JetPack<第四篇>：Lifecycle

Lifecycle：管理并监听Activity生命周期

【1】依赖

def lifecycle_version = "2.2.0"
implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-viewmodel�savedstate:$lifecycle_version"
annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-service:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-process:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-reactivestreams:$lifecycle_version"

【2】完整实现步骤

第一步：将 Activity 做为被观察者

class LifecycleActivity : AppCompatActivity(), LifecycleOwner {

    // 注册类
    private val lifecycle : LifecycleRegistry = LifecycleRegistry(this@LifecycleActivity)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        ....
        
    }

    // 重写 Lifecycle 回调方法
    override fun getLifecycle(): Lifecycle {
        return super.getLifecycle()
    }
}

即，实现 LifecycleOwner 接口。

第二步：构建观察者

class MyObserver : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun onResume() {
        Log.d("MyObserver", "Lifecycle call onResume")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun onPause() {
        Log.d("MyObserver", "Lifecycle call onPause")
    }
}

上面的两个注解中的枚举对应了两个不同的生命周期，所有的枚举如下：

    ON_CREATE,
    ON_START,
    ON_RESUME,
    ON_PAUSE,
    ON_STOP,
    ON_DESTROY,
    ON_ANY

使用 OnLifecycleEvent 注解指定指定生命周期的回调方法。

第三步：添加观察者（订阅）

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    ...
    getLifecycle().addObserver(MyObserver())
    // 使用上面定义的 LifecycleRegistry 对象也可以
    lifecycle.addObserver(MyObserver())

}

第四步：调试生命周期和监听回调方法的执行顺序。

【3】简单实现方式

上面完整的实现方式是可以实现的更加简单，原因是当前Activity父类或者间接父类已经完成了一部分。

完整代码如下：

class LifecycleActivity : AppCompatActivity(), LifecycleOwner {

    private lateinit var binding : ActivityLifecycleBinding

    private val lifecycle : LifecycleRegistry = LifecycleRegistry(this@LifecycleActivity)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView<ActivityLifecycleBinding>(this@LifecycleActivity, R.layout.activity_lifecycle)
        getLifecycle().addObserver(MyObserver())
        // 或者
        lifecycle.addObserver(MyObserver())
    }

    override fun getLifecycle(): Lifecycle {
        return super.getLifecycle()
    }

    // ...
    // MyObserver 和 生命周期方法省略
}

第一步：省略 LifecycleOwner 接口的实现，因为 LifecycleActivity 的间接父类已经实现了 LifecycleOwner 接口。
LifecycleOwner 接口的实现方法 getLifecycle 也会被删除。

class LifecycleActivity : AppCompatActivity() {

    private lateinit var binding : ActivityLifecycleBinding

    private val lifecycle : LifecycleRegistry = LifecycleRegistry(this@LifecycleActivity)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView<ActivityLifecycleBinding>(this@LifecycleActivity, R.layout.activity_lifecycle)
        getLifecycle().addObserver(MyObserver())
        // 或者
        lifecycle.addObserver(MyObserver())
    }
    // ...
    // MyObserver 和 生命周期方法省略
}

第二步：在 ComponentActivity 中， 已经创建了 LifecycleRegistry 实例，
如果您的Activity实现了 ComponentActivity，那么可以省略 LifecycleRegistry 实例的创建。

class LifecycleActivity : AppCompatActivity() {

    private lateinit var binding : ActivityLifecycleBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView<ActivityLifecycleBinding>(this@LifecycleActivity, R.layout.activity_lifecycle)
        getLifecycle().addObserver(MyObserver())
    }
    // ...
    // MyObserver 和 生命周期方法省略
}

如果是Kotlin， 还可以将：
        getLifecycle().addObserver(MyObserver())
写成：
        lifecycle.addObserver(MyObserver())

最终，简化之后的完成代码如下：

class LifecycleActivity : AppCompatActivity() {

    private lateinit var binding : ActivityLifecycleBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView<ActivityLifecycleBinding>(this@LifecycleActivity, R.layout.activity_lifecycle)
        lifecycle.addObserver(MyObserver())
    }

    override fun onResume() {
        Log.d("MyObserver", "before onResume")
        super.onResume()
        Log.d("MyObserver", "after onResume")
    }

    override fun onPause() {
        Log.d("MyObserver", "before onPause")
        super.onPause()
        Log.d("MyObserver", "after onPause")
    }

    class MyObserver : LifecycleObserver {

        @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
        fun onResume() {
            Log.d("MyObserver", "Lifecycle call onResume")
        }

        @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
        fun onPause() {
            Log.d("MyObserver", "Lifecycle call onPause")
        }
    }
}

【4】另一种实现方式

优雅的管理Activity - 简书 (jianshu.com)

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