为什么要使用Lifecycle？

有时候我们的很多操作和生命周期是相关联的，就好比我们如果用mvp架构，Presenter如何感知Activity或者Fragment的生命周期？可能会想到这样做： IPresenter中定义和生命周期相关的几个方法，然后在MainActivity中调用对应的方法。可以是可以，但是这样做会有个问题：在MainActivity中的每个生命周期方法中都要调用一次IPresenter中的接口，很不灵性。所以Lifecycle就诞生了。

如何使用Lifecycle？

IPresenter实现LifecycleObserver接口

interface IPresenter : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun onCreate(@NotNull owner: LifecycleOwner)

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun onDestroy(@NotNull owner: LifecycleOwner)

    @OnLifecycleEvent(Lifecycle.Event.ON_ANY)
    fun onLifecycleChanged(@NotNull owner: LifecycleOwner, @NotNull event: Lifecycle.Event)
}

BasePresenter在对应的生命周期处理View

abstract class BasePresenter<V : IView>(view: IView) : IPresenter {
    @Suppress("UNCHECKED_CAST")
    protected var mViewRef: WeakReference<V>? = WeakReference(view as V)

    override fun onCreate(owner: LifecycleOwner) {

    }

    override fun onDestroy(owner: LifecycleOwner) {
        mViewRef?.clear()
    }

    override fun onLifecycleChanged(owner: LifecycleOwner, event: Lifecycle.Event) {

    }
}

BaseActivity

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        presenter = createPresenter()
        lifecycle.addObserver(presenter)
    }

最后贴一下官方样例

class MyActivity : Activity(), LifecycleOwner {

    private lateinit var lifecycleRegistry: LifecycleRegistry

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

        lifecycleRegistry = LifecycleRegistry(this)
        lifecycleRegistry.markState(Lifecycle.State.CREATED)
    }

    public override fun onStart() {
        super.onStart()
        lifecycleRegistry.markState(Lifecycle.State.STARTED)
    }

    override fun getLifecycle(): Lifecycle {
        return lifecycleRegistry
    }
}

总结一下

Activity/Fragment实现了LifecycleOwner接口，也就是成为了生命周期拥有者；BasePresenter实现了LifecycleObserver接口，观察生命周期变化；通过lifecycle.addObserver(presenter)相关联，owner生命周期发生变化时会分发到已经订阅的observer。当然我们自己的类也可以通过LifeCycleRegistry成为LifeCycleOwner。