【Android Tip】
Android 獲取當(dāng)前Activity對(duì)象的方式：
背景：（當(dāng)你不能使用this獲取Activity對(duì)象）如何方便地當(dāng)前Activity對(duì)象
思路：

1.維護(hù)對(duì)象棧

維護(hù)一個(gè)Activity的対象棧，在每個(gè)Activity的生命周期方法執(zhí)行的時(shí)候，控制add和remove，棧頂元素就是當(dāng)前的Activity對(duì)象。為了代碼的復(fù)用，這個(gè)操作可以寫(xiě)在BaseActivity中，這個(gè)思路很正常。代碼就不貼了。

2. 反射

使用反射來(lái)獲取當(dāng)前Activity對(duì)象。（個(gè)人認(rèn)為是相對(duì)優(yōu)雅和解耦的方式）
查看源碼發(fā)現(xiàn) Activity Thread 這個(gè)類(lèi)管理著所有的Activity對(duì)象，也就持有所有的Activity對(duì)象，使用反射獲得當(dāng)前ActivityThread對(duì)象
，然后就能拿到當(dāng)前的Activity對(duì)象
示例：

public static Activity getCurrentActivity () {
    try {
        Class activityThreadClass = Class.forName("android.app.ActivityThread");
        Object activityThread = activityThreadClass.getMethod("currentActivityThread").invoke(
                null);
        Field activitiesField = activityThreadClass.getDeclaredField("mActivities");
        activitiesField.setAccessible(true);
        Map activities = (Map) activitiesField.get(activityThread);
        for (Object activityRecord : activities.values()) {
            Class activityRecordClass = activityRecord.getClass();
            Field pausedField = activityRecordClass.getDeclaredField("paused");
            pausedField.setAccessible(true);
            if (!pausedField.getBoolean(activityRecord)) {
                Field activityField = activityRecordClass.getDeclaredField("activity");
                activityField.setAccessible(true);
                Activity activity = (Activity) activityField.get(activityRecord);
                return activity;
            }
        }
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (InvocationTargetException e) {
        e.printStackTrace();
    } catch (NoSuchMethodException e) {
        e.printStackTrace();
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
    return null;
}

由于源碼的設(shè)計(jì)并不是用來(lái)做當(dāng)前的操作，所以源碼修改可能導(dǎo)致該方法不能用，而且，評(píng)論區(qū)也出現(xiàn)了拿不到activitys的情況，說(shuō)明該方法穩(wěn)定性也不是最佳。

3. （推薦）Framework回調(diào)

Android自 API 14開(kāi)始引入了一個(gè)方法，即Applicationd的registerActivityLifecycleCallbacks方法，用來(lái)監(jiān)聽(tīng)所有Activity的生命周期回調(diào)，比如onActivityCreated,onActivityResumed等。
So，一個(gè)簡(jiǎn)單的實(shí)現(xiàn)如下：

public class BaseApplication extends Application {
    //為避免內(nèi)存泄漏使用弱引用
    private WeakReference<Activity> mCurrentActivity;
    @Override
    public void onCreate () {
        super.onCreate();
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
            registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
                @Override
                public void onActivityCreated (Activity activity, Bundle bundle) {
                    //do nothing
                }

                @Override
                public void onActivityStarted (Activity activity) {
                    //do nothing
                }

                @Override
                public void onActivityResumed (Activity activity) {
                    mCurrentActivity = new WeakReference<>(activity);
                }

                @Override
                public void onActivityPaused (Activity activity) {
                    //do nothing
                }

                @Override
                public void onActivityStopped (Activity activity) {
                    //do nothing
                }

                @Override
                public void onActivitySaveInstanceState (Activity activity, Bundle bundle) {
                    //do nothing
                }

                @Override
                public void onActivityDestroyed (Activity activity) {
                    //do nothing
                }
            });
        }
    }
}

思路來(lái)自:stackoverflow