QPromise 类提供存储计算结果的办法，访问通过 QFuture . 更多...
| 头： | #include <QPromise> |
| CMake: |
find_package(Qt6 COMPONENTS Core REQUIRED)
target_link_libraries(mytarget PRIVATE Qt6::Core) |
| qmake: | QT += core |
| Since: | Qt 6.0 |
注意： 此类的所有函数 thread-safe .
| QPromise (QPromise<T> && other ) | |
| QPromise () | |
| QPromise<T> & | operator= (QPromise<T> && other ) |
| ~QPromise () | |
| bool | addResult (const T & result , int index = -1) |
| bool | addResult (T && result , int index = -1) |
| void | finish () |
| QFuture<T> | future () const |
| bool | isCanceled () const |
| void | setException (const QException & e ) |
| void | setException (std::exception_ptr e ) |
| void | setProgressRange (int minimum , int maximum ) |
| void | setProgressValue (int progressValue ) |
| void | setProgressValueAndText (int progressValue , const QString & progressText ) |
| void | start () |
| void | suspendIfRequested () |
| void | swap (QPromise<T> & other ) |
QPromise provides a simple way to communicate progress and results of the user-defined computation to QFuture in an asynchronous fashion. For the communication to work, QFuture must be constructed by QPromise.
You can use QPromise based workloads as an alternative to Qt Concurrent framework when fine-grained control is needed or high-level communication primitive to accompany QFuture is sufficient.
The simplest case of promise and future collaboration would be a single result communication:
QPromise<int> promise;
QFuture<int> future = promise.future();
QScopedPointer<QThread> thread(QThread::create([] (QPromise<int> promise) {
promise.start(); // notifies QFuture that the computation is started
promise.addResult(42);
promise.finish(); // notifies QFuture that the computation is finished
}, std::move(promise)));
thread->start();
future.waitForFinished(); // blocks until QPromise::finish is called
future.result(); // returns 42
By design, QPromise is a move-only object. This behavior helps to ensure that whenever the promise is destroyed, the associated future object is notified and will not wait forever for the results to become available. However, this is inconvenient if one wants to use the same promise to report results from different threads. There is no specific way to do that at the moment, but known mechanisms exist, such as the use of smart pointers or raw pointers/references. QSharedPointer is a good default choice if you want to copy your promise and use it in multiple places simultaneously. Raw pointers or references are, in a sense, easier, and probably perform better (since there is no need to do a resource management) but may lead to dangling.
Here is an example of how a promise can be used in multiple threads:
QSharedPointer<QPromise<int>> sharedPromise(new QPromise<int>());
QFuture<int> future = sharedPromise->future();
// ...
// here, QPromise is shared between threads via a smart pointer
QScopedPointer<QThread> threads[] = {
QScopedPointer<QThread>(QThread::create([] (auto sharedPromise) {
sharedPromise->addResult(0, 0); // adds value 0 by index 0
}, sharedPromise)),
QScopedPointer<QThread>(QThread::create([] (auto sharedPromise) {
sharedPromise->addResult(-1, 1); // adds value -1 by index 1
}, sharedPromise)),
QScopedPointer<QThread>(QThread::create([] (auto sharedPromise) {
sharedPromise->addResult(-2, 2); // adds value -2 by index 2
}, sharedPromise)),
// ...
};
// start all threads
for (auto& t : threads)
t->start();
// ...
future.resultAt(0); // waits until result at index 0 becomes available. returns value 0
future.resultAt(1); // waits until result at index 1 becomes available. returns value -1
future.resultAt(2); // waits until result at index 2 becomes available. returns value -2
另请参阅 QFuture .
添加 result to the internal result collection at index position. If index is unspecified, result is added to the end of the collection.
返回
true
当
result
is added to the collection.
返回
false
when this promise is in canceled or finished state or when
result
is rejected.
addResult
() rejects
result
if there's already another result in the collection stored at the same index.
You can get a result at a specific index by calling QFuture::resultAt ().
