state. The following signals will, if applicable, be emitted in the given order: started(), progressRangeChanged(), progressValueChanged(), progressTextChanged(), resultsReadyAt(), resultReadyAt(), paused(), canceled(), and finished(). Of these, resultsReadyAt() and resultReadyAt() may be emitted several times to cover all available results. progressValueChanged() and progressTextChanged() will only be emitted once for the latest available progress value and text.

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).