Kotlin 中未从一种状态传送到另一种状态的变量的值

Question

初学者问题 - 知道为什么 SELECT 中的 lemonSize 值在切换柠檬水状态时没有传递到 SQUEEZE 吗？ 任何帮助是极大的赞赏 ：）

该应用程序/代码是 Google Kotlin 课程介绍的一部分，该课程用于创建一个在屏幕之间旋转的柠檬水应用程序https://developer.android.com/codelabs/basic-android-kotlin-training-project-lemonade?continue= https%3A%2F%2Fdeveloper.android.com%2Fcourses%2Fpathways%2Fandroid-basics-kotlin-four%23codelab-https%3A%2F%2Fdeveloper.android.com%2Fcodelabs%2Fbasic-android-kotlin-training-project-柠檬水#0

这就是我认为问题所在（我在下面附上了日志）：

    // Default lemonSize to -1
    private var lemonSize = -1

    // Default the squeezeCount to -1
    private var squeezeCount = -1

    private var lemonTree = LemonTree()


when (lemonadeState) {
            SELECT -> {

                Log.d(TAG, "lemonadeState at beginning of SELECT *should be SELECT* lemonadeState = $lemonadeState")


                // Set the lemonSize (the number of squeezes needed) by calling the pick() method
                var lemonSize = LemonTree()//UPDATE
                Log.d(TAG, "randomly generated lemonSize using LemonTree -> lemonSize = $lemonSize")

                // Setting the squeezeCount (the number of times the user has squeezed the lemon) to 0.
                squeezeCount = 0
                Log.d(TAG, " setting squeezeCount to 0 -> squeezeCount = $squeezeCount")

                // Transition to the SQUEEZE state
                lemonadeState = SQUEEZE
                Log.d(TAG, "lemonadeState at bottom of SELECT *should be SQUEEZE* lemonadeState = $lemonadeState")
            }


            // TODO: When the image is clicked in the SQUEEZE state the squeezeCount needs to be
            //  INCREASED by 1 and lemonSize needs to be DECREASED by 1.
            //  - If the lemonSize has reached 0, it has been juiced and the state should become DRINK
            //  - Additionally, lemonSize is no longer relevant and should be set to -1
            SQUEEZE -> {


                Log.d(TAG, "at top of SQUEEZE *lemonSize should be 2-4* lemonSize = $lemonSize")
                Log.d(TAG, "at top of SQUEEZE *lemonadeState should be SQUEEZE* lemonadeState = $lemonadeState")

                //squeezeCount needs to be INCREASED by 1
                squeezeCount += 1

                //lemonSize needs to be DECREASED by 1.
                Log.d(TAG, "decreasing lemonSize by 1")
                Log.d(TAG, "lemonSize before decreasing by 1 lemonSize = $lemonSize")
                lemonSize = lemonSize - 1
                Log.d(TAG, "lemonSize after decreasing by 1 lemonSize = $lemonSize")

                //lemonSize = 0//REMOVE once done with testing
                //Log.d(TAG, "lemonSizeHere = $lemonSize ** REMOVE")

                //If the lemonSize has reached 0, it has been juiced and the state should become DRINK
                if (lemonSize == 0) {
                    lemonadeState = DRINK
                    //lemonSize = -1
                    Log.d(TAG, "lemonadeState when lemonSize == 0 lemonadeState = $lemonadeState")
                }
            }

/**
 * A Lemon tree class with a method to "pick" a lemon. The "size" of the lemon is randomized
 * and determines how many times a lemon needs to be squeezed before you get lemonade.
 */
class LemonTree {
    fun pick(): Int {
        return (2..4).random()
    }
}

