https://github.com/flaviotordini/promises

Javascript-style Promise implementation for Qt
https://github.com/flaviotordini/promises

async asynchronous asynchronous-programming promise promises qt qt6

Last synced: about 2 months ago
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Javascript-style Promise implementation for Qt

• Host: GitHub
• URL: https://github.com/flaviotordini/promises
• Owner: flaviotordini
• License: mit
• Created: 2021-06-17T13:47:58.000Z (over 3 years ago)
• Default Branch: master
• Last Pushed: 2024-05-14T06:59:47.000Z (8 months ago)
• Last Synced: 2024-05-14T07:55:07.093Z (8 months ago)
• Topics: async, asynchronous, asynchronous-programming, promise, promises, qt, qt6
• Language: C++
• Homepage:
• Size: 21.5 KB
• Stars: 0
• Watchers: 1
• Forks: 2
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

        
# Qt Promises

A very simple Javascript-style Promise implementation for Qt. The main idea is to use QObject features (i.e. signal and slots, deleteLater) to implement promises.

I wrote this Promise implementation in Qt 5 times. I'm not sure whether Qt 6 has something comparable in terms of simplicity and lightness. Anyway, it works great in Qt 6 too and I'm using it in my apps at https://flavio.tordini.org

In the Qt world a signal is emitted by an object instance. Sometimes that's exactly what's needed, like in the Observer design pattern (aka Publisher/Subscriber). In other cases the asynchronous response should be bound to a specific function call and having an instance-wide signal can be a cause of concurrency problems. Using promises to return async data from a function is a better solution: in Qt terms, it basically creates a temporary object with its own signal connection which is not shared with other users of a function.

## Example usage

The main class is Promise. It is template-based. Use it like this:

```c++

Promise &myAsyncFunction() {

  auto promise = new Promise(this);

  // any asynchronous work you might have to do:

  connect(myObj, &MyObject::finished, this, [promise](auto result) {

    if (myObj.hasError()) promise->reject(myObj.errorMessage());

    else promise->resolve(result);

  });

  myObj.doSomething();

  return *promise;

}

myAsyncFunction().then([](auto result) {

  qDebug() << "Success!" << result;

}).onFailed([](auto errorMessage)) {

  qWarning() << "Error!" << errorMessage;

}).finally([] {

  // optionally do something in both cases

});

```

## EmptyPromise

`EmptyPromise` can be used when there is no data as the result of the async operation.

## VariantPromise

`VariantPromise` is not template-based but QVariant-based. It can be useful when the data result is a QVariant or any QVariant-supported data type, especially if there is a need to avoid templates for any reason.