Two-way data binding

Using one-way data binding, you can set a value on an attribute and set a listener that reacts to a change in that attribute:

<CheckBox
    android:id="@+id/rememberMeCheckBox"
    android:checked="@{viewmodel.rememberMe}"
    android:onCheckedChanged="@{viewmodel.rememberMeChanged}"
/>

Two-way data binding provides a shortcut to this process:

<CheckBox
    android:id="@+id/rememberMeCheckBox"
    android:checked="@={viewmodel.rememberMe}"
/>

The @={} notation, which importantly includes the "=" sign, receives data changes to the property and listen to user updates at the same time.

In order to react to changes in the backing data, you can make your layout variable an implementation of Observable, usually BaseObservable, and use a @Bindable annotation, as shown in the following code snippet:

class LoginViewModel : BaseObservable {
    // val data = ...

    @Bindable
    fun getRememberMe(): Boolean {
        return data.rememberMe
    }

    fun setRememberMe(value: Boolean) {
        // Avoids infinite loops.
        if (data.rememberMe != value) {
            data.rememberMe = value

            // React to the change.
            saveData()

            // Notify observers of a new value.
            notifyPropertyChanged(BR.remember_me)
        }
    }
}

public class LoginViewModel extends BaseObservable {
    // private Model data = ...

    @Bindable
    public Boolean getRememberMe() {
        return data.rememberMe;
    }

    public void setRememberMe(Boolean value) {
        // Avoids infinite loops.
        if (data.rememberMe != value) {
            data.rememberMe = value;

            // React to the change.
            saveData();

            // Notify observers of a new value.
            notifyPropertyChanged(BR.remember_me);
        }
    }
}

Because the bindable property's getter method is called getRememberMe(), the property's corresponding setter method automatically uses the name setRememberMe().

For more information on using BaseObservable and @Bindable, see Work with observable data objects.

Two-way data binding using custom attributes

The platform provides two-way data binding implementations for the most common two-way attributes and change listeners, which you can use as part of your app. If you want to use two-way data binding with custom attributes, you need to work with the @InverseBindingAdapter and @InverseBindingMethod annotations.

For example, if you want to enable two-way data binding on a "time" attribute in a custom view called MyView, complete the following steps:

1. Annotate the method that sets the initial value and updates when the value changes using @BindingAdapter:

   Kotlin

   @BindingAdapter("time")
   @JvmStatic fun setTime(view: MyView, newValue: Time) {
       // Important to break potential infinite loops.