Android 数据绑定：BindingAdapter 与 InverseBindingAdapter

Question

两者有什么区别？

什么时候应该使用其中之一？

当我定义一个 BindingAdapter 时，我必须创建一个反函数吗？

Answer 1

引用我自己，来自 The Busy Coder's Guide to Android Development:

Two-way binding works well in cases where the way you store the data in the models lines up well with the getters and setters of the associated widget. A boolean field in the model works well with the checked property of a CompoundButton like a Switch, as CompoundButton has an isChecked() method returning a boolean and a setChecked() accepting a boolean.

A BindingAdapter allows you to create other mappings between data types and properties, but only for the classic model->view binding. To accomplish the same thing in the reverse direction, you wind up creating an InverseBindingAdapter. As the name suggests, this serves the same basic role as a BindingAdapter, but in the inverse direction, taking data from the widget and preparing it for the model using custom code. Here, the "preparing it for the model" means converting it into a suitable data type for a setter, Observable field, etc. for your model.

This is fairly unusual.

The example used in some places is "what if I want to tie a float to an EditText?". The InverseBindingAdapter would look something like this:

@InverseBindingAdapter(attribute = "android:text")
public static float getFloat(EditText et) {
  try {
    return(Float.parseFloat(et.getText().toString()));
  }
  catch (NumberFormatException e) {
    return(0.0f); // because, um, what else can we do?
  }
}

The problem is if the user types in something that is not a valid floating-point number, like snicklefritz. parseFloat() will fail with a NumberFormatException. You should let the user know that their data entry was invalid. However, two-way data binding does not support this, with a default value (e.g., 0.0f) being handed to the model instead.

所以，回答你的问题：

What is the difference between the two?

BindingAdapter 有助于填充属性，其中数据类型和 View setter 不是数据绑定知道如何自行处理的东西。

InverseBindingAdapter 有助于在双向绑定中填充视图模型，其中数据类型和 View 吸气剂不是数据绑定知道如何自行处理的东西。

When should use one or the other? When I define a BindingAdapter do I have to create an inverse?

当您想要的数据类型（例如 float）不是数据绑定必须知道如何填充到小部件属性（例如，android:text 在 EditText) 上，但你还是想绑定它。

如果你这样做，和你想做双向绑定，用户在 UI 中的更改会自动更新你的视图模型，很可能你将需要一个匹配的 InverseBindingAdapter 来从属性（例如 EditText 的文本）转换为所需的数据类型（例如 float）。

Android 数据绑定：BindingAdapter 与 InverseBindingAdapter

Android databinding: BindingAdapter vs InverseBindingAdapter

android

android-databinding