[Android面試題-3] Activity的四種加載模式

題目：請簡單介紹一些Activity的四種加載模式。   分析：四種加載模式分別為standard, singleTop,singleTask,singleInstance，設置的位置在Androidmanifest.xml中Activity元素的Android：launchMode屬性。   1.standard：執行如下代碼   [java] view plaincopy Intent intent = new Intent();    intent.setClass(ActA.this, ActA.class);    startActivity(intent);    在棧中會產生一個新的Activity實例。 2.singleTop：同樣執行上述代碼，系統會檢測到棧頂已經存在滿足需要的Activity實例，所以將intent發送給棧頂的實例，而不會產生一個新的Activity實例。但是如果執行如下代碼的話則會產生一個新的Activity實例，因為棧頂不存在滿足需要的實例。   [java] view plaincopy Intent intent = new Intent();    intent.setClass(ActA.this, ActB.class);    startActivity(intent);    3.singleTask：和singleTop不同的是，每次發送intent的時候，系統會檢測整個棧來尋找滿足需要的Activity實例，如果存在，將intent發送給它，不存在則產生新的。   4.singleInstance：singleTask相當於是在一個應用中某一類的Activity只存在一個實例，大家共享這個實例；singleInstance相當於是多個應用共享一個Activity的實例。例如，一個導游的應用中調用了地圖應用，你在導游應用中打開地圖界面定位到中關村，然後按Home鍵將導游應用放到後台，然後再打開手機上的百度地圖，會發現百度地圖此時正好被定為到了中關村，這說明此導游應用和百度地圖共享了同一個Activity的實例。   下面是官方文檔中的介紹，有興趣的讀者可以查閱：   standard：The default mode, which will usually create a new instance of the activity when it is started, though this behavior may change with the introduction of other options such as Intent.FLAG_ACTIVITY_NEW_TASK.     singleTop：If, when starting the activity, there is already an instance of the same activity class in the foreground that is interacting with the user, then re-use that instance. This existing instance will receive a call toActivity.onNewIntent() with the new Intent that is being started.       singleTask:If, when starting the activity, there is already a task running that starts with this activity, then instead of starting a new instance the current task is brought to the front. The existing instance will receive a call toActivity.onNewIntent() with the new Intent that is being started, and with the Intent.FLAG_ACTIVITY_BROUGHT_TO_FRONT flag set. This is a superset of the singleTop mode, where if there is already an instance of the activity being started at the top of the stack, it will receive the Intent as described there (without the FLAG_ACTIVITY_BROUGHT_TO_FRONT flag set). See the Tasks and Back Stack document for more details about tasks.       singleInstance:Only allow one instance of this activity to ever be running. This activity gets a unique task with only itself running in it; if it is ever launched again with the same Intent, then that task will be brought forward and its Activity.onNewIntent() method called. If this activity tries to start a new activity, that new activity will be launched in a separate task. See the Tasks and Back Stack document for more details about tasks.     由於筆者水平有限，給各面試題提供的思路或代碼難免會有錯誤，還請讀者批評指正。另外，熱忱歡迎讀者能夠提供更多、更好的面試題，本人將感激不盡。如有任何意見或建議，歡迎在評論中告知。 博主徐方磊對本博客文章享有版權。網絡轉載請注明出處http://blog.csdn.net/shishengshi。整理出版物請和作者聯系。