Android开发使用Lambda表达式

前言

AndroidStudio从2.1开始官方通过Jack支持Java8，从而使用Lambda等特性。

Lambda表达式能有效减少代码量 使代码的结构更清晰 那么怎样在项目中使用呢。

有两种方式：

• 使用第三方的Java8兼容插件
• 使用官方Jack支持Java8

使用第三方的Java8兼容插件(建议)

Github地址:retrolambda

• 使用JDK1.8
• 在project的根build.gradle里添加：

dependencies {
	classpath 'me.tatarka:gradle-retrolambda:3.7.0'
}

• 然后在module的build.gradle里添加：

apply plugin: 'me.tatarka.retrolambda'

android {
	compileOptions {
		sourceCompatibility JavaVersion.VERSION_1_8
		targetCompatibility JavaVersion.VERSION_1_8
	}
}

使用官方Jack支持Java8(不建议)

配置

android {
  //必须24以上
  buildToolsVersion "24.0.3"
  ...
  //JDK1.8
  compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
  }

  defaultConfig {
    ...
    //使用jack
    jackOptions {
      enabled true
    }
  }
  ...
}

但是如果打开jackOptions，意味着ButterKnife，Dagger等基于APT的注解框架将无法使用(当然很多库例如ButterKnife的新版本已经不再依赖apt了)。

会提示如下错误：

Error:Could not find property 'options' on task ':app:compileDebugJavaWithJack'.

所以要去除apt插件

apply plugin: 'android-apt'

需要 Build Tools 24.0.0 or later

Error:Execution failed for task ':app:jillDebugPackagedLibraries'.
> Jack requires Build Tools 24.0.0 or later

很直观的缺点

• build超级慢 耗内存，而且没有Instant Run

总的来说 还是不建议使用这种形式(要求太多了)

如果你还不了解Jack & Jill

那么现在你已经不用了解了!

Google 又弃坑了，Jack+Jill vs. javac+dx

官方post：

Future of Java 8 Language Feature Support on Android

原文：

At Google, we always try to do the right thing. Sometimes this means adjusting our plans. We know how much our Android developer community cares about good support for Java 8 language features, and we’re changing the way we support them.

We’ve decided to add support for Java 8 language features directly into the current javac and dx set of tools, and deprecate the Jack toolchain. With this new direction, existing tools and plugins dependent on the Java class file format should continue to work. Moving forward, Java 8 language features will be natively supported by the Android build system. We’re aiming to launch this as part of Android Studio in the coming weeks, and we wanted to share this decision early with you.

We initially tested adding Java 8 support via the Jack toolchain. Over time, we realized the cost of switching to Jack was too high for our community when we considered the annotation processors, bytecode analyzers and rewriters impacted. Thank you for trying the Jack toolchain and giving us great feedback. You can continue using Jack to build your Java 8 code until we release the new support. Migrating from Jack should require little or no work.

We hope the new plan will pave a smooth path for everybody to take advantage of Java 8 language features on Android. We’ll share more details when we release the new support in Android Studio.

实例对比

点击事件

• 使用前

btn = (Button) findViewById(R.id.btn);
btn.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
          Toast.makeText(getApplicationContext(), "按钮点击了", Toast.LENGTH_LONG).show()
      }
});

• 使用后

btn = (Button) findViewById(R.id.btn);
btn.setOnClickListener(v -> {
    Toast.makeText(getApplicationContext(), "按钮点击了", Toast.LENGTH_LONG).show();
});

Snackbar的点击事件

final Snackbar snackbar = Snackbar.make(outLayoyt, "服务器地址尚未设置!", Snackbar.LENGTH_LONG);
snackbar.setAction("设置",v -> {

});

线程处理

• 使用前

new Thread(new Runnable() {
    @Override
    public void run() {
        Log.i("TAG", "定义一个简单测试例子...");
    }
}).start();

• 使用后

new Thread(
    () -> {
        Log.i("TAG", "使用Lambda表达式的例子...");
    }
).start();