"lateinit"或“偷懒"定义全局android.widget var/val时

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问题描述

定义全局android.widget变量时，例如TextView，最好使用lateinit或by lazy?起初我以为使用by lazy是首选，因为它是不可变的，但我不确定.

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示例:

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时使用一个使用另一个是否有好处?使用by lazy定义android.widget val是否有任何陷阱?决定只是基于您想要可变还是不变的值?

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. widget属性将是只读的，因此是技术上的最终版本(用Java术语).但是，没有记录保证onCreate()对于一个实例仅被调用一次.同样，findViewById()可能返回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.

因此，最好使用lateinit，并且会告诉您是否在onCreate()之前使用了val.

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.然后，您完全不必担心变量.

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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