参考：《Professional Android 4 Application Development》
深入了解Android Activity
每一个Android Activity都对应于一个用户界面（UI）。每个Android Application都有一个main Activity，而这个main Activity大多由多个Fragment组成，而这些Fragment后面往往都由一个或多个secondary Activity支持。当用户在不同的界面（窗口）切换时，Android会生成对应的新Activity。
创建Activity
Android提供了Activity类，在编写自己的Activity时，程序员需要继承Activity并重载里面的方法。例如：
package cn.jubincn.activities; import android.app.Activity;
import android.os.Bundle; public class MyActivity extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
}
空白的Activity对应的是一个空白屏幕，因此我们需要添加Fragment，Layout和View等UI元素来定制自己的Activity。大部分Activity会占据整个屏幕，除此之外，还有一些半透明的Activity和浮动的Activity。
Android应用程序中，用户交互界面和数据的显示是由View来提供的。Android中的layout类，也叫ViewGroup，可以将View打包进行管理。Fragment则用来组织界面元素，从而更方便地适配不同的界面。给Activity设置界面的方法有两种，一种是创建View类，将其设置到Activity中；另一种是将layout文件设置到Activity中，然后Inflate对应的Activity。
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
TextView textView = new TextView(this);
setContentView(textView);
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
<activity android:label="@string/app_name" android:name=".MyActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
Activity生命周期
Activity的生命周期可以决定Process的优先级，并且Activity需要对生命周期事件进行适当地响应才能提供更好的用户体验。
Activity Stacks
Activity的状态决定于它在Activity Stack中的位置。Activity Stack遵循Stack的“先进后出”的规则：当一个Activity刚被创建时，它会移到栈顶；若用户点击”go back”按钮，或关闭当
前Activity时，栈顶的Activity会被弹出，第二个Activity成为新的栈顶Activity。
Activity State
Activity的生命周期中，有这几种状态：
- Active：在栈顶的Activity，具有最高的优先级，Android会竭力满足它对资源的需求。当其他Activity变为Active状态时，此Activity的状态会变为Paused。
- Paused：处于这个状态的Activity的部分或全部对用户可见，但无法接受用户的输入。
- Stopped：当Activity对用户不可见时，它将转为Stopped状态。此时尽管Activity仍会保留在内存中，但当系统资源紧张时，它会被回收掉。
- Inactive：Activity在启动之前，或被kill之后，处于Inactive状态。
监听并响应Activity State的变化
Activity需要在State发生变化时作出相应的反应，为此Android为Activity的生命周期事件提供了很多相关的ActionHandler，下图展示了Activity的几个生命周期：
对应的代码：
package cn.jubincn.activities;
import android.app.Activity;
import android.os.Bundle;
public class MyStateChangeActivity extends Activity {
// Called at the start of the full lifetime.
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Initialize Activity and inflate the UI.
}
// Called after onCreate has finished, use to restore UI state
@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
super.onRestoreInstanceState(savedInstanceState);
// Restore UI state from the savedInstanceState.
// This bundle has also been passed to onCreate. Will only be called if the Activity has been killed by the system since it was last visible.
}
// Called before subsequent visible lifetimes for an Activity process.
@Override
public void onRestart(){
super.onRestart();
// Load changes knowing that the Activity has already been visible within this process.
}
// Called at the start of the visible lifetime.
@Override
public void onStart(){
super.onStart();
// Apply any required UI change now that the Activity is visible.
}
// Called at the start of the active lifetime.
@Override
public void onResume(){
super.onResume();
// Resume any paused UI updates, threads, or processes required
// by the Activity but suspended when it was inactive.
}
// Called to save UI state changes at the end of the active lifecycle.
@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
// Save UI state changes to the savedInstanceState.
// This bundle will be passed to onCreate and onRestoreInstanceState if the process is killed and restarted by the run time.
super.onSaveInstanceState(savedInstanceState);
}
// Called at the end of the active lifetime.
@Override
public void onPause(){
// Suspend UI updates, threads, or CPU intensive processes that don’t need to be updated when the Activity isn't the active foreground Activity.
super.onPause();
}
// Called at the end of the visible lifetime.
@Override
public void onStop(){
// Suspend remaining UI updates, threads, or processing that aren’t required when the Activity isn’t visible.
// Persist all edits or state changes as after this call the process is likely to be killed.
super.onStop();
}
// Sometimes called at the end of the full lifetime.
@Override
public void onDestroy(){
// Clean up any resources including ending threads, closing database connections etc.
super.onDestroy();
}
}