Android Activity 源码解析
Activity 是用得最多的了，平时也只是熟练的使用，知道 launchMode、intent-filte、screenOrientation，然后看下官网的资料，7个步骤，如何切换这些。现在就读一下底层的源码吧。
类注释中就有很好的说明了，按例，先从类注释开始，主要有这些
* <p>Topics covered here:
* <ol>
* <li><a href="#Fragments">Fragments</a>
* <li><a href="#ActivityLifecycle">Activity Lifecycle</a>
* <li><a href="#ConfigurationChanges">Configuration Changes</a>
* <li><a href="#StartingActivities">Starting Activities and Getting Results</a>
* <li><a href="#SavingPersistentState">Saving Persistent State</a>
* <li><a href="#Permissions">Permissions</a>
* <li><a href="#ProcessLifecycle">Process Lifecycle</a>
* </ol>
Activity 生命周期的7个方法：
<pre class="prettyprint">
* public class Activity extends ApplicationContext {
* protected void onCreate(Bundle savedInstanceState);
*
* protected void onStart();
*
* protected void onRestart();
*
* protected void onResume();
*
* protected void onPause();
*
* protected void onStop();
*
* protected void onDestroy();
* }
* </pre>
捡关键的来说：
onResume() <td>Called when the activity will interacting with the user. At this point your activity is at the top of the activity stack, with user input going to <p>Always followed by <code>onPause()</code>.</td>
onResume()的方法调用后，就会放在 activity stack 的栈顶，用户就可以进行交互。
Unless you specify otherwise, a configuration change (such as a change
* in screen orientation, language, input devices, etc) will cause your
* current activity to be <em>destroyed</em>, going through the normal activity
* lifecycle process of {@link #onPause},
* {@link #onStop}, and {@link #onDestroy} as appropriate.
屏幕方向切换、系统语言、输入设备将会引起当前的 activity 调用 destroyed()。
###代码： 类继承结构
java.lang.Object
↳ android.content.Context
↳ android.content.ContextWrapper
↳ android.view.ContextThemeWrapper
↳ android.app.Activity