QFuture Class
template <typename T> class QFutureThe QFuture class represents the result of an asynchronous computation. More...
| Header: | #include <QFuture> |
| qmake: | QT += core |
| Since: | Qt 4.4 |
This class was introduced in Qt 4.4.
Note: All functions in this class are thread-safe with the following exceptions:
Public Types
| class | const_iterator |
| typedef | ConstIterator |
Public Functions
| QFuture(const QFuture<T> &other) | |
| QFuture() | |
| QFuture<T> & | operator=(const QFuture<T> &other) |
| ~QFuture() | |
| const_iterator | begin() const |
| void | cancel() |
| const_iterator | constBegin() const |
| const_iterator | constEnd() const |
| const_iterator | end() const |
| bool | isCanceled() const |
| bool | isFinished() const |
| bool | isPaused() const |
| bool | isResultReadyAt(int index) const |
| bool | isRunning() const |
| bool | isStarted() const |
| void | pause() |
| int | progressMaximum() const |
| int | progressMinimum() const |
| QString | progressText() const |
| int | progressValue() const |
| T | result() const |
| T | resultAt(int index) const |
| int | resultCount() const |
| QList<T> | results() const |
| void | resume() |
| void | setPaused(bool paused) |
| void | togglePaused() |
| void | waitForFinished() |
| T | operator T() const |
| bool | operator!=(const QFuture<T> &other) const |
| bool | operator==(const QFuture<T> &other) const |
Detailed Description
To start a computation, use one of the APIs in the Qt Concurrent framework.
QFuture allows threads to be synchronized against one or more results which will be ready at a later point in time. The result can be of any type that has a default constructor and a copy constructor. If a result is not available at the time of calling the result(), resultAt(), or results() functions, QFuture will wait until the result becomes available. You can use the isResultReadyAt() function to determine if a result is ready or not. For QFuture objects that report more than one result, the resultCount() function returns the number of continuous results. This means that it is always safe to iterate through the results from 0 to resultCount().
QFuture provides a Java-style iterator (QFutureIterator) and an STL-style iterator (QFuture::const_iterator). Using these iterators is another way to access results in the future.
QFuture also offers ways to interact with a running computation. For instance, the computation can be canceled with the cancel() function. To pause the computation, use the setPaused() function or one of the pause(), resume(), or togglePaused() convenience functions. 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.
Progress information is provided by the progressValue(), progressMinimum(), progressMaximum(), and progressText() functions. The waitForFinished() function causes the calling thread to block and wait for the computation to finish, ensuring that all results are available.
The state of the computation represented by a QFuture can be queried using the isCanceled(), isStarted(), isFinished(), isRunning(), or isPaused() functions.
QFuture is a lightweight reference counted class that can be passed by value.
QFuture<void> is specialized to not contain any of the result fetching functions. Any QFuture<T> can be assigned or copied into a QFuture<void> as well. This is useful if only status or progress information is needed - not the actual result data.
To interact with running tasks using signals and slots, use QFutureWatcher.
Note: Some APIs (see QtConcurrent::run() allow scheduling the computation to a specific thread pool. However, QFuture implements a work-stealing algorithm to prevent deadlocks and optimize thread usage. As a result, computations can be executed directly in the thread which requests the QFuture's result.
See also QFutureWatcher and Qt Concurrent.
Member Type Documentation
typedef QFuture::ConstIterator
Qt-style synonym for QFuture::const_iterator.
Member Function Documentation
QFuture::QFuture(const QFuture<T> &other)
Constructs a copy of other.
See also operator=().
QFuture::QFuture()
Constructs an empty, canceled future.
QFuture<T> &QFuture::operator=(const QFuture<T> &other)
Assigns other to this future and returns a reference to this future.
QFuture::~QFuture()
Destroys the future.
