Use publishers to filter numbers in a string field

Question

I'm building a wrapper for a textField that is used to introduce quantities. I'm trying to build everything with Combine. One of the use cases consists in that if the stringValue sent by the text field has a letter, I filter the letters and reassign the new value to the same var, so the text field filters these values. There's also a code to change this value to an int so other components can read the int value. Here's the code:

class QuantityPickerViewModel: ObservableObject {
    private var subscriptions: Set<AnyCancellable> = Set<AnyCancellable>()
    @Published var stringValue: String = ""
    @Published var value : Int? = nil
    
    init(initialValue: Int?) {
        $stringValue
            .removeDuplicates()
            .print("pre-filter")
            .map {
                $0.filter {$0.isNumber}
            }
            .print("post-filter")
            .map {
                Int($0)
            }
            .assign(to: \.value, on: self)
            .store(in: &subscriptions)

        $value.map {
            $0 != nil ? String($0!): ""
        }
        .print("Value")
        .assign(to: \.stringValue, on:self)
        .store(in: &subscriptions)
    
        value = initialValue
    }
}

I verify the behavior using tests, I'll just the test that fails:

class QuantityPickerViewModelTest: AppTestCase {
    var model: QuantityPickerViewModel!
    override func setUpWithError() throws {
        super.setUp()
        model = QuantityPickerViewModel(initialValue: 10)
    }
    
    func test_changeStringValueWithLetters_filtersLettersAndChangesValue() {
        model.stringValue = "30a"
        
        XCTAssertEqual(model.value, 30)
        XCTAssertEqual(model.stringValue, "30") // fails saying stringValue is still "30a"
    }
}

The output of the test is:

Test Case '-[SourdoughMasterTests.QuantityPickerViewModelTest test_changeStringValueWithLetters_filtersLettersAndChangesValue]' started.
pre-filter: receive subscription: (RemoveDuplicates)
post-filter: receive subscription: (Print)
post-filter: request unlimited
pre-filter: request unlimited
pre-filter: receive value: ()
post-filter: receive value: ()
Value: receive subscription: (PublishedSubject)
Value: request unlimited
Value: receive value: ()
Value: receive value: (10)
pre-filter: receive value: (10)
post-filter: receive value: (10)
Value: receive value: (10)
pre-filter: receive value: (30a)
post-filter: receive value: (30)
Value: receive value: (30)
pre-filter: receive value: (30)
post-filter: receive value: (30)
Value: receive value: (30)
/Users/jpellat/workspace/SourdoughMaster/SourdoughMasterTests/QuantityPickerViewModelTest.swift:54: error: -[SourdoughMasterTests.QuantityPickerViewModelTest test_changeStringValueWithLetters_filtersLettersAndChangesValue] : XCTAssertEqual failed: ("30a") is not equal to ("30")

Does anyone know why the value has not been assigned? Thanks

Answer 1

It's not a Combine issue per-se that causes this, but it seems that a Published publisher emits before the value is actually set on the property. So, basically the "30a" is overwriting whatever is set in the assign.

In any case, this circular pipeline chain seems a bit fishy. I also don't think you actually need Combine here at all - it could just be solved with two computed properties and a common stored property:

@Published 
private var _value: Int? = nil

var value: Int? {
   get { _value }
   set { _value = newValue }
}

var stringValue: String {
   get { _value?.description ?? "" }
   set {
      _value = Int(newValue.filter { "0"..."9" ~= $0 })
   }
}

Answer 2

Even if I consider New Dev's solution better, and hence the official answer, I post my Combine solution in case someone is curious. I Basically eliminate the loop by separating user input and user output. To the interface, it looks the same but I can create a userInput -> value -> user output structure:

class QuantityPickerViewModel: ObservableObject {
    private var subscriptions: Set<AnyCancellable> = Set<AnyCancellable>()
    var stringValue: String {
        get {
            userOutput
        }
        set {
            userInput = newValue
        }
    }
    @Published var value : Int? = nil
    @Published private var userInput: String = ""
    private var userOutput: String = ""
    
    init(initialValue: Int?) {
        $userInput
            .map {
                $0.filter {$0.isNumber}
            }
            .map {
                Int($0)
            }
            .assign(to: \.value, on: self)
            .store(in: &subscriptions)
        
        $value
            .map {
                $0 == nil ? "": String($0!)
            }
            .assign(to: \.userOutput, on: self)
            .store(in: &subscriptions)
        
        value = initialValue
    }
}

Also here are the tests in case you're curious about the specs:

class QuantityPickerViewModelTest: XCTestCase {
    var model: QuantityPickerViewModel!
    override func setUpWithError() throws {
        super.setUp()
        model = QuantityPickerViewModel(initialValue: 10)
    }

    func test_initWith10_valueAfterInit_is10() {
        XCTAssertEqual(model.value, 10)
        XCTAssertEqual(model.stringValue, "10")
    }

    func test_initWithNil_valueAfterInit_isNilAndEmptyString() {
        model = QuantityPickerViewModel(initialValue: nil)
        XCTAssertNil(model.value)
        XCTAssertEqual(model.stringValue, "")
    }
    
    func test_changeStringValue_changesValue() {
        model.stringValue = "20"
        
        XCTAssertEqual(model.value, 20)
        XCTAssertEqual(model.stringValue, "20")
    }
    
    func test_changeValue_changesStringValue() {
        model.value = 20
        
        XCTAssertEqual(model.value, 20)
        XCTAssertEqual(model.stringValue, "20")
    }
    
    func test_changeValueToNil_changesStringValueToEmpty() {
        model.value = nil
        
        XCTAssertEqual(model.value, nil)
        XCTAssertEqual(model.stringValue, "")
    }
    
    func test_changeStringValueWithLetters_filtersLettersAndChangesValue() {
        model.stringValue = "30a"
        
        XCTAssertEqual(model.value, 30)
        XCTAssertEqual(model.stringValue, "30")
    }
}