Lifecycle原理

lifecycle通过观察者模式加反射注解模式实习对生命周期的监听。其中实现了LifecycleOwner的为被观察者，实现了LifecycleObserver的微观察者，通过在被观察者中
getLifecycle().addObserver(new TestPresenter());
实现绑定。

一 、LifecycleObserver被观察者

@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void onCreateX(LifecycleOwner owner) {

}

@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void onStartX(LifecycleOwner owner) {

}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void onResumeX(LifecycleOwner owner) {

}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void onPauseX(LifecycleOwner owner) {

}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void onStopX(LifecycleOwner owner) {

}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void onDestoryX(LifecycleOwner owner) {

}

@OnLifecycleEvent(Lifecycle.Event.ON_ANY)
public void onANYX(LifecycleOwner owner) {

}

在被观察者中创建对应生命周期的方法，并用OnLifecycleEvent注解，后面在add的时候会通过注解来获取方法。

二、getLifecycle().addObserver绑定流程

关键的方法是createInfo(Class klass, @Nullable Method[] declaredMethods)
在方法中通过反射获取observe的方法

Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);

然后通过过滤注解得到对应的TYPE和方法名，包装后放入集合中。

   for (Method method : methods) {
        OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
        if (annotation == null) {
            continue;
        }
        hasLifecycleMethods = true;
        Class<?>[] params = method.getParameterTypes();
        int callType = CALL_TYPE_NO_ARG;
        if (params.length > 0) {
            callType = CALL_TYPE_PROVIDER;
            if (!params[0].isAssignableFrom(LifecycleOwner.class)) {
                throw new IllegalArgumentException(
                        "invalid parameter type. Must be one and instanceof LifecycleOwner");
            }
        }

这里要注意下在注解参数为一个的时候是 int callType = CALL_TYPE_NO_ARG;后面会用到。

三、实现了LifecycleOwner的ComponentActivity调用观察者

getStateAfter这里有个状态机的概念

static State getStateAfter(Event event) {
    switch (event) {
        case ON_CREATE:
        case ON_STOP:
            return CREATED;
        case ON_START:
        case ON_PAUSE:
            return STARTED;
        case ON_RESUME:
            return RESUMED;
        case ON_DESTROY:
            return DESTROYED;
        case ON_ANY:
            break;
    }
    throw new IllegalArgumentException("Unexpected event value " + event);
}