“迟到"或“懒惰"定义全局 android.widget var/val 时

问题描述

定义全局 android.widget 变量时，例如TextView，使用lateinit还是by lazy更可取?我最初认为使用 by lazy 会更受欢迎，因为它是不可变的，但我不完全确定.

When defining a global android.widget variable, e.g. TextView, is it preferable to use lateinit or by lazy? I initially thought using by lazy would be preferred as its immutable but I'm not entirely sure.

by lazy 例子:

class MainActivity: AppCompatActivity() {

    val helloWorldTextView by lazy { findViewById(R.id.helloWorldTextView) as TextView }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        updateTextView(helloWorldTextView)
    }

    fun updateTextView(tv: TextView?) {
        tv?.setText("Hello?")
    }
}

lateinit 示例:

class MainActivity: AppCompatActivity() {

    lateinit var helloWorldTextView: TextView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        helloWorldTextView = findViewById(R.id.helloWorldTextView) as TextView
        updateTextView(helloWorldTextView)
    }

    fun updateTextView(tv: TextView?) {
        tv?.setText("Hello?")
    }
}

在定义全局 android.widget var/val 时，使用一个比另一个有什么好处吗?使用 by lazy 定义 android.widget val 有什么陷阱吗?决定是否仅基于您想要可变值还是不可变值?

Are there any benefits of using one over the other when defining a global android.widget var/val? Are there any pitfalls with using by lazy to define a android.widget val? Is the decision just based on whether you want a mutable or immutable value?

推荐答案

by lazy 有一个陷阱.小部件属性将是只读的，因此技术上是最终的(在 Java 术语中).但是没有文件保证 onCreate() 只为一个实例调用一次.findViewById() 也可以返回 null.

There's one pitfall with by lazy. The widget property would be read-only and therefore technically final (in Java terms). But there's no documented guarantee that onCreate() is called only once for an instance. Also findViewById() could return null.

所以最好使用 lateinit，你会得到一个异常，告诉你 val 是否在 onCreate() 之前使用.

So using lateinit is preferable and you'll get an exception to tell you if the val was used before onCreate().

第三种可能性是 Android 综合属性.那么你根本不需要担心变量.

A third possibility would be Android synthetic properties. Then you don't need to worry about the variables at all.

import kotlinx.android.synthetic.main.activity_main.*

helloWorldTextView.text = "Hello?"

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