空 Android 数据绑定

Question

While studying android data-binding, my colleague told me that the android data-binding can be null in few cases, also when one layout includes another layout with data-binding, the generated data-binding file annotate the binding of another layout as @Nullable. My question is can data-binding be null and if yes when?

Answer 1

Data binding is a blueprint. A class that is created at compile time when it sees the "layout" tag. The blueprint class will be named LayoutNameBinding Pascal Case.

Just like any other class, it is non-existent until you reserve memory for it and new it up.

So when you use the data binding utility on the onCreate it creates the class and you can store that in a local variable for using later.

example:

   override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = DataBindingUtil.setContentView(this, R.layout.activity_main)

    binding.activity = this
    binding.iBindingRecyclerView = this
    binding.navHeader?.activity = this

    setupNavigationDrawer()
}

"layout root xml files that have other layout root xml files nested in them or named include layouts" will be added as classes inside the parent NameOfLayoutBinding class. These will not be null as they are auto generated at compile time so when you new up the parent, the children will exist.

So a databinding will not be null if you are referring to the auto generated class and if you are newing it up in your onCreate method appropriately.

Now a failed binding event due to a null object can happen if you didn't pass in the variable that you are binding to, but that is not the question you asked.

Hope that helps, if you meant something different, please digress.

Answer 2

Finally, after so many years found an explaination for this. In case you are manually executing the DataBinding.inflate function then the returned DataBinding won't be null.

For auto generated bindings (ie when you include a layout and binding is generated and assigned to a variable in the parent layout binding), the data binding can be null based on some situation.

Eg: Let's say we have layout as follow:

Portrait Mode layout: /src/res/layout/activity.xml

<LinearLayout ...>
    <include
      android:id="@+id/main_content"
      layout="@layout/main_content_layout"
    />

</LinearLayout>

And for Lanscape mode: /src/res/layout-land/activity.xml

<LinearLayout ...>
    <include
      android:id="@+id/sidebar"
      layout="@layout/sidebar_layout"
    />

    <include
      android:id="@+id/main_content"
      layout="@layout/main_content_layout"
    />

</LinearLayout>

Now here as the multiple layout files are for same purpose but with different configuration (lanscape mode and portrait mode) the Android DataBinding will generate ActivityBinding.java file. Now here the developer would need to access binding for both sidebar and main content using object of ActivityBinding.java class. As the sidebar is not present in portrait mode layout file, the binding file won't have any reference. Hence the reference for sidebar binding would be kept as Nullable.

Hence, for the layout files with same name for different configuration and with different view hierarchy, the inner binding object generated can have null value, due to which the data binding may have Nullable binding fields.