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1f30224578
Introduce a new `QtPromise::connect()` helper that allows to create a promise resolved from a single signal and optionally, rejected by another one (from a different object or not). The promise type is determined by the type of the first signal argument (other arguments are currently ignored). A `QPromise<void>` is returned if the resolve signal doesn't provide any argument. If the rejection is emitted before the promise is resolved, the promise will be rejected with the value of the first argument (other arguments being ignored). If the rejection signal doesn't provide any argument, the promise will be rejected with `QPromiseUndefinedException` if the signal is emitted. Additionally, the promise will be automatically rejected with `QPromiseContextException` if the source object is destroyed before the promise is resolved.
90 lines
2.8 KiB
Markdown
90 lines
2.8 KiB
Markdown
# Qt Signals
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QtPromise supports creating promises that are resolved or rejected by regular [Qt signals](https://doc.qt.io/qt-5/signalsandslots.html).
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::: warning IMPORTANT
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A promise connected to a signal will be resolved (fulfilled or rejected) **only one time**, no matter if the signals are emitted multiple times. Internally, the promise is disconnected from all signals as soon as one signal is emitted.
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:::
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## Resolve Signal
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The [`QtPromise::connect()`](helpers/connect.md) helper allows to create a promise resolved from a single signal:
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```cpp
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// [signal] Object::finished(const QByteArray&)
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auto output = QtPromise::connect(obj, &Object::finished);
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// output type: QPromise<QByteArray>
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output.then([](const QByteArray& data) {
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// {...}
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});
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```
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If the signal doesn't provide any argument, a `QPromise<void>` is returned:
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```cpp
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// [signal] Object::done()
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auto output = QtPromise::connect(obj, &Object::done);
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// output type: QPromise<void>
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output.then([]() {
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// {...}
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});
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```
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::: tip NOTE
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QtPromise currently only supports single argument signals, which means that only the first argument is used to fulfill or reject the connected promise, other arguments being ignored.
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:::
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## Reject Signal
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The [`QtPromise::connect()`](helpers/connect.md) helper also allows to reject the promise from another signal:
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```cpp
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// [signal] Object::finished(const QByteArray& data)
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// [signal] Object::error(ObjectError error)
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auto output = QtPromise::connect(obj, &Object::finished, &Object::error);
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// output type: QPromise<QByteArray>
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output.then([](const QByteArray& data) {
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// {...}
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}).fail(const ObjectError& error) {
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// {...}
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});
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```
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If the rejection signal doesn't provide any argument, the promise will be rejected
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with [`QPromiseUndefinedException`](../exceptions/undefined), for example:
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```cpp
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// [signal] Object::finished()
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// [signal] Object::error()
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auto output = QtPromise::connect(obj, &Object::finished, &Object::error);
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// output type: QPromise<QByteArray>
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output.then([]() {
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// {...}
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}).fail(const QPromiseUndefinedException& error) {
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// {...}
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});
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```
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A third variant allows to connect the resolve and reject signals from different objects:
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```cpp
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// [signal] ObjectA::finished(const QByteArray& data)
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// [signal] ObjectB::error(ObjectBError error)
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auto output = QtPromise::connect(objA, &ObjectA::finished, objB, &ObjectB::error);
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// output type: QPromise<QByteArray>
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output.then([](const QByteArray& data) {
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// {...}
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}).fail(const ObjectBError& error) {
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// {...}
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});
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```
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Additionally to the rejection signal, promises created using [`QtPromise::connect()`](helpers/connect.md) are automatically rejected with [`QPromiseContextException`](exceptions/context.md) if the sender is destroyed before fulfilling the promise.
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See [`QtPromise::connect()`](helpers/connect.md) for more details.
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