Kotlin属性访问语法如何用于Java类(即EditText setText)? [英] How does Kotlin property access syntax work for Java classes (i.e. EditText setText)?

问题描述

我正在尝试将我的Android项目切换到Kotlin.我有一个EditText(TextView的子类)，我想以编程方式设置提示和文本.提示按预期方式工作.但是，对于文本，如果尝试使用Kotlin setter语法进行操作，则会遇到类型不匹配的异常:

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

如果看一下声明，我们会发现从TextView继承的相同签名:

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)

我给人的印象是x.y = z是x.setY(z)的快捷方式，但是显然这种印象是错误的. setText()被当作普通方法而不是二传手，但是这两种方法之间的区别是什么使编译器的行为有所不同?我唯一能想到的是TextView具有mHint属性，但我认为情况并非如此.

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.

我不太了解的另一件事是，android.text.Editable是从哪里来的?没有相应的setText(Editable)方法，也没有这种类型的公共字段.

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.

推荐答案

为Java getter/setter对生成综合属性时，Kotlin首先会寻找getter.该吸气剂足以创建具有该吸气剂类型的合成属性.另一方面，如果仅显示设置器，则不会创建该属性.

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.

当二传手开始比赛时，财产创造变得更加困难.原因是getter和setter可能具有不同的类型.而且，可以在子类中覆盖获取器和/或设置器.

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.

在您的情况下，TextView类包含一个getter CharSequence getText()和一个setter void setText(CharSequence).如果您具有类型为TextView的变量，则您的代码可以正常工作.但是您有一个EditText类型的变量.并且EditText类包含重写的getter Editable getText()，这意味着您可以为EditText获取Editable并将Editable设置为EditText.因此，Kotlin合理地创建了类型为Editable的合成属性text. String类不是Editable，这就是为什么您不能将String实例分配给EditText类的text属性的原因.

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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