valueStream throws missingData when string property is set to empty string

[mfazekas]

Description

When using the @_spi(RiveExperimental) API, setting a ViewModel string property to an empty string ("") causes valueStream to throw a missingData error. This terminates the AsyncThrowingStream permanently — the listener is killed and cannot receive any further updates.

An empty string is a valid value and should be emitted through the stream, not treated as missing data.

Root cause

The bug originates in the ObjC++ bridge at RiveViewModelInstanceData.mm:90:

case rive::DataType::string:
case rive::DataType::enumType:
    if (!data.stringValue.empty())  // ← filters out empty strings
    {
        _stringValue = [NSString stringWithUTF8String:data.stringValue.c_str()];
    }
    break;

When the C++ runtime delivers an empty string, data.stringValue.empty() returns true, so _stringValue is never set and stays nil.

On the Swift side, ViewModelInstanceService.swift:671-684 checks data.stringValue, which is nil, so all type checks fail and it falls through to:

streamContinuation.finish(throwing: ViewModelInstanceError.missingData)
streamContinuations.removeValue(forKey: requestID)

This kills the stream permanently.

Provide a Repro

@_spi(RiveExperimental) import RiveRuntime

let nameProp = StringProperty(path: "childVm/name")

// Start listening
Task { @MainActor in
    let stream = vmi.valueStream(of: nameProp)
    do {
        for try await val in stream {
            print("Value: '\(val)'")
        }
    } catch {
        // THIS IS HIT: "missingData"
        print("Stream error: \(error)")
    }
}

// Set to empty string — stream throws missingData and terminates
vmi.setValue(of: nameProp, to: "")

Unit tests reproducing the bug

// In ViewModelInstanceTests.swift

@MainActor
func test_valueStream_withStringProperty_throwsMissingDataOnEmptyString() async throws {
    let mockCommandQueue = MockCommandQueue()

    let viewModelInstance = makeViewModelInstance(mockCommandQueue: mockCommandQueue)

    let property = StringProperty(path: "childVm/name")
    let stream = viewModelInstance.valueStream(of: property)

    let subscribeCall = mockCommandQueue.subscribeToViewModelPropertyCalls[0]
    let observer = mockCommandQueue.getObserver(for: 99)
    let requestID = subscribeCall.requestID

    let errorExpectation = expectation(description: "Stream throws missingData")

    let task = Task {
        do {
            for try await _ in stream {}
            XCTFail("Stream should have thrown, not ended normally")
        } catch let error as ViewModelInstanceError {
            guard case .missingData = error else {
                XCTFail("Expected missingData, got \(error)")
                return
            }
            errorExpectation.fulfill()
        }
    }

    // Simulate what happens when the ObjC++ bridge receives an empty string:
    // RiveViewModelInstanceData.mm filters out empty strings via `if (!data.stringValue.empty())`
    // which leaves _stringValue as nil.
    let emptyStringData = MockRiveViewModelInstanceData(stringValue: nil)
    observer?.onViewModelDataReceived(99, requestID: requestID, data: emptyStringData)

    await fulfillment(of: [errorExpectation], timeout: 1.0)
    task.cancel()
}

@MainActor
func test_value_withStringProperty_throwsMissingDataOnEmptyString() async throws {
    let mockCommandQueue = MockCommandQueue()
    var capturedObserver: ViewModelInstanceListener?

    let viewModelInstance = makeViewModelInstance(mockCommandQueue: mockCommandQueue) { observer in
        capturedObserver = observer
    }

    mockCommandQueue.stubRequestViewModelInstanceString { instanceHandle, path, requestID in
        let mockData = MockRiveViewModelInstanceData(stringValue: nil)
        capturedObserver?.onViewModelDataReceived(instanceHandle, requestID: requestID, data: mockData)
    }

    let property = StringProperty(path: "childVm/name")

    do {
        _ = try await viewModelInstance.value(of: property)
        XCTFail("Expected ViewModelInstanceError.missingData to be thrown")
    } catch let error as ViewModelInstanceError {
        guard case .missingData = error else {
            XCTFail("Expected missingData error, got \(error)")
            return
        }
    }
}

Source .riv/.rev file

Any .riv file with a ViewModel containing a string property will reproduce this. Nested paths (e.g. "childVm/name") are confirmed affected.

Expected behavior

Setting a string property to "" should emit "" through the stream. An empty string is a valid value.

Suggested fix

In RiveViewModelInstanceData.mm, always set _stringValue for string/enum types:

case rive::DataType::string:
case rive::DataType::enumType:
    _stringValue = [NSString stringWithUTF8String:data.stringValue.c_str()];
    break;

[NSString stringWithUTF8String:""] correctly produces @"" (non-nil empty NSString).

Device & Versions (please complete the following information)

• Device: iOS Simulator (iPhone 16)
• iOS version: iOS 18.2
• rive-ios: latest main (397b290)
• API: @_spi(RiveExperimental)

Additional context

• The same bug likely affects EnumProperty since it shares the same code path
• The one-shot value(of:) API also returns missingData for empty strings (same root cause)
• Current workaround is wrapping stream iteration in a retry loop, but this introduces gaps where updates can be missed