QFutureWatcher Class

template <typename T> class QFutureWatcher

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

Header: #include <QFutureWatcher>
CMake: find_package(Qt6 COMPONENTS Core REQUIRED)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core
Inherits: QObject

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 isRunning() const
bool isStarted() const
bool isSuspended() const
bool isSuspending() 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 resume()
void setSuspended(bool suspend)
void suspend()
void toggleSuspended()

Signals

void canceled()
void finished()
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()
void suspended()
void suspending()

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(), isSuspending(), isSuspended(), waitForFinished(), result(), and resultAt(). The cancel(), setSuspended(), suspend(), resume(), and toggleSuspended() functions are slots in QFutureWatcher.

Status changes are reported via the started(), finished(), canceled(), suspending(), suspended(), 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 suspended. 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 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.

[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 setSuspended(false).

See also suspend().

[signal] void QFutureWatcher::resumed()

This signal is emitted when the watched future is resumed.

[slot, since 6.0] void QFutureWatcher::setSuspended(bool suspend)

If suspend is true, this function suspends the asynchronous computation represented by the future(). If the computation is already suspended, this function does nothing. QFutureWatcher will not immediately stop delivering progress and result ready signals when the future is suspended. At the moment of suspending there may still be computations that are in progress and cannot be stopped. Signals for such computations will still be delivered.

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

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

This function was introduced in Qt 6.0.

See also suspended(), suspend(), resume(), and toggleSuspended().

[signal] void QFutureWatcher::started()

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

[slot, since 6.0] void QFutureWatcher::suspend()

Suspends the asynchronous computation represented by this future. This is a convenience method that simply calls setSuspended(true).

This function was introduced in Qt 6.0.

See also resume().

[signal, since 6.0] void QFutureWatcher::suspended()

This signal is emitted when suspend() took effect, meaning that there are no more running computations. After receiving this signal no more result ready or progress reporting signals are expected.

This function was introduced in Qt 6.0.

See also setSuspended(), suspend(), and suspended().

[signal, since 6.0] void QFutureWatcher::suspending()

This signal is emitted when the state of the watched future is set to suspended.

Note: This signal only informs that suspension has been requested. It doesn't indicate that all background operations are stopped. Signals for computations that were in progress at the moment of suspending will still be delivered. To be informed when suspension actually took effect, use the suspended() signal.

This function was introduced in Qt 6.0.

See also setSuspended(), suspend(), and suspended().

[slot, since 6.0] void QFutureWatcher::toggleSuspended()

Toggles the suspended state of the asynchronous computation. In other words, if the computation is currently suspending or suspended, calling this function resumes it; if the computation is running, it is suspended. This is a convenience method for calling setSuspended(!(isSuspending() || isSuspended())).

This function was introduced in Qt 6.0.

See also setSuspended(), suspend(), 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::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.

[since 6.0] bool QFutureWatcher::isSuspended() const

Returns true if a suspension of the asynchronous computation has been requested, and it is in effect, meaning that no more results or progress changes are expected.

This function was introduced in Qt 6.0.

See also suspended(), setSuspended(), and isSuspending().

[since 6.0] bool QFutureWatcher::isSuspending() const

Returns true if the asynchronous computation has been suspended with the suspend() function, but the work is not yet suspended, and computation is still running. Returns false otherwise.

To check if suspension is actually in effect, use isSuspended() instead.

This function was introduced in Qt 6.0.

See also setSuspended(), toggleSuspended(), and isSuspended().

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().

template <typename U, typename> 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().

template <typename U, typename> 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), i.e. until isFinished() returns true.

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