如何将CoreML模型输出的MLMultiArray持久化到Core Data？

方案一：用Core Data Transformable类型（自定义转换器） #

Transformable类型完全适用，但直接依赖默认转换器可能存在兼容性问题，建议自定义ValueTransformer来处理MLMultiArray与可存储数据的双向转换：

1. 实现自定义转换器

class MLMultiArrayTransformer: ValueTransformer {
    override class func transformedValueClass() -> AnyClass {
        return Data.self
    }
    
    override class func allowsReverseTransformation() -> Bool {
        return true
    }
    
    override func transformedValue(_ value: Any?) -> Any? {
        guard let multiArray = value as? MLMultiArray else { return nil }
        guard multiArray.dataType == .float32 else { return nil }
        
        let count = multiArray.count
        let floatPointer = multiArray.dataPointer.bindMemory(to: Float32.self, capacity: count)
        let buffer = UnsafeBufferPointer(start: floatPointer, count: count)
        return Data(buffer: buffer)
    }
    
    override func reverseTransformedValue(_ value: Any?) -> Any? {
        guard let data = value as? Data else { return nil }
        
        let count = data.count / MemoryLayout<Float32>.stride
        guard let multiArray = try? MLMultiArray(shape: [NSNumber(value: count)], dataType: .float32) else {
            return nil
        }
        
        data.copyBytes(to: multiArray.dataPointer.bindMemory(to: Float32.self, capacity: count))
        return multiArray
    }
}

2. 注册转换器
   在App启动阶段（比如AppDelegate或SceneDelegate的初始化方法）注册转换器：

ValueTransformer.setValueTransformer(MLMultiArrayTransformer(), forName: NSValueTransformerName(rawValue: "MLMultiArrayTransformer"))

3. 配置Core Data实体

• 在Core Data模型编辑器中添加属性，类型选择Transformable
• 在属性的「Transformer」栏填写MLMultiArrayTransformer（与注册名称一致）
• 可选勾选「Allow External Storage」，适合存储较大的数组

4. 读写示例

// 存储
let predictions = try model.prediction(from: input)
let record = PredictionRecord(context: viewContext)
record.predictionArray = predictions.yourMultiArrayOutput

// 读取
if let storedArray = record.predictionArray as? MLMultiArray {
    // 使用存储的MLMultiArray
}

方案二：转换为Data后存储到Binary Data字段 #

如果不想用Transformable，直接将MLMultiArray转为Data，存储到Core Data的Binary Data类型字段，可控性更强：

1. 配置Core Data实体
   添加一个Binary Data类型的属性，可选勾选「Allow External Storage」；建议额外添加一个Transformable属性存储数组的shape（还原时需要）。

2. 转换工具方法

// MLMultiArray转Data
func multiArrayToData(_ array: MLMultiArray) -> Data? {
    guard array.dataType == .float32 else { return nil }
    let count = array.count
    let floatPointer = array.dataPointer.bindMemory(to: Float32.self, capacity: count)
    let buffer = UnsafeBufferPointer(start: floatPointer, count: count)
    return Data(buffer: buffer)
}

// Data转MLMultiArray（需要传入原shape）
func dataToMultiArray(_ data: Data, shape: [NSNumber]) -> MLMultiArray? {
    let expectedCount = shape.reduce(1) { $0 * $1.intValue }
    let actualCount = data.count / MemoryLayout<Float32>.stride
    guard expectedCount == actualCount else { return nil }
    
    guard let array = try? MLMultiArray(shape: shape, dataType: .float32) else {
        return nil
    }
    
    data.copyBytes(to: array.dataPointer.bindMemory(to: Float32.self, capacity: actualCount))
    return array
}

3. 读写示例

// 存储
let predictions = try model.prediction(from: input)
let record = PredictionRecord(context: viewContext)
record.arrayData = multiArrayToData(predictions.yourMultiArrayOutput)
record.arrayShape = predictions.yourMultiArrayOutput.shape

// 读取
if let data = record.arrayData, let shape = record.arrayShape as? [NSNumber], let array = dataToMultiArray(data, shape: shape) {
    // 使用还原后的MLMultiArray
}

方案三：结构化存储（适合简单维度数组） #

如果你的MLMultiArray是一维或固定结构的，可转为JSON字符串存储到Core Data的String字段：

// MLMultiArray转JSON字符串
func multiArrayToJSON(_ array: MLMultiArray) -> String? {
    guard array.dataType == .float32 else { return nil }
    let count = array.count
    let floatPointer = array.dataPointer.bindMemory(to: Float32.self, capacity: count)
    let values = Array(UnsafeBufferPointer(start: floatPointer, count: count))
    
    guard let jsonData = try? JSONSerialization.data(withJSONObject: values) else {
        return nil
    }
    return String(data: jsonData, encoding: .utf8)
}

// JSON字符串转MLMultiArray
func jsonToMultiArray(_ jsonString: String) -> MLMultiArray? {
    guard let jsonData = jsonString.data(using: .utf8), let values = try? JSONSerialization.jsonObject(with: jsonData) as? [Float32] else {
        return nil
    }
    
    guard let array = try? MLMultiArray(shape: [NSNumber(value: values.count)], dataType: .float32) else {
        return nil
    }
    
    let floatPointer = array.dataPointer.bindMemory(to: Float32.self, capacity: values.count)
    for (index, value) in values.enumerated() {
        floatPointer[index] = value
    }
    return array
}

读写时直接调用上述方法即可，存储到String类型的Core Data属性中。

内容的提问来源于stack exchange，提问作者MazzzyStar