Android LiveData和DataBinding

Question

我遇到数据绑定问题

我的ViewModel包含两个属性。一个用于日期，另一个用于文本表示

class MyViewModel(context: Application): AndroidViewModel(context) {
    val dateValue = MutableLiveData<Date>()
    val dateText = MediatorLiveData<String>().apply {
        this.addSource(dateValue) {
            it?.let {
                this.value = DateHelper.DisplayFormat.format(it)
            }
        }
    }
}

layout包含EditText，它绑定到文本表示

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <data>
        <import type="android.view.View" />
        <variable name="viewModel" type=".MyViewModel" />
    </data>

    <LinearLayout>

        <EditText android:id="@+id/dateText"
            android:text="@{ viewModel.dateText }"
            android:focusable="false" android:hint="choose" />

    </LinearLayout>
</layout>

当我点击EditText时，我会显示设置dateValue

的datepicker

override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
    val listener = DatePickerDialog.OnDateSetListener { _: DatePicker, year: Int, month: Int, dayOfMonth: Int ->
        val result = Calendar.getInstance().apply {
            this[year, month] = dayOfMonth
        }
        viewModel.dateValue.value = result.time
    }
    val c = Calendar.getInstance()
    if (dateField.value != null) c.time = viewModel.dateValue.value
    return DatePickerDialog(activity!!, listener,
            c.get(Calendar.YEAR), c.get(Calendar.MONTH), c.get(Calendar.DAY_OF_MONTH))
}

但是当设置了日期时，EditText中的文本仍为空。 哪里有问题？是否应自动刷新EditText？