1.介绍

   LifeCycle是可以感知Activity和Fragment生命周期函数的组件，这样的话可以在页面开启和销毁的时候做一些事情，比如一个定时器功能在页面可见的时候计时，页面不可见的时候暂停。

2.实战

3.普通写法以及打印的日志

class LifeCycleActivity : AppCompatActivity(){
    var isRun:Boolean=true
    var count:Int = 0
    private val TAG:String = "LifeCycleActivity"

    @SuppressLint("SetTextI18n")
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_lifecycle)
    }

    override fun onResume() {
        super.onResume()
        isRun = true
        Thread{
            run {
                while (isRun){
                    Thread.sleep(1000)
                    count++
                    Log.e(TAG, "onResume: $count")
                }
            }
        }.start()
    }

    override fun onPause() {
        super.onPause()
        isRun = false
        Log.e(TAG, "onPause")
    }

}

这种方法固然能实现，但是不优雅不符合编程的分层思想，Activity中不适合做太多的逻辑判断和耗时操作，扯远了哈！！！这就又说到设计模式了。

4.来看看LifeCycle的实现方式吧

1).创建一个TimeDown的类并实现LifecycleObserver接口

class TimeDown : LifecycleObserver {
    
}

2).创建两个方法并加入LifeCyle枚举类型的注解，看注解的名字就知道是和活动的生命周期是同步的。

class TimeDown : LifecycleObserver {
    private val TAG: String = "LifecycleObserver"
    private var isStart: Boolean = true
    private var count:Int = 0
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun onResume() {
        isStart = true
        Thread {
            run {
                while (isStart) {
                    Thread.sleep(1000)
                    count++
                    Log.e(TAG, "onStart: $count")
                }
            }
        }.start()
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun onPause() {
        isStart = false
        Log.e(TAG, "onStop")
    }
}

源码中如下

 @SuppressWarnings("WeakerAccess")
    public enum Event {
        /**
         * Constant for onCreate event of the {@link LifecycleOwner}.
         */
        ON_CREATE,
        /**
         * Constant for onStart event of the {@link LifecycleOwner}.
         */
        ON_START,
        /**
         * Constant for onResume event of the {@link LifecycleOwner}.
         */
        ON_RESUME,
        /**
         * Constant for onPause event of the {@link LifecycleOwner}.
         */
        ON_PAUSE,
        /**
         * Constant for onStop event of the {@link LifecycleOwner}.
         */
        ON_STOP,
        /**
         * Constant for onDestroy event of the {@link LifecycleOwner}.
         */
        ON_DESTROY,
        /**
         * An {@link Event Event} constant that can be used to match all events.
         */
        ON_ANY
    }

3).Activity中引用

class LifeCycleActivity : AppCompatActivity(){


    @SuppressLint("SetTextI18n")
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_lifecycle)
        lifecycle.addObserver(TimeDown())
    }

}

有的同学可能会问这个lifecycle哪里来的？这个其实是Activity的基类中已经实现了LifecycleOwner接口，并在Activity中提供了一个get方法获取lifecycle,所以我们获取这个然后将我们的timedown通过观察者的模式添加上，就等于和Activity的生命周期绑定上啦！！！

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner,
        ViewModelStoreOwner,
        HasDefaultViewModelProviderFactory,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner {
	.......
    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    }
}

到这里LifeCycle就讲完啦！！！