MutableMap< String，Any?当我在Kotlin中使用委托时，将值传递给var吗? [英] How does MutableMap<String, Any?> pass value to var when I use delegate in Kotlin?

问题描述

以下示例代码来自网页.我了解这些var _id，城市和国家/地区可以通过代表获得价值.

The following sample code is from a webpage. I understand that these vars _id, city and country get value by delegate.

我不明白地图如何:MutableMap将值传递给这些vars _id，城市和国家/地区?

I can't understand how map: MutableMap pass value to these vars _id, city and country ?

我必须先传递这些键的值才能进行映射吗?例如map ["_ id"] = 132L，map ["city"] ="Wuha"和map ["country"] ="USA"?

Do I must pass value of these key to map first ? such as map["_id"]=132L, map["city"]="Wuha" and map["country"]="USA"?

如果没有map ["_ id"]会发生什么?代码var _id: Long by map会导致错误吗?

What happened if there is no map["_id"] ? will the code var _id: Long by map cause error?

class CityForecast(val map: MutableMap<String, Any?>, val dailyForecast: List<DayForecast>) {
    var _id: Long by map
    var city: String by map
    var country: String by map

    constructor(id: Long, city: String, country: String, dailyForecast: List<DayForecast>)
        : this(HashMap(), dailyForecast) {
        this._id = id
        this.city = city
        this.country = country
    }
}

推荐答案

首先，您需要了解委托属性的工作方式.对象不直接拥有委​​托的属性.它仅拥有委托.例如，var _id: Long by map实际上看起来像

First, you need to understand how delegated property works. Object does not own the delegated property directly. It only owns the delegate. For example, var _id: Long by map is actually look like

var _id: Long
    get() {
        val value = map["_id"] as? Long
        if (value == null)
            throw Excecption()
        return value        
    }
    set(value) {
        map["_id"] = value
    }

我必须先传递这些键的值才能进行映射吗?例如map ["_ id"] = 132L，map ["city"] ="Wuha"和map ["country"] ="USA"?

Do I must pass value of these key to map first ? such as map["_id"]=132L, map["city"]="Wuha" and map["country"]="USA"?

否，就像lateinit一样，这是一个运行时检查.

No, it is a run-time check, just like lateinit.

如果没有map ["_ id"]会发生什么?代码var _id:Long by map会导致错误吗?

What happened if there is no map["_id"] ? will the code var _id: Long by map cause error?

当然会有一个例外.

请注意，对于普通班级，建议不.它是为JSON反序列化而设计的(尽管我仍然更喜欢GSON).不建议这样做，因为您正在创建潜在的运行时异常，可以避免这种异常，包括缺少值和类型检查.

Note that this is NOT recommended for normal class. It is designed for JSON deserialization (I still prefer GSON though). It is not recommended because you are creating potential run-time exception which can be avoid, including missing value and type checking.

此外，如果不需要显式使用HashMap()，则应使用mutableMapOf()而不是HashMap().

Also, you should use mutableMapOf() instead of HashMap() if you does not need HashMap() explicitly.

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