Android registerActivityLifecycleCallbacks方法分析

在Application中有一个registerActivityLifecycleCallbacks方法，实现注册Activity生命周期回调，首先看下在Application源码中实现：

  private ArrayList<ActivityLifecycleCallbacks> mActivityLifecycleCallbacks =
            new ArrayList<ActivityLifecycleCallbacks>();

    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);
    }

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

    public void unregisterActivityLifecycleCallbacks(ActivityLifecycleCallbacks callback) {
        synchronized (mActivityLifecycleCallbacks) {
            mActivityLifecycleCallbacks.remove(callback);
        }
    }

代码很简单，一个ArrayList保存所有注册回调接口实现对象，ActivityLifecycleCallbacks声明和Activity生命周期相关的方法，那在哪里调用这些方法呢？接着再看下Application中的源码，发现如下方法：

 /* package */ 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);
            }
        }
    }

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

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

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

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

    /* package */ 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);
            }
        }
    }

    /* package */ void dispatchActivityDestroyed(Activity activity) {
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ActivityLifecycleCallbacks)callbacks[i]).onActivityDestroyed(activity);
            }
        }
    }

各种dispatch相关的方法，这里是回调ArrayList接口实现对象的方法，那哪里调用这些方法了，等等，我们想下，既然在Activity生命周期会回调这些方法，那么Activity中应该存在调用的逻辑？好的，那我们看下Activity中各生命周期的方法：

//关键代码 这里我们看下onStart方法
    @CallSuper
    protected void onStart() {
        if (DEBUG_LIFECYCLE) Slog.v(TAG, "onStart " + this);
        mCalled = true;

        mFragments.doLoaderStart();

        getApplication().dispatchActivityStarted(this);//这不是调用dispatch相关的方法，哈哈

        if (mAutoFillResetNeeded) {
            AutofillManager afm = getAutofillManager();
            if (afm != null) {
                afm.onVisibleForAutofill();
            }
        }
    }

可以看到在onStart生命周期方法中调用了getApplication().dispatchxxxx方法，同理，如果大家再去看的话可以看到其它相关生命周期方法的回调，这里就不一一贴上源码了。到这里，我们就知道了调用逻辑。那注册生命周期回调registerActivityLifecycleCallbacks有什么用呢？
这里说个特殊需求的实现：实现在断网情况下显示一个弹窗提示用户？这个需求想必有很多同鞋遇到的，这里我们不再讨论通过广播来监听网络状态的改变，我们只讨论如何来显示弹窗，大部分做法会在BaseActivity中写上相关弹窗的代码，
这里提供另外一种思路，就是注册registerActivityLifecycleCallbacks回调，可以得到Activity栈顶的activity来显示相关的弹窗。有兴趣的同鞋可以试下哈。