Note that this neither waits nor cancels the asynchronous computation. Use waitForFinished() or QFutureSynchronizer when you need to ensure that the computation is completed before the future is destroyed.
const_iterator QFuture::begin() const
Returns a const STL-style iterator pointing to the first result in the future.
See also constBegin() and end().
void QFuture::cancel()
Cancels the asynchronous computation represented by this future. Note that the cancellation is asynchronous. Use waitForFinished() after calling cancel() when you need synchronous cancellation.
Results currently available may still be accessed on a canceled future, but new results will not become available after calling this function. Any QFutureWatcher object that is watching this future will not deliver progress and result ready signals on a canceled future.
Be aware that not all running asynchronous computations can be canceled. For example, the future returned by QtConcurrent::run() cannot be canceled; but the future returned by QtConcurrent::mappedReduced() can.
const_iterator QFuture::constBegin() const
Returns a const STL-style iterator pointing to the first result in the future.
See also begin() and constEnd().
const_iterator QFuture::constEnd() const
Returns a const STL-style iterator pointing to the imaginary result after the last result in the future.
See also constBegin() and end().
const_iterator QFuture::end() const
Returns a const STL-style iterator pointing to the imaginary result after the last result in the future.
See also begin() and constEnd().
bool QFuture::isCanceled() const
Returns true if the asynchronous computation has been canceled with the cancel() function; otherwise returns false.
Be aware that the computation may still be running even though this function returns true. See cancel() for more details.
bool QFuture::isFinished() const
Returns true if the asynchronous computation represented by this future has finished; otherwise returns false.
bool QFuture::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 QFuture::isResultReadyAt(int index) const
Returns true if the result at index is immediately available; otherwise returns false.
See also resultAt() and resultCount().
bool QFuture::isRunning() const
Returns true if the asynchronous computation represented by this future is currently running; otherwise returns false.
bool QFuture::isStarted() const
Returns true if the asynchronous computation represented by this future has been started; otherwise returns false.
void QFuture::pause()
Pauses the asynchronous computation represented by this future. This is a convenience method that simply calls setPaused(true).
See also resume().
int QFuture::progressMaximum() const
Returns the maximum progressValue().
See also progressValue() and progressMinimum().
int QFuture::progressMinimum() const
Returns the minimum progressValue().
See also progressValue() and progressMaximum().
QString QFuture::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 QFuture::progressValue() const
Returns the current progress value, which is between the progressMinimum() and progressMaximum().
See also progressMinimum() and progressMaximum().
T QFuture::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() and results().
T QFuture::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(), results(), and resultCount().
int QFuture::resultCount() const
Returns the number of continuous results available in this future. The real number of results stored might be different from this value, due to gaps in the result set. It is always safe to iterate through the results from 0 to resultCount().
See also result(), resultAt(), and results().
QList<T> QFuture::results() const
Returns all results from the future. If the results are not immediately available, this function will block and wait for them to become available.
See also result(), resultAt(), and resultCount().
void QFuture::resume()
Resumes the asynchronous computation represented by this future. This is a convenience method that simply calls setPaused(false).
See also pause().
void QFuture::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. Any QFutureWatcher object that is watching this future will stop delivering progress and result ready signals while the future is paused. Signal delivery will continue once the future 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 future returned by QtConcurrent::run() cannot be paused; but the future returned by QtConcurrent::mappedReduced() can.
See also isPaused(), pause(), resume(), and togglePaused().
void QFuture::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 is paused. This is a convenience method for calling setPaused(!isPaused()).
See also setPaused(), pause(), and resume().
void QFuture::waitForFinished()
Waits for the asynchronous computation to finish (including cancel()ed computations).
T QFuture::operator T() 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 result() or resultAt(0).
See also result(), resultAt(), and results().
bool QFuture::operator!=(const QFuture<T> &other) const
Returns true if other is not a copy of this future; otherwise returns false.
bool QFuture::operator==(const QFuture<T> &other) const
Returns true if other is a copy of this future; otherwise returns false.