日志：

 2022-06-17 16:20:46.837 8549-8578/com.example.lemonade W/OpenGLRenderer: Failed to initialize 101010-2 format, error = EGL_SUCCESS
    2022-06-17 16:20:49.788 8549-8549/com.example.lemonade D/MainActivity: lemonadeState at beginning of SELECT *should be SELECT* lemonadeState = select
    2022-06-17 16:20:49.789 8549-8549/com.example.lemonade D/MainActivity: randomly generated lemonSize using LemonTree -> lemonSize = com.example.lemonade.LemonTree@5c31497
    2022-06-17 16:20:49.790 8549-8549/com.example.lemonade D/MainActivity:  setting squeezeCount to 0 -> squeezeCount = 0
    2022-06-17 16:20:49.793 8549-8549/com.example.lemonade D/MainActivity: lemonadeState at bottom of SELECT *should be SQUEEZE* lemonadeState = squeeze
    2022-06-17 16:21:04.177 8549-8549/com.example.lemonade D/MainActivity: at top of SQUEEZE *lemonSize should be 2-4* lemonSize = -1
    2022-06-17 16:21:04.179 8549-8549/com.example.lemonade D/MainActivity: at top of SQUEEZE *lemonadeState should be SQUEEZE* lemonadeState = squeeze
    2022-06-17 16:21:04.184 8549-8549/com.example.lemonade D/MainActivity: lemonSize before decreasing by 1 lemonSize = -1
    2022-06-17 16:21:04.186 8549-8549/com.example.lemonade D/MainActivity: lemonSize after decreasing by 1 lemonSize = -2

Answer 1

因此，您正在创建一个名为lemonSize的顶级变量，该变量已初始化为-1 ：

// Default lemonSize to -1
private var lemonSize = -1

该 var 是一个Int ，它是通过您为其分配一个整数值这一事实推断出来的。 这应该是你现在柠檬的大小，对吧？

但是在您的SELECT分支中，您正在执行以下操作：

// Set the lemonSize (the number of squeezes needed) by calling the pick() method
var lemonSize = LemonTree()//UPDATE

这里有两个问题 - 首先，您正在创建一个名为lemonSize的新变量，它只存在于这些花括号的范围内，即当您的状态为SELECT时执行的代码。 无论您对该变量做什么，一旦您退出该代码块，它就会消失。

因此，当您到达SQUEEZE分支时，当它指的是lemonSize时，它​​正在查看顶级分支，它仍然是-1 。 您在SELECT中创建的lemonSize变量已经消失，并且无论如何对SQUEEZE块都不可见。

---

因此，您应该在SELECT中做的是更改顶级“lemonSize”变量的值（无论如何，这就是您为squeezeCount和lemonadeState所做的）：

// no 'var' this time, we're setting the top-level one
// THIS LINE WON'T WORK THOUGH! see below
lemonSize = LemonTree()

这就是您遇到其他问题的地方-这不起作用，因为您正在调用LemonTree() ，它是LemonTree类的构造函数。 您正在创建一个LemonTree对象并将其分配给lemonSize变量——正如我们之前看到的，它已被推断为持有一个Int类型。 而且LemonTree不是Int ！ 它不能去那里

您当前的代码运行良好，因为您基本上是在创建一个新的局部变量，它恰好被称为lemonSize ，并为其分配了一个LemonTree - 所以您创建了一个具有推断LemonTree类型的变量。 而且你不用它做任何事情，所以它从来都不是问题（尽管在你的日志中你可以看到它打印出来很奇怪 - com.example.lemonade.LemonTree@5c31497意味着它是一个具有参考 ID 的LemonTree对象，不是一个号码！）

---

所以就像那行上面的评论所说，你真的需要调用pick()来获取你的lemonSize 。 您可以在此处创建一个新的LemonTree对象以从中进行选择——但您已经将一个对象放入顶级变量lemonTree中。 您可能只是想使用它，对吗？

最后，您可能想要这样做：

SELECT -> {
    // pick a new current lemon and record its size
    lemonSize = lemonTree.pick()
    ...
}

现在你的顶级lemonSize状态变量有了一个新的值，一切都可以看到。

希望这是有道理的！ 这里有一些概念需要熟悉