如何将数据绑定变量作为 xml 属性传递给自定义视图

Question

我正在尝试创建一个简单的表单布局，它使用包含 LiveData 的 ViewModel 绑定到每个表单字段，以便输入的数据可以通过配置更改和后退导航（在多步表单的情况下）保持不变。此外，我有一个自定义表单字段视图，它有自己的布局 xml 来进行一些必需的处理。但是，我无法通过表单布局传递数据以绑定到自定义视图布局。

这是我的 ViewModel、Fragment、Field 对象及其布局的简化版本：

class FormViewModel : ViewModel() {
    @Bindable val email = MutableLiveData<String>().apply { value = "" }
}

class FormFragment : BaseFragment() {

    private val formViewModel: FormViewModel by lazy {
        ViewModelProviders.of(requireActivity()).get(FormViewModel::class.java)
    }

    override fun onCreateView(inflater: LayoutInflater, 
                              container: ViewGroup?, 
                              savedInstanceState: Bundle?) : View? =
        (DataBindingUtil
            .inflate(inflater, R.layout.fragment_form, container, false) as FragmentFormBinding)
            .apply {
                lifecycleOwner = requireActivity()
                viewModel = formViewModel
            }
            .root
}

class Field @JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0)
    : LinearLayout(context, attrs, defStyleAttr) {

    @Bindable var fieldData: MutableLiveData<String> = MutableLiveData<String>().apply { value = "" }

    init {
        val inflater = context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) as LayoutInflater
        (DataBindingUtil
            .inflate(inflater, R.layout.field, this, true) as FieldBinding)
            .apply { fieldData = this@Field.fieldData }
            .root
    }
}

字段.xml

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

    <data>
        <variable
                name="fieldData"
                type="androidx.lifecycle.MutableLiveData&lt;String>"/>
    </data>

    <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">
        <com.google.android.material.textfield.TextInputLayout
                android:id="@+id/inputField"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                tools:hint="Email">

            <com.google.android.material.textfield.TextInputEditText
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:hint="Email"
                    android:text="@={ fieldData }" />

        </com.google.android.material.textfield.TextInputLayout>
    </LinearLayout>
</layout>

我想要做的是将实时数据从 ViewModel 绑定到表单字段，方法是将其作为属性传递给表单 xml 中的自定义表单字段的实例，如下所示：

<?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">
    <data>
        <variable name="viewModel"
                type="packageName.FormViewModel"/>
    </data>

    <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:padding="32dp">

        <packageName.Field
                android:id="@+id/emailField"
                app:fieldData="@{ viewModel.email }"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="60dp" />

    </LinearLayout>
</layout>

但是，当我尝试此操作时，出现以下错误：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: packageName, PID: 27543
    java.lang.RuntimeException: Failed to call observer method
        at androidx.lifecycle.ClassesInfoCache$MethodReference.invokeCallback(ClassesInfoCache.java:226)
        at androidx.lifecycle.ClassesInfoCache$CallbackInfo.invokeMethodsForEvent(ClassesInfoCache.java:194)
        at androidx.lifecycle.ClassesInfoCache$CallbackInfo.invokeCallbacks(ClassesInfoCache.java:185)
        at androidx.lifecycle.ReflectiveGenericLifecycleObserver.onStateChanged(ReflectiveGenericLifecycleObserver.java:37)
        at androidx.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:361)
        at androidx.lifecycle.LifecycleRegistry.addObserver(LifecycleRegistry.java:188)
        at androidx.databinding.ViewDataBinding.setLifecycleOwner(ViewDataBinding.java:394)
        at packageName.FormFragment.onCreateView(FormFragment.kt:24)
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2669)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1321)
        at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2515)
        at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:2290)
        at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:2246)
        at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:2143)
        at androidx.fragment.app.FragmentManager$3.run(FragmentManager.java:417)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
     Caused by: java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter <set-?>
        at packageName.Field.setFieldData(Field.kt)
        at packageName.databinding.FragmentFormBindingImpl.executeBindings(FragmentFormBindingImpl.java:127)
        at androidx.databinding.ViewDataBinding.executeBindingsInternal(ViewDataBinding.java:472)
        at androidx.databinding.ViewDataBinding.executePendingBindings(ViewDataBinding.java:444)
        at androidx.databinding.ViewDataBinding$OnStartListener.onStart(ViewDataBinding.java:1685)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at androidx.lifecycle.ClassesInfoCache$MethodReference.invokeCallback(ClassesInfoCache.java:216)
        at androidx.lifecycle.ClassesInfoCache$CallbackInfo.invokeMethodsForEvent(ClassesInfoCache.java:194) 
        at androidx.lifecycle.ClassesInfoCache$CallbackInfo.invokeCallbacks(ClassesInfoCache.java:185) 
        at androidx.lifecycle.ReflectiveGenericLifecycleObserver.onStateChanged(ReflectiveGenericLifecycleObserver.java:37) 
        at androidx.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:361) 
        at androidx.lifecycle.LifecycleRegistry.addObserver(LifecycleRegistry.java:188) 
        at androidx.databinding.ViewDataBinding.setLifecycleOwner(ViewDataBinding.java:394) 
        at packageName.FormFragment.onCreateView(FormFragment.kt:24) 
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2669) 
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1321) 
        at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2515) 
        at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:2290) 
        at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:2246) 
        at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:2143) 
        at androidx.fragment.app.FragmentManager$3.run(FragmentManager.java:417) 
        at android.os.Handler.handleCallback(Handler.java:739) 
        at android.os.Handler.dispatchMessage(Handler.java:95) 
        at android.os.Looper.loop(Looper.java:135) 
        at android.app.ActivityThread.main(ActivityThread.java:5221) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at java.lang.reflect.Method.invoke(Method.java:372) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 

作为替代方案，这可以通过包含字段布局来轻松实现，如下所示，但在这种情况下，我无法在 Field 对象中进行任何我想做的设置

<?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">

    <data>
        <variable name="viewModel"
                type="packageName.FormViewModel"/>
    </data>

    <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:padding="32dp">

        <include
                layout="@layout/field"
                android:id="@+id/emailField"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                app:fieldData="@{ viewModel.email }" />
    </LinearLayout>
</layout>

我猜这与 xml 膨胀和生成数据绑定变量的顺序有关，但我在调试时遇到了困难

编辑：我只是注意到我没有在 Field 数据绑定中设置生命周期所有者但是即使我没有在 Field 类中膨胀布局也会出现问题

Answer 1

尝试使用@BindingAdapter("...")而不是 @Bindable注释。

我制作了一个示例存储库来反映并解决您的问题，也许可以看看。

https://github.com/phamtdat/BindingAdapterSample