# QPromise::filter

Since: 0.4.0

QPromise<Sequence<T>>::filter(Filter filterer) -> QPromise<Sequence<T>>

// With:
// - Sequence: STL compatible container (e.g. QVector, etc.)
// - Filterer: Function(T value, int index) -> bool

IMPORTANT

This method only applies to promise with sequence value.

Iterates over all the promise values (i.e. Sequence<T>) and filters the sequence (opens new window) to another using the given filterer function. If filterer returns true, a copy of the item is put in the output sequence, otherwise, the item will not appear in output. If filterer throws, output is rejected with the new exception.

If filterer returns a promise (or QFuture), the output promise is delayed until all the promises are resolved. If any of the promises fail, 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.filter([](const QUrl& url, ...) {
    return url.isValid();  // Keep only valid URLs
}).filter([](const QUrl& url, ...) {
    return QPromise<bool>{[&](auto resolve, auto reject) {
        // resolve(true) if `url` is reachable, else resolve(false)
    }};
});

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

// 'output' type: QPromise<QList<QUrl>>
output.then([](const QList<QUrl>& res) {
    // 'res' contains only reachable URLs
});

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 filterer.