# QPromise::map

Since: 0.4.0

QPromise<Sequence<T>>::map(Mapper mapper) -> QPromise<QVector<R>>

// With:
// - Sequence: STL compatible container (e.g. QVector, etc.)
// - Mapper: Function(T value, int index) -> R | QPromise<R>

IMPORTANT

This method only applies to promise with sequence value.

Iterates over all the promise values (i.e. Sequence<T>) and maps the sequence (opens new window) to another using the given mapper function. The type returned by mapper determines the type of the output promise. If mapper throws, output is rejected with the new exception.

If mapper returns a promise (or QFuture), the output promise is delayed until all the promises are resolved. If any of the promises fails, output immediately rejects with the error of the promise that rejected, whether or not the other promises are resolved.

QPromise<QList<QUrl>> input = {...}

auto output = input.map([](const QUrl& url, int index) {
    return QPromise<QByteArray>{[&](auto resolve, auto reject) {
        // download content at 'url' and resolve
        // {...}
    }};
}).map([](const QByteArray& value, ...) {
    // process the downloaded QByteArray
    // {...}
    return DownloadResult{value};
});

// 'output' resolves as soon as all promises returned by
// 'mapper' are fulfilled or at least one is rejected.

// 'output' type: QPromise<QVector<DownloadResult>>
output.then([](const QVector<DownloadResult>& res) {
    // {...}
});

NOTE

The order of the output sequence values is guarantee to be the same as the original sequence, regardless of completion order of the promises returned by mapper.

This function is provided for convenience and is similar to:

promise.then([](const Sequence<T>& values) {
    return QtPromise::map(values, [](const T& value, int index) {
        return // {...}
    });
});