Application中ActivityLifecycleCallbacks的原理分析

ActivityLifecycleCallbacks是Application中的一个接口，既然是在Application中，不得不说说Application类，别急，一个个来说道说道

ActivityLifecycleCallbacks接口，如下： public interface ActivityLifecycleCallbacks {
void onActivityCreated(Activity activity, Bundle savedInstanceState);
void onActivityStarted(Activity activity);
void onActivityResumed(Activity activity);
void onActivityPaused(Activity activity);
void onActivityStopped(Activity activity);
void onActivitySaveInstanceState(Activity activity, Bundle outState);
void onActivityDestroyed(Activity activity);
}有很多方法声明，仔细一看，很熟悉啊，没错，这些方法与Activity的整个生命周期相关，而且，通过命名，可以大体猜测，这些方法会在Activity周期方法调用完后被执行
接下来，看看Application中还有哪儿些与ActivityLifecycleCallbacks相关的，代码如下：

public void registerActivityLifecycleCallbacks(ActivityLifecycleCallbacks callback) {
synchronized (mActivityLifecycleCallbacks) {
mActivityLifecycleCallbacks.add(callback);
}
}

public void unregisterActivityLifecycleCallbacks(ActivityLifecycleCallbacks callback) {
synchronized (mActivityLifecycleCallbacks) {
mActivityLifecycleCallbacks.remove(callback);
}
}这里，两个方法，一个注册ActivityLifecycleCallbacks，一个反注册ActivityLifecycleCallbacks，还有一个mActivityLifecycleCallbacks，可以看出是个成员变量
在Application中定义mActivityLifecycleCallbacks，代码如下：

private ArrayList<ActivityLifecycleCallbacks> mActivityLifecycleCallbacks = new ArrayList<ActivityLifecycleCallbacks>();继续阅读，会发现如下方法：
void dispatchActivityCreated(Activity activity, Bundle savedInstanceState) {
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
((ActivityLifecycleCallbacks)callbacks[i]).onActivityCreated(activity,
savedInstanceState);
}
}
}

void dispatchActivityStarted(Activity activity) {
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
((ActivityLifecycleCallbacks)callbacks[i]).onActivityStarted(activity);
}
}
}

void dispatchActivityResumed(Activity activity) {
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
((ActivityLifecycleCallbacks)callbacks[i]).onActivityResumed(activity);
}
}
}

void dispatchActivityPaused(Activity activity) {
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
((ActivityLifecycleCallbacks)callbacks[i]).onActivityPaused(activity);
}
}
}

void dispatchActivityStopped(Activity activity) {
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
((ActivityLifecycleCallbacks)callbacks[i]).onActivityStopped(activity);
}
}
}

void dispatchActivitySaveInstanceState(Activity activity, Bundle outState) {
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
((ActivityLifecycleCallbacks)callbacks[i]).onActivitySaveInstanceState(activity,
outState);
}
}
}

void dispatchActivityDestroyed(Activity activity) {
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
((ActivityLifecycleCallbacks)callbacks[i]).onActivityDestroyed(activity);
}
}
}以上这些方法，都有个共同点，就是调用了collectActivityLifecycleCallbacks方法，而且遍历callbacks，类型转换为ActivityLifecycleCallbacks，回调其方法
collectActivityLifecycleCallbacks方法做了什么呢？将list转化为数组，代码如下：

private ArrayList<ComponentCallbacks> mComponentCallbacks = new ArrayList<ComponentCallbacks>();
private Object[] collectComponentCallbacks() {
Object[] callbacks = null;
synchronized (mComponentCallbacks) {
if (mComponentCallbacks.size() > 0) {
callbacks = mComponentCallbacks.toArray();
}
}
return callbacks;
}那么问题来了，Applicatipn中的dispatchActivityXXX的这些方法，是谁调用呢？
如果对Activity源码了解的话，很容易发现如下代码：

protected void onCreate(@Nullable Bundle savedInstanceState) {
//省略
<pre name="code" class="java"> getApplication().dispatchActivityCreated(this, savedInstanceState);
//... }

protected void onStart() {
//...
getApplication().dispatchActivityStarted(this);
}

protected void onResume() {
//...
getApplication().dispatchActivityResumed(this);
//...
}

onPause()，onStop()，onDestroy()，同样调用getApplication().dispatchActivityPaused(this)

整理整个执行过程：

        需要定义一个类A，实现ActivityLifecycleCallbacks，并且实现接口方法

        registerActivityLifecycleCallbacks注册类A对象，

        系统调用Activity的attach方法，绑定Application上下文，

        系统调用Activity生命周期方法，相应的调用Application中的dispatchActivityXXX方法，dispatchActivityXXX方法内部，回调A对象中方法的具体实现，从而完成，对Activity生命周期完整的跟踪监听

优点：官方提供对Activity生命周期的统一监听，避免代码在某类中的显示调用