問題描述

我正在嘗試將我的 Android 項目切換到 Kotlin.我有一個 EditText(TextView 的子類)，我想以編程方式為其設置提示和文本.提示按預期工作.但是，對于文本，如果我嘗試使用 Kotlin setter 語法進行操作，則會出現(xiàn)類型不匹配異常:

I'm trying to switch my Android project to Kotlin. I have an EditText (a subclass of TextView) for which I want to set a hint and text programmatically. The hint works as expected. For text, though, I'm getting a type mismatch exception if I try to do it using Kotlin setter syntax:

    val test = EditText(context)

    test.setHint("hint")    // Lint message: "Use property access syntax"
    test.hint = "hint"      // ok

    test.setText("text")    // ok (no lint message)
    test.text = "text"      // Type mismatch: inferred type is kotlin.String but android.text.Editable! was expected

如果我們查看聲明，我們會發(fā)現(xiàn)繼承自 TextView 的相同簽名:

If we look at the declaration, we'll find identical signatures inherited from TextView:

    public final void setHint(CharSequence hint)

    public final void setText(CharSequence text)

我的印象是 x.y = z 是 x.setY(z) 的快捷方式，但顯然這種印象是錯誤的.setText() 被視為普通方法而不是setter，但是這兩種方法有什么區(qū)別使編譯器表現(xiàn)不同呢?我唯一能想到的是 TextView 有一個 mHint 屬性，但我不認為可能是這種情況.

I had an impression that x.y = z was a shortcut for x.setY(z) but apparently that impression was wrong. setText() is treated as a normal method rather than a setter, but what's the difference between these two methods that makes the compiler behave differently? The only one I can think of is that TextView has an mHint property but I don't think it might be the case.

還有一點我不太明白的是，android.text.Editable 是從哪里來的?沒有對應的setText(Editable)方法，也沒有這種類型的公共字段.

Another thing I don't quite understand is, where does android.text.Editable come from? There is no corresponding setText(Editable) method, nor is there a public field of this type.

推薦答案

在為 Java getter/setter 對生成合成屬性時，Kotlin 首先查找 getter.getter 足以創(chuàng)建具有 getter 類型的合成屬性.另一方面，如果只有 setter 存在，則不會創(chuàng)建該屬性.

When generating a synthetic property for a Java getter/setter pair Kotlin first looks for a getter. The getter is enough to create a synthetic property with a type of the getter. On the other hand the property will not be created if only a setter presents.

當 setter 出現(xiàn)時，屬性創(chuàng)建變得更加困難.原因是 getter 和 setter 可能有不同的類型.此外，getter 和/或 setter 可以在子類中被覆蓋.

When a setter comes into play property creation becomes more difficult. The reason is that the getter and the setter may have different type. Moreover, the getter and/or the setter may be overridden in a subclass.

在您的情況下，TextView 類包含一個 getter CharSequence getText() 和一個 setter void setText(CharSequence).如果你有一個 TextView 類型的變量，你的代碼就可以正常工作.但是你有一個 EditText 類型的變量.并且 EditText 類包含一個重寫的 getter Editable getText()，這意味著您可以為 EditTextEditablecode> 并將 Editable 設置為 EditText.因此，Kotlin 合理地創(chuàng)建了 Editable 類型的合成屬性 text.String 類不是 Editable，這就是為什么不能將 String 實例分配給 text 屬性的原因code>EditText 類.

In your case the TextView class contains a getter CharSequence getText() and a setter void setText(CharSequence). If you had a variable of type TextView your code would work fine. But you have a variable of type EditText. And the EditText class contains an overridden getter Editable getText(), which means that you can get an Editable for an EditText and set an Editable to an EditText. Therefore, Kotlin reasonably creates a synthetic property text of type Editable. The String class is not Editable, that's why you cannot assign a String instance to the text property of the EditText class.

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