Kotlin初始化：如何失败

Question

I need to parse Firebase DataSnapshot (a JSON object) into a data class, whose properties include enum and list. So I prefer parsing it manually by passing the DataSnapshot into a secondary constructor. However, I want the construction to fail if some fields are missing in the DataSnapshot . In Swift, it has failable initializer that returns null so I know it has failed. How to do that in Kotlin?

The following is my attempt, but I assume you can't just return from a constructor, right?

data class Unit(val name: String, val type: UnitType, val components: List<Component>) {
    constructor(snapshot: DataSnapshot) {
        name = snapshot.child("name").value as? String ?: return
        val typeString = snapshot.child("type").value as? String ?: return
        type = UnitType.values().firstOrNull { it.abbrv == typeString } ?: return
        ...
    }

}

Answer 1

There is the require function which was designed for this purpose:

// ...
init {
    require(foo == "bar") {
        "$foo should be equal to 'bar'"
    }
}

There is also requireNotNull , check , error and some others. These are called Preconditions in Kotlin. There is no magic here, they are just some helper functions. You can check the actual source here , it is very useful. There is a related blog post about the topic here .

Answer 2

No, you can't just return from a constructor - it doesn't make much sense conceptually - but since I'm fairly naive on Kotlin syntax, I'll answer to a more broad paradigm here.

Your data class is not responsible for error checking or error handling. It is only responsible for storing data. Therefore, you should rely on a factory pattern to parse the result of a DataSnapshot and conditionally instantiate a new instance of Unit if and only if no fields are missing.