How to combine multiple nullable NSPredicates?

Question

For eg, Something like:

    var finalPredicate = NSPredicate(format: "")

    if (screen != nil) {
        screenPredicate = NSPredicate(format: "screen = %@", screen!)
        finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: [screenPredicate!])
    }

    if (feature != nil) {
        featurePredicate = NSPredicate(format: "feature = %@", feature!)
        finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: [finalPredicate, featurePredicate!])
    }

    if (shouldDisplayPredicate != nil) {
        shouldDisplayPredicate = NSPredicate(format: "shouldDisplay = %@", shouldDisplay!)
        finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: [finalPredicate, shouldDisplayPredicate!])
    }
    if (hasDisplayed != nil) {
        displayPredicate = NSPredicate(format: "hasDisplayed = %@", hasDisplayed!)
        finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: [finalPredicate, displayPredicate!])
    }

Is there a better way to do this where the predicates could be null or not ?

Thanks

score 2 · Accepted Answer · edited May 23 '17 at 11:51

First, you should avoid the forced unwrapping and replace

if screen != nil {
    // ... add predicate for `screen!` ...
}

by an optional binding:

if let screenValue = screen {
    // ... add predicate for `screenValue` ...
}

Compare When should I compare an optional value to nil? for a good overview of that topic. The same can be achieved more compactly using the map() method of Optional:

screen.map { /* ... add predicate for `$0` ... }

The closure is called only if screen != nil, and then $0 inside the closure is the unwrapped value.

Second, it is simpler to fill an array with all required predicates first, and create the compound predicate only once. This also allows you to check if any search attribute was set at all.

Your code then becomes

var predicates: [NSPredicate] = []
if let screenValue = screen {
    predicates.append(NSPredicate(format: "screen = %@", screenValue))
}
if let featureValue = feature {
    predicates.append(NSPredicate(format: "feature = %@", featureValue))
}
// ... other search attributes ...
if !predicates.isEmpty {
    let finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates:predicates)
}

or

var predicates: [NSPredicate] = []
screen.map { predicates.append(NSPredicate(format: "screen = %@", $0)) }
feature.map { predicates.append(NSPredicate(format: "feature = %@", $0)) }
// ... other search attributes ...
if !predicates.isEmpty {
    let finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates:predicates)
}

The map option looks neat though a bit hard to read for the untrained (in swift) eye. — ofeida, Aug 22 '16 at 12:26

How to combine multiple nullable NSPredicates?

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