Kotlinの拡張関数は当然データバインディングでは使えない

## 拡張関数

```java:model/User.java
public class User extends RealmObject {
  @PrimaryKey public long id;
  public String name;
}
```

みたいなどっかの誰かが作ったJavaのモデルクラスに

```kotlin:UserExtension.kt
fun User.displayName(): String {
  return if (TextUtils.isEmpty(name)) "名無しの権兵衛" : name
}
```

Kotlinで拡張メソッドを生やす。

## 拡張関数をデータバインディングで使・・・えない

```xml:res/layout/activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<layout>
    <data>
        <variable
            name="lastUser"
            type="io.github.yusukeiwaki.kotlindatabindingplayground.User"/>
    </data>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal">
            <EditText
                android:id="@+id/editor_username"
                android:layout_width="0px"
                android:layout_height="wrap_content"
                android:layout_weight="1"/>
            <Button
                android:id="@+id/btn_add_user"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />
        </LinearLayout>
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{lastUser.displayName()}"/>
    </LinearLayout>
</layout>
```

`android:text="@{lastUser.displayName()}"` とやろうとすると、

```
Error:java.lang.IllegalStateException: failed to analyze: android.databinding.tool.util.LoggedErrorException: Found data binding errors.
****/ data binding error ****msg:cannot find method displayName() in class io.github.yusukeiwaki.kotlindatabindingplayground.User file:/Users/yi01/AndroidStudioProjects/KotlinDatabindingPlayground/app/src/main/res/layout/activity_main.xml loc:29:28 - 29:49 ****\ data binding error ****

	at org.jetbrains.kotlin.analyzer.AnalysisResult.throwIfError(AnalysisResult.kt:57)
	at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.compileModules(KotlinToJVMBytecodeCompiler.kt:137)
	at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:158)
	at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:61)
	at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.java:107)
	at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.java:51)
	at org.jetbrains.kotlin.cli.common.CLITool.exec(CLITool.kt:92)
	at org.jetbrains.kotlin.daemon.CompileServiceImpl$compile$1$2.invoke(CompileServiceImpl.kt:386)
	at org.jetbrains.kotlin.daemon.CompileServiceImpl$compile$1$2.invoke(CompileServiceImpl.kt:96)
	at org.jetbrains.kotlin.daemon.CompileServiceImpl$doCompile$$inlined$ifAlive$lambda$2.invoke(CompileServiceImpl.kt:892)
	at org.jetbrains.kotlin.daemon.CompileServiceImpl$doCompile$$inlined$ifAlive$lambda$2.invoke(CompileServiceImpl.kt:96)
	at org.jetbrains.kotlin.daemon.common.DummyProfiler.withMeasure(PerfUtils.kt:137)
	at org.jetbrains.kotlin.daemon.CompileServiceImpl.checkedCompile(CompileServiceImpl.kt:919)
	at org.jetbrains.kotlin.daemon.CompileServiceImpl.doCompile(CompileServiceImpl.kt:891)
	at org.jetbrains.kotlin.daemon.CompileServiceImpl.compile(CompileServiceImpl.kt:385)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:346)
	at sun.rmi.transport.Transport$1.run(Transport.java:200)
	at sun.rmi.transport.Transport$1.run(Transport.java:197)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.rmi.transport.Transport.serviceCall(Transport.java:196)
	at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:568)
	at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:826)
	at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:683)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)
Caused by: android.databinding.tool.util.LoggedErrorException: Found data binding errors.
****/ data binding error ****msg:cannot find method displayName() in class io.github.yusukeiwaki.kotlindatabindingplayground.User file:/Users/yi01/AndroidStudioProjects/KotlinDatabindingPlayground/app/src/main/res/layout/activity_main.xml loc:29:28 - 29:49 ****\ data binding error ****

	at android.databinding.tool.processing.Scope.assertNoError(Scope.java:112)
	at android.databinding.annotationprocessor.ProcessDataBinding.process(ProcessDataBinding.java:91)
	at com.sun.tools.javac.processing.JavacProcessingEnvironment.callProcessor(JavacProcessingEnvironment.java:794)
	at com.sun.tools.javac.processing.JavacProcessingEnvironment.access$200(JavacProcessingEnvironment.java:91)
	at com.sun.tools.javac.processing.JavacProcessingEnvironment$DiscoveredProcessors$ProcessorStateIterator.runContributingProcs(JavacProcessingEnvironment.java:627)
	at com.sun.tools.javac.processing.JavacProcessingEnvironment$Round.run(JavacProcessingEnvironment.java:1033)
	at com.sun.tools.javac.processing.JavacProcessingEnvironment.doProcessing(JavacProcessingEnvironment.java:1198)
	at com.sun.tools.javac.main.JavaCompiler.processAnnotations(JavaCompiler.java:1170)
	at com.sun.tools.javac.main.JavaCompiler.processAnnotations(JavaCompiler.java:1068)
	at org.jetbrains.kotlin.kapt3.AnnotationProcessingKt.doAnnotationProcessing(annotationProcessing.kt:73)
	at org.jetbrains.kotlin.kapt3.AnnotationProcessingKt.doAnnotationProcessing$default(annotationProcessing.kt:42)
	at org.jetbrains.kotlin.kapt3.AbstractKapt3Extension.runAnnotationProcessing(Kapt3Extension.kt:205)
	at org.jetbrains.kotlin.kapt3.AbstractKapt3Extension.analysisCompleted(Kapt3Extension.kt:166)
	at org.jetbrains.kotlin.kapt3.ClasspathBasedKapt3Extension.analysisCompleted(Kapt3Extension.kt:82)
	at org.jetbrains.kotlin.cli.jvm.compiler.TopDownAnalyzerFacadeForJVM$analyzeFilesWithJavaIntegration$2.invoke(TopDownAnalyzerFacadeForJVM.kt:96)
	at org.jetbrains.kotlin.cli.jvm.compiler.TopDownAnalyzerFacadeForJVM.analyzeFilesWithJavaIntegration(TopDownAnalyzerFacadeForJVM.kt:106)
	at org.jetbrains.kotlin.cli.jvm.compiler.TopDownAnalyzerFacadeForJVM.analyzeFilesWithJavaIntegration$default(TopDownAnalyzerFacadeForJVM.kt:83)
	at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler$analyze$1.invoke(KotlinToJVMBytecodeCompiler.kt:376)
	at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler$analyze$1.invoke(KotlinToJVMBytecodeCompiler.kt:67)
	at org.jetbrains.kotlin.cli.common.messages.AnalyzerWithCompilerReport.analyzeAndReport(AnalyzerWithCompilerReport.kt:96)
	at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.analyze(KotlinToJVMBytecodeCompiler.kt:367)
	at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.compileModules(KotlinToJVMBytecodeCompiler.kt:132)
	... 30 more
```

そんなメソッドないぞー！って怒られる

## どーすりゃええの？

Kotlinで定義したバインディングアダプタを使うか、Kotlinで定義したカスタムビューを使えば、拡張関数を使える

```kotlin:widgets/UserNameTextView.kt
class UserNameTextView : TextView {
    constructor(context: Context) : super(context) {}

    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs) {}

    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr) {}

    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int, defStyleRes: Int) : super(context, attrs, defStyleAttr, defStyleRes) {}

    fun setUser(user: User?) {
        setText(user?.displayName() ?: "-")
    }
}
```

```xml:res/layout/activity_main.xml
    ...

    <io.github.yusukeiwaki.kotlindatabindingplayground.UserNameTextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:user="@{lastUser}"/>

    ...
```

## まとめ

そもそもデータバインディングで使うようなものは拡張関数なんかで定義するべきではない！