在哪种情况下，Kotlin 构造函数参数中需要 val/var? [英] In which situation val/var is necessary in Kotlin constructor parameter?

问题描述

正确的代码:

class MainActHandler(val weakActivity: WeakReference): Handler() {覆盖乐趣 handleMessage(味精:消息?){val trueAct = weakActivity.get() ?: 返回if (msg?.what == ConversationMgr.MSG_WHAT_NEW_SENTENCE){val 语句 = msg.obj 作为字符串?trueAct.conversation.text = 句子}super.handleMessage(msg)}}

无法解析代码:

class MainActHandler(weakActivity: WeakReference): Handler() {覆盖乐趣 handleMessage(味精:消息?){val trueAct = weakActivity.get() ?: 返回if (msg?.what == ConversationMgr.MSG_WHAT_NEW_SENTENCE){val 语句 = msg.obj 作为字符串?trueAct.conversation.text = 句子}super.handleMessage(msg)}}

无法解析代码截图

唯一的区别是val"已被删除，无法解析.

可能重要的是它是一个内部类.

但是

这个在构造函数参数中没有val/var"的类正在工作:

class BookInfo(convrMgr: ConversationMgr, id: String, queue: RequestQueue, queueTag:String) {val 标签 = "BookInfo"var 标题:字符串?="在里面 {val url = "https://api.douban.com/v2/book/$id"//从提供的 URL 请求字符串响应.val stringRequest = StringRequest(Request.Method.GET, url,Response.Listener{ 响应 ->Log.d(TAG + "响应", response.substring(0))//从字符串值解析 JSONval 解析器 = Parser()val jsonObj: JsonObject =parser.parse(StringBuilder(response.substring(0))) 作为 JsonObject//书籍属性的初始书名.title = jsonObj.string("title")Log.d(TAG + "书名", 书名)convrMgr.addNewMsg(title)},Response.ErrorListener { 错误 ->Log.e(TAG + "错误", error.toString()) })//在请求上设置标签.stringRequest.tag = queueTag//将请求添加到RequestQueue.queue.add(stringRequest)}

}

如果我在 "queue: RequestQueue" 之前添加 var/val，我会得到建议:

构造函数参数永远不会被用作较少的属性.此检查报告可以删除 'val' 或 'var' 的主构造函数参数.在主构造函数中不必要地使用 'val' 和 'var' 会消耗不必要的内存."

我只是对此感到困惑.

解决方案

当您在构造函数中编写 val/var 时，它会在类中声明一个属性.当你不写它时，它只是一个传递给主构造函数的参数，你可以在其中访问 init 块中的参数或使用它来初始化其他属性.例如，

class User(val id: Long, email: String) {val hasEmail = email.isNotBlank()//这里可以访问email在里面 {//电子邮件可以在这里访问}有趣的 getEmail(){//这里无法访问邮箱}}

<块引用>

构造函数参数从不用作属性

这个建议是说除了初始化之外你不要在适当的地方使用这个属性.因此，它建议您从类中删除此属性.

Right code:

class MainActHandler(val weakActivity: WeakReference<Activity>): Handler() {
    override fun handleMessage(msg: Message?) {
        val trueAct = weakActivity.get() ?: return
        if (msg?.what == ConversationMgr.MSG_WHAT_NEW_SENTENCE){
            val sentence = msg.obj as String?
            trueAct.conversation.text = sentence
        }
        super.handleMessage(msg)
    }
}

cannot be resolved code:

class MainActHandler(weakActivity: WeakReference<Activity>): Handler() {
    override fun handleMessage(msg: Message?) {
        val trueAct = weakActivity.get() ?: return
        if (msg?.what == ConversationMgr.MSG_WHAT_NEW_SENTENCE){
            val sentence = msg.obj as String?
            trueAct.conversation.text = sentence
        }
        super.handleMessage(msg)
    }
}

cannot be resolved code screenshot

The only difference is the "val" has been deleted and cannot be resolve.

Which might be important is that it's a inner class.

BUT

This one class without "val/var" in constructor parameter is working:

class BookInfo(convrMgr: ConversationMgr, id: String, queue: RequestQueue, queueTag:String) {

val TAG = "BookInfo"
var title: String? = ""

init {
    val url = "https://api.douban.com/v2/book/$id"
    // Request a string response from the provided URL.
    val stringRequest = StringRequest(Request.Method.GET, url,
            Response.Listener<String> { response ->
                Log.d(TAG + " Response", response.substring(0))
                // Parse JSON from String value
                val parser = Parser()
                val jsonObj: JsonObject =
                        parser.parse(StringBuilder(response.substring(0))) as JsonObject
                // Initial book title of book properties.
                title = jsonObj.string("title")
                Log.d(TAG + " Book title", title)
                convrMgr.addNewMsg(title)
            },
            Response.ErrorListener { error -> Log.e(TAG + " Error", error.toString()) })
    // Set the tag on the request.
    stringRequest.tag = queueTag
    // Add the request to the RequestQueue.
    queue.add(stringRequest)
}

}

And if I add var/val before "queue: RequestQueue", I'll get suggestion:

"Constructor parameter is never used as a property less. This inspection reports primary constructor parameters that can have 'val' or 'var' removed. Unnecessary usage of 'val' and 'var' in primary constructor consumes unnecessary memory."

I am just confused about it.

解决方案

When you write val/var within the constructor, it declares a property inside the class. When you do not write it, it is simply a parameter passed to the primary constructor, where you can access the parameters within the init block or use it to initialize other properties. For example,

class User(val id: Long, email: String) {
    val hasEmail = email.isNotBlank()    //email can be accessed here
    init {
        //email can be accessed here
    }

    fun getEmail(){
        //email can't be accessed here
    }
}

Constructor parameter is never used as a property

This suggestion is saying that you do not use this property in place apart from the initialization. So, it suggests you to remove this property from the class.

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