Kotlin属性与吸气剂.我可以不指定初始值吗? [英] Kotlin property with getter. Can I not specify an initial value?

问题描述

我想创建一个单例类，但是不幸的是，Android几乎所有内容都需要Context，因此我需要它来创建实例.所以我只是假设用户名为init()，然后返回该实例.如下所示，如果_instance为null，则将引发异常，因此get方法不能返回null.

I want to create a singleton class, but unfortunately, Android needs Context for almost anything so I need it to create an instance. So I just assumed the user called init(), and then return the instance. As you see below, if the _instance is null, an exception will be thrown, so the get method cannot return null.

但是Kotlin说我必须初始化instance.事实是，没有上下文就无法创建MyClass.因此，我不想指定初始值.我该怎么办?

But Kotlin says I must initialise instance. The things is, that MyClass cannot be created without a context. So I would like not to specify an initial value. How can I do that?

companion object
{
    protected var _instance:MyClass? = null;
    fun init(context:Context)
    {
        _instance = MyClass(context)
    }
    var instance:MyClass //<---This causes a compile error.
        get()
        {
            if(_instance==null) throw RuntimeException("Call init() first.");
            return _instance!!;
        }
}

推荐答案

将var更改为val，它应该可以工作:

Change the var to val and it should work:

....
val instance: MyClass
....

可变属性(var)不仅假定使用getter，而且假定使用setter.由于您未提供设置器，因此将生成默认设置器set(value) { field = value }.尽管在这种情况下毫无用处，但默认的setter使用field，因此需要对其进行初始化.

A variable property (var) not only assumes a getter, but also a setter. Since you provided no setter, a default one was generated set(value) { field = value }. Despite is uselessness in this situation, the default setter uses field, thus requires its initialization.

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