在 各个 Activity 之间传参 我们一般需要 使用 getIntent().get**Extra() 去获取上个页面的参数，代码比较重复，于是可以考虑像 Butterknife 那样子去简化 findViewById。
考虑到代码执行效率问题，使用 kotlinpoet 处理并生成 Kotlin 代码
代码如下：

//package com.android.extrainjector
interface IExtraInjector {
    fun inject()
}

class ExtrasInjector {
    companion object {
        @JvmStatic
        fun inject(injector: IExtraInjector) {
            injector.inject()
        }
    }
}

//注解声明
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.SOURCE)
public @interface Extra {
    String value() default "";
    String defaultValue() default "";
}

大致调用的时候 我们在 Activity 的 onCreate 里面调用

class MainActivity : AppCompatActivity() {

    @Extra("msg")
    var msg: String? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        ExtrasInjector.inject(MainActivity_ExtraInjector(this))
    }
}

MainActivity_ExtraInject 是 自动生成的 Kotlin 代码文件。

class MainActivity_ExtraInjector(target: Any) : IExtraInjector {
    var target: Any? = null
    init {
        this.target = target
    }

    override fun inject() {
        if (target is MainActivity) {
            val t  = target as MainActivity
            t.msg = t.getIntent().getStringExtra("msg")
        }
    }
}

新建一个 Java Lib 库，命名 compiler 存放 ExtraInjectorProcessor，然后记得在声明 ExtraInjectorProcessor

class ExtraInjectorProcessor : AbstractProcessor() {

    private var elementUtils : Elements? = null

    @Synchronized
    override fun init(processingEnv: ProcessingEnvironment) {
        super.init(processingEnv)
        elementUtils = processingEnv.elementUtils
    }

    override fun process(annotations: Set<TypeElement>, roundEnv: RoundEnvironment): Boolean {
        val elements = roundEnv
                .getElementsAnnotatedWith(Extra::class.java)
        val map = HashMap<String, MutableList<Element>>()
        category(map, elements)
        if (map.size > 0) {
            generateCode(map)
        }
        return true
    }

    private fun isSubClass(element: Element, type: String): Boolean{
        return processingEnv.typeUtils.isSubtype(element.asType(), elementUtils!!
                .getTypeElement(type).asType())
    }

    private fun category(map : HashMap<String, MutableList<Element>>, list: Set<Element>) {
        list.forEach {
            if (it.kind.isField) {
                val parent = it.enclosingElement
                if (isSubClass(parent, "android.app.Activity")) {
                    val key = "${(parent as TypeElement).asClassName().packageName}.${parent.simpleName}"
                    var value = map[key]
                    if (value == null) {
                        value = ArrayList()
                    }
                    value.add(it)
                    map[key] = value
                }
            }
        }
    }

    private fun generateCode(map : HashMap<String, MutableList<Element>>) {
        map.forEach { key, value ->
            val constructMethod = FunSpec.constructorBuilder()
                    .addParameter(ParameterSpec.builder("target", Any::class).build())
                    .addStatement("this.target = target")
                    .build()
            val parentElement = elementUtils!!.getTypeElement(key)
            val className = key.split(".").last()
            val loadMethodBuilder = FunSpec.builder("inject")
                    .addModifiers(KModifier.OVERRIDE)
                    .beginControlFlow("if (target is %T)", parentElement)
                    .addStatement("val t  = target as %T", parentElement)
            value.forEach {
                autowear(loadMethodBuilder, it)
            }
            loadMethodBuilder.endControlFlow()
            val codePackageName = elementUtils!!.getPackageOf(parentElement)
                    .qualifiedName.toString()
            val generateClassName = "${className}_ExtraInjector"
            val generateFileName = "${className}_ExtraInjector"
            val file = FileSpec.builder(codePackageName, generateFileName)
                    .addType(TypeSpec.classBuilder(generateClassName)
                            .addKdoc("Generated by ExtraInjectorProcessor. Do not edit it!\n")
                            .primaryConstructor(constructMethod)
                            .addSuperinterface(ClassName("com.android.extrainjector", "IExtraInjector"))
                            .addProperty(PropertySpec.varBuilder("target", Any::class.asTypeName().asNullable())
                                    .initializer("null")
                                    .build())
                            .addFunction(loadMethodBuilder.build())
                            .build())
                    .build()

            try {
                file.writeFile()
            } catch (e: Exception) {
                e.printStackTrace()
            }
        }
    }

    private fun autowear(method: FunSpec.Builder, element: Element) {
        val extra = element.getAnnotation(Extra::class.java)
        var name = extra.value
        val field = element.simpleName
        if (name.isEmpty())
            name = field.toString()
        var placeholder = ""
        var defaultValue = extra.defaultValue
        when {
            element.asType().toString() == "java.lang.String" -> placeholder = "String"
            isSubClass(element, "android.os.Parcelable") -> {
                placeholder = "Parcelable"
            }
            (element.asType().toString().contains("java.util.List")) -> {
                //method.addStatement("System.out.print(\$S)", "java.util.List")
                placeholder = "ParcelableArrayList"
            }
            else ->
                when(element.asType().kind.ordinal) {
                    TypeKind.INT.ordinal -> {
                        placeholder = "Int"
                        defaultValue = if (defaultValue.isEmpty())
                            ", 0"
                        else
                            ", $defaultValue"
                    }
                    TypeKind.BOOLEAN.ordinal -> {
                        placeholder = "Boolean"
                        defaultValue = if (defaultValue.isEmpty())
                            ", false"
                        else
                            ", $defaultValue"
                    }
                    TypeKind.DOUBLE.ordinal -> {
                        placeholder = "Double"
                        defaultValue = if (defaultValue.isEmpty())
                            ", 0"
                        else
                            ", $defaultValue"
                    }
                    TypeKind.FLOAT.ordinal -> {
                        placeholder = "Float"
                        defaultValue = if (defaultValue.isEmpty())
                            ", 0F"
                        else
                            ", $defaultValue"
                    }
                }
        }
        var statement = ""
        if (placeholder.isNotEmpty()) {
            statement = "t.$field = t.getIntent().get${placeholder}Extra(\"$name\" $defaultValue)"

        }
        method.addStatement(statement, element.asType())
    }

    override fun getSupportedAnnotationTypes(): Set<String> {
        return setOf(Extra::class.java.canonicalName)
    }

    override fun getSupportedSourceVersion(): SourceVersion {
        return SourceVersion.latestSupported()
    }

    private fun FileSpec.writeFile() {
        val kaptKotlinGeneratedDir = processingEnv.options["kapt.kotlin.generated"]
        val outputFile = File(kaptKotlinGeneratedDir).apply {
            mkdirs()
        }
        writeTo(outputFile.toPath())
    }

}

这里先将注解的地方分类，同个文件里面是一起生成的同个文件，然后 可以设置 default value 这里我直接用字符串代替，比较通用吧。
工程 build 之后应该就能看到生成的 Injector 文件了。
建议 对照生成的代码文件 跟 ExtraInjectorProcessor 里面的代码对比 应该更好体会理解。