Kotlin 中未從一種狀態傳送到另一種狀態的變量的值

Question

初學者問題 - 知道為什么 SELECT 中的 lemonSize 值在切換檸檬水狀態時沒有傳遞到 SQUEEZE 嗎？ 任何幫助是極大的贊賞 ：）

該應用程序/代碼是 Google Kotlin 課程介紹的一部分，該課程用於創建一個在屏幕之間旋轉的檸檬水應用程序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-檸檬水#0

這就是我認為問題所在（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，您正在創建一個名為lemonSize的頂級變量，該變量已初始化為-1 ：

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

該 var 是一個Int ，它是通過您為其分配一個整數值這一事實推斷出來的。 這應該是你現在檸檬的大小，對吧？

但是在您的SELECT分支中，您正在執行以下操作：

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

這里有兩個問題 - 首先，您正在創建一個名為lemonSize的新變量，它只存在於這些花括號的范圍內，即當您的狀態為SELECT時執行的代碼。 無論您對該變量做什么，一旦您退出該代碼塊，它就會消失。

因此，當您到達SQUEEZE分支時，當它指的是lemonSize時，它​​正在查看頂級分支，它仍然是-1 。 您在SELECT中創建的lemonSize變量已經消失，並且無論如何對SQUEEZE塊都不可見。

---

因此，您應該在SELECT中做的是更改頂級“lemonSize”變量的值（無論如何，這就是您為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()

這就是您遇到其他問題的地方-這不起作用，因為您正在調用LemonTree() ，它是LemonTree類的構造函數。 您正在創建一個LemonTree對象並將其分配給lemonSize變量——正如我們之前看到的，它已被推斷為持有一個Int類型。 而且LemonTree不是Int ！ 它不能去那里

您當前的代碼運行良好，因為您基本上是在創建一個新的局部變量，它恰好被稱為lemonSize ，並為其分配了一個LemonTree - 所以您創建了一個具有推斷LemonTree類型的變量。 而且你不用它做任何事情，所以它從來都不是問題（盡管在你的日志中你可以看到它打印出來很奇怪 - com.example.lemonade.LemonTree@5c31497意味着它是一個具有參考 ID 的LemonTree對象，不是一個號碼！）

---

所以就像那行上面的評論所說，你真的需要調用pick()來獲取你的lemonSize 。 您可以在此處創建一個新的LemonTree對象以從中進行選擇——但您已經將一個對象放入頂級變量lemonTree中。 您可能只是想使用它，對嗎？

最后，您可能想要這樣做：

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

現在你的頂級lemonSize狀態變量有了一個新的值，一切都可以看到。

希望這是有道理的！ 這里有一些概念需要熟悉