qt5-legatus-legacy/Lts/Common/LtsUtils/LtsUtilsCore/promise_examples.cpp

#include "promise_examples.h"
#include "promise.h"

#include <QTimer>
#include <QTest>


#include <QDebug>


namespace LtsUtils
{

// First Example
void PromiseExample1_Class::doSomethingAsync1()
{
  QTimer::singleShot(200, this, [this]()
  {
    emit signalDone1(true);
  });
}


void PromiseExample1_Class::doSomethingAsync2()
{
  QTimer::singleShot(450, this, [this]()
  {
    emit signalDone2(true, 1234);
  });
}


PromiseExample1::PromiseExample1(QObject *parent)
  : QObject(parent)
  , m_success(false)
{
  /* We have to work with a class that works asynchronous and need many emits and we have to wait for some results to come in.
   * But we want the developer to use this function without rewriting everything that could be handled by a Promise that could be provided. */

  auto func = [this](ResultFunction resolve /* call this function if the promise was successful */
      , ResultFunction reject /* call this function if the promise was not successful */)
  {
    // we create the class (or you could use it from somewhere else)
    PromiseExample1_Class *c = new PromiseExample1_Class(this);

    // we connect to the done signal for the first action (before actually calling the function that can return this signal)
    connect(c, &PromiseExample1_Class::signalDone1,
            this, [this, c, resolve, reject](bool didThisWork)
    {
      // did the first action not work?
      if (!didThisWork)
      {
        // something went wrong and we have to exit here with a reject
        reject(QStringLiteral("First Action did not succeed")); // .. did not work.. at least tell the enduser what happened (the reject function has to print the string, though)   <= promise is done, but failed early
        return;
      }
      else // yes, it worked! we can continue!
      {
        // after the first action succeeded, we can call the next action, that would otherwise be useless to call if the first call failed
        // we do the same thing as with the last signal connect here

        connect(c, &PromiseExample1_Class::signalDone2,
                this, [resolve, reject](bool didThisWork, int someNumberThatReturns)
        {
          if (didThisWork) // did the secondary action succeed?
            resolve(someNumberThatReturns); // .. then return the returned number to the resolve function   <= this is the final point where the promise is done!
          else
            reject(QStringLiteral("Second Action did not succeed")); // .. did not work.. at least tell the enduser what happened (the reject function has to print the string, though)   <= promise is done, but failed

          // everything done!
        });

        c->doSomethingAsync2(); // now, finally, execute the second action - should call signalDone2 soon!
      }
    });

    c->doSomethingAsync1(); // now, finally, execute this async stuff and then signalDone1 should be called soon!
  };
  Promise *p = new Promise(func, this);


  // the promise was created, but will not execute until Qt is back into the event loop
  // and to receive resolve(..) or reject(..) messages, we have to connect to them first (but note that this works without connects)

  p->resolved([this](const QVariant &value)
  {
    // "someNumberThatReturns" is inside value
    qDebug() << "example1 promise succeeded! 'someNumberThatReturns' was" << value;

    m_success = true;

  }).rejected([](const QVariant &value) // <= you can directly call rejected(..) on resolved(..) to save time, as resolved(..) returns the promise itself, just like in js
  {
    // one or both of the actions didn't work - tell the user where it failed
    qDebug() << "example1 promise failed.." << value;
  });
}
// End of First Example


// Second Example
PromiseExample2::PromiseExample2(QObject *parent)
  : QObject(parent)
  , m_success(false)
{
  /* There is an asynchonous function inside the Promise that might not resolve because of some error.
   * A timeout is set for the Promise to reject so it doesn't get stuck. */

  Promise *p = new Promise([](ResultFunction , ResultFunction )
  {
    // never resolve for this test case
  }, this);

  p->setTimeout(1000);

  p->resolved([](const QVariant &)
  {
    QFAIL("This test case's promise should never resolve");
  });

  p->rejected([this](const QVariant &val)
  {
    QVERIFY2(val == QStringLiteral("timeout"), "Promise didn't reject with string \"timeout\"");
    qDebug() << "Promise rejected as expected";
    m_success = true;
  });
}
// End of Second Example


// 3rd Example
PromiseExample3::PromiseExample3(QObject *parent)
  : QObject(parent)
  , m_success(true)
  , m_alreadyRejected(false)
{
  /* The promise's function failed with a reject, additionally a timeout was set
   * to reject a promise that might never resolve/reject. After the promise was rejected
   * the timeout should never trigger (because the promise was already rejected). */

  Promise *p = new Promise([](ResultFunction , ResultFunction reject)
  {
    reject(QString()); // immediately reject
  }, this);

  p->setTimeout(1000); // set timeout

  p->resolved([](const QVariant &)
  {
    QFAIL("This test case's promise should never resolve");
  });

  p->rejected([this](const QVariant &val)
  {
    qDebug() << "rejected with val" << val;
    QVERIFY2(val == QString(), "Promise didn't reject with an expected empty string.");
    QVERIFY2(!m_alreadyRejected, "Promise rejected more than once");

    m_alreadyRejected = true;
    qDebug() << "Promise rejected as expected";
  });
}
// End of 3rd Example


} // namespace LtsUtils