SWIFT: How do I create a predicate with an Int val

问题:

I hope that no one tells me to RTFM because I have been struggling with this for some time here and on Apple Developer and doing lots of searching.

I am getting an EXC-BAD-ACCESS error on this statement:

var thisPredicate = NSPredicate(format: "(sectionNumber == %@"), thisSection)

thisSection has an Int value of 1 and displays the value 1 when I hover over it. But in the debug area I see this:

thisPredicate = (_ContiguousArrayStorage ...)

Another predicate using a String shows as ObjectiveC.NSObject Why is this happening?

回答1:

You might try String Interpolation Swift Standard Library Reference. That would look something like this:

let thisSection = 1
let thisPredicate = NSPredicate(format: "sectionNumber == \(thisSection)")

回答2:

You will need to change %@ for %i and remove the extra parenthesis:

Main problem here is that you are putting an Int where it's expecting an String.

Here's an example based on this post:

class Person: NSObject {
    let firstName: String
    let lastName: String
    let age: Int

    init(firstName: String, lastName: String, age: Int) {
        self.firstName = firstName
        self.lastName = lastName
        self.age = age
    }

    override var description: String {
        return "\(firstName) \(lastName)"
    }
}

let alice = Person(firstName: "Alice", lastName: "Smith", age: 24)
let bob = Person(firstName: "Bob", lastName: "Jones", age: 27)
let charlie = Person(firstName: "Charlie", lastName: "Smith", age: 33)
let quentin = Person(firstName: "Quentin", lastName: "Alberts", age: 31)
let people = [alice, bob, charlie, quentin]


let thisSection = 33
let thisPredicate = NSPredicate(format: "age == %i", thisSection)

let _people = (people as NSArray).filteredArrayUsingPredicate(thisPredicate)
_people

Another workaround would be to make thisSection's value an String, this can be achieved by String Interpolation or via description property of the Int.. lets say:

Changing:

let thisPredicate = NSPredicate(format: "age == %i", thisSection)

for

let thisPredicate = NSPredicate(format: "age == %@", thisSection.description)

or

let thisPredicate = NSPredicate(format: "age == %@", "\(thisSection)")

of course, you can always bypass this step and go for something more hardcoded (but also correct) as:

let thisPredicate = NSPredicate(format: "sectionNumber == \(thisSection)")

But take into account that for some weird reason String Interpolation (this kind of structure: "\(thisSection)") where leading to retain cycles as stated here