注意： It is possible to specify an arbitrary index and request result at that index. However, some QFuture methods operate with continuous results. For instance, iterative approaches that use QFuture::resultCount () 或 QFuture::const_iterator . In order to get all available results without thinking if there are index gaps or not, use QFuture::results ().
Move constructs a new QPromise from other .
另请参阅 operator= ().
Constructs a QPromise with a default state.
Move assigns other to this promise and returns a reference to this promise.
Destroys the promise.
注意： The promise implicitly transitions to a canceled state on destruction unless finish () is called beforehand by the user.
Reports that the computation is finished. Once finished, no new results will be added when calling addResult (). This method accompanies start ().
另请参阅 QFuture::isFinished (), QFuture::waitForFinished ()，和 start ().
Returns a future associated with this promise.
Returns whether the computation has been canceled with the
QFuture::cancel
() function. The returned value
true
indicates that the computation should be finished and
finish
() called.
注意： After cancellation, results currently available may still be accessed by a future, but new results will not be added when calling addResult ().
Sets exception e to be the result of the computation.
注意： You can set at most one exception throughout the computation execution.
注意： This method must not be used after QFuture::cancel () 或 finish ().
另请参阅 isCanceled ().
这是重载函数。
Sets the progress range of the computation to be between minimum and maximum .
若 maximum 小于 minimum , minimum 变为唯一合法值。
The progress value is reset to be minimum .
The progress range usage can be disabled by using setProgressRange(0, 0). In this case progress value is also reset to 0.
另请参阅 QFuture::progressMinimum (), QFuture::progressMaximum ()，和 QFuture::progressValue ().
Sets the progress value of the computation to progressValue . It is possible to only increment the progress value. This is a convenience method for calling setProgressValueAndText (progressValue, QString()).
In case of the progressValue falling out of the progress range, this method has no effect.
另请参阅 QFuture::progressValue () 和 setProgressRange ().
Sets the progress value and the progress text of the computation to progressValue and progressText respectively. It is possible to only increment the progress value.
注意： This function has no effect if the promise is in canceled or finished state.
另请参阅 QFuture::progressValue (), QFuture::progressText (), QFuture::cancel ()，和 finish ().
Reports that the computation is started. Calling this method is important to state the beginning of the computation as QFuture methods rely on this information.
注意： Extra attention is required when start() is called from a newly created thread. In such case, the call might naturally be delayed due to the implementation details of the thread scheduling.
另请参阅 QFuture::isStarted (), QFuture::waitForFinished ()，和 finish ().
Conditionally suspends current thread of execution and waits until resumed or canceled by the corresponding methods of QFuture . This method does not block unless the computation is requested to be suspended by QFuture::suspend () or another related method. If you want to check that the execution has been suspended, use QFuture::isSuspended ().
注意：
When using the same promise in multiple threads,
QFuture::isSuspended
() becomes
true
as soon as at least one thread with the promise suspends.
The following code snippets show the usage of suspension mechanism:
// Create promise and future
QPromise<int> promise;
QFuture<int> future = promise.future();
promise.start();
// Start a computation thread that supports suspension and cancellation
QScopedPointer<QThread> thread(QThread::create([] (QPromise<int> promise) {
for (int i = 0; i < 100; ++i) {
promise.addResult(i);
promise.suspendIfRequested(); // support suspension
if (promise.isCanceled()) // support cancellation
break;
}
promise.finish();
}, std::move(promise)));
thread->start();
QFuture::suspend () requests the associated promise to suspend:
future.suspend();
后于
QFuture::isSuspended
() becomes
true
, you can get intermediate results:
future.resultCount(); // returns some number between 0 and 100
for (int i = 0; i < future.resultCount(); ++i) {
// process results available before suspension
}
When suspended, you can resume or cancel the awaiting computation:
future.resume(); // resumes computation, this call will unblock the promise
// alternatively, call future.cancel() to stop the computation
future.waitForFinished();
future.results(); // returns all computation results - array of values from 0 to 99
另请参阅 QFuture::resume (), QFuture::cancel (), QFuture::setSuspended ()，和 QFuture::toggleSuspended ().
Swaps promise other with this promise. This operation is very fast and never fails.