学步园

    Activity是Android应用中最常用的。Activity有其固有的状态和生命周期。但是Android系统对Activity状态切换的处理往往并不是我们所期望的那样，所以了解并掌握Activity的生命周期，对于开发高质量的Android应用是十分必要的。

    Android系统会将当前存在的Activity组织成一个栈。当显示新的Activity时，将其创建后放入栈顶。如果用户按了返回键，顶层的Activity就会被移走，下面的一个Activity就显示出来了。

    一个Activity一共有四种状态：

    Active： 激活状态。Activity处于栈顶，其可见并拥有焦点。当其他Activity被激活时，此Activity便会进入暂停状态。

    Paused：暂停状态。Activity可见，但没有焦点。通常是它前面还有一个透明的或者不是全屏的Activity。

    Stopped：停止状态。Activity完全不可见时，便处于此状态。
    Inactive： 未激活状态。Activity被关闭后的状态，它也会被从Activity栈中移除。

    对Activity在各种状态之间的切换，Android系统提供了完整的事件处理函数。

import android.app.Activity;<br /> import android.os.Bundle;</p> <p>public class MyActivity extends Activity<br /> {<br /> private String LOG_TAG = "MyApplication";</p> <p> @Override<br /> public void onCreate(Bundle savedInstanceState)<br /> {<br /> // Initialize activity.<br /> super.onCreate(savedInstanceState);<br /> }</p> <p> // Called after onCreate has finished, use to restore UI state<br /> @Override<br /> public void onRestoreInstanceState(Bundle savedInstanceState)<br /> {<br /> // Restore UI state from the savedInstanceState.<br /> // This bundle has also been passed to onCreate.<br /> super.onRestoreInstanceState(savedInstanceState);<br /> }</p> <p> // Called before subsequent visible lifetimes<br /> // for an activity process.<br /> @Override<br /> public void onRestart()<br /> {<br /> // Load changes knowing that the activity has already<br /> // been visible within this process.<br /> super.onRestart();<br /> }</p> <p> // Called at the start of the visible lifetime.<br /> @Override<br /> public void onStart()<br /> {<br /> // Apply any required UI change now that the Activity is visible.<br /> super.onStart();<br /> }</p> <p> // Called at the start of the active lifetime.<br /> @Override<br /> public void onResume()<br /> {<br /> // Resume any paused UI updates, threads, or processes required<br /> // by the activity but suspended when it was inactive.<br /> super.onResume();<br /> }</p> <p> // Called to save UI state changes at the<br /> // end of the active lifecycle.<br /> @Override<br /> public void onSaveInstanceState(Bundle savedInstanceState)<br /> {<br /> // Save UI state changes to the savedInstanceState.<br /> // This bundle will be passed to onCreate if the process is<br /> // killed and restarted.<br /> super.onSaveInstanceState(savