Kotlin 优雅的接口实现

1. 日常遇到的冗余的接口方法实现

日常开发中，经常会要实现接口，但是很多场景中，只需要用到其中一两个方法，例如 ActivityLifecycleCallbacks，它有很多个接口需要实现，但是很多时候我们只需要用到其中的一两个

    val myActivityLifecycleCallbacks = object : Application.ActivityLifecycleCallbacks  {/*** 例如我们只需要监听 Activity 的创建和销毁，那么 onActivityStarted， onActivityResumed， onActivityPaused* onActivityStopped，onActivityStopped，onActivitySaveInstanceState 这 6 个方法是完全没必要是做实现的，* 即使是空实现*/override fun onActivityCreated(activity: Activity, savedInstanceState: Bundle?) {TODO("Not yet implemented")}override fun onActivityStarted(activity: Activity) {TODO("Not yet implemented")}override fun onActivityResumed(activity: Activity) {TODO("Not yet implemented")}override fun onActivityPaused(activity: Activity) {TODO("Not yet implemented")}override fun onActivityStopped(activity: Activity) {TODO("Not yet implemented")}override fun onActivitySaveInstanceState(activity: Activity, outState: Bundle) {TODO("Not yet implemented")}override fun onActivityDestroyed(activity: Activity) {TODO("Not yet implemented")}}

如果有多个不同业务需要实现这个接口，就这样很容易产生代码冗余。有没有一种优雅的方式，只需要实现自己需要的方法而不再需要去关注其他方法？有的，那就是利用 Java 的动态代理和 kotlin 的委托模式

2. 利用 Java 的动态代理和 Kotlin 的委托模式

首先需要实现一个通用的动态代理，新建一个 Kotlin 文件 DelegateObject.kt，这里通过 inline 和 reified 关键字，获取到泛型的 class 信息

import java.lang.reflect.InvocationHandler
import java.lang.reflect.Proxyinline fun <reified T> noOpDelegate() : T {val javaClass = T::class.javareturn Proxy.newProxyInstance(javaClass.classLoader, arrayOf(javaClass), no_op_invocationHandler) as T
}val no_op_invocationHandler = InvocationHandler { _, _, _ -> }

这样就可以获取到任意一个接口的一个对象，只是没有具体的实现。接着再利用 Kotlin 的 by 关键字实现对象委托

    val myActivityLifecycleCallbacks = object : Application.ActivityLifecycleCallbacks by noOpDelegate() {}

由于 Kotlin 委托模式的原理，实际上在编译期间也是会生成 ActivityLifecycleCallbacks 的所有方法，先来看看转译后的实现

   private final Application.ActivityLifecycleCallbacks myActivityLifecycleCallbacks = (Application.ActivityLifecycleCallbacks)(new Application.ActivityLifecycleCallbacks() {// $FF: synthetic fieldprivate final Application.ActivityLifecycleCallbacks $$delegate_0;{int $i$f$noOpDelegate = false;Class javaClass$iv = Application.ActivityLifecycleCallbacks.class;Object var10001 = Proxy.newProxyInstance(javaClass$iv.getClassLoader(), new Class[]{javaClass$iv}, DelegateObjectKt.getNo_op_invocationHandler());if (var10001 == null) {throw new NullPointerException("null cannot be cast to non-null type android.app.Application.ActivityLifecycleCallbacks");} else {this.$$delegate_0 = (Application.ActivityLifecycleCallbacks)var10001;}}public void onActivityCreated(@NonNull @NotNull Activity activity, @Nullable @org.jetbrains.annotations.Nullable Bundle savedInstanceState) {Intrinsics.checkNotNullParameter(activity, "activity");this.$$delegate_0.onActivityCreated(activity, savedInstanceState);}public void onActivityDestroyed(@NonNull @NotNull Activity activity) {Intrinsics.checkNotNullParameter(activity, "activity");this.$$delegate_0.onActivityDestroyed(activity);}public void onActivityPaused(@NonNull @NotNull Activity activity) {Intrinsics.checkNotNullParameter(activity, "activity");this.$$delegate_0.onActivityPaused(activity);}public void onActivityResumed(@NonNull @NotNull Activity activity) {Intrinsics.checkNotNullParameter(activity, "activity");this.$$delegate_0.onActivityResumed(activity);}public void onActivitySaveInstanceState(@NonNull @NotNull Activity activity, @NonNull @NotNull Bundle outState) {Intrinsics.checkNotNullParameter(activity, "activity");Intrinsics.checkNotNullParameter(outState, "outState");this.$$delegate_0.onActivitySaveInstanceState(activity, outState);}public void onActivityStarted(@NonNull @NotNull Activity activity) {Intrinsics.checkNotNullParameter(activity, "activity");this.$$delegate_0.onActivityStarted(activity);}public void onActivityStopped(@NonNull @NotNull Activity activity) {Intrinsics.checkNotNullParameter(activity, "activity");this.$$delegate_0.onActivityStopped(activity);}});

现在已经将 ActivityLifecycleCallbacks 的匿名内部类对象委托给了 noOpDelegate 生成的代理对象。这样需要用到具体哪个方法时，只需要再次重写即可，例如文章最开始的例子可以变为

    val myActivityLifecycleCallbacks = object : Application.ActivityLifecycleCallbacks by noOpDelegate() {override fun onActivityCreated(activity: Activity, savedInstanceState: Bundle?) {TODO("Not yet implemented")}override fun onActivityDestroyed(activity: Activity) {TODO("Not yet implemented")}}

经过精简的代码可以使代码更加简洁，可以更好的聚焦业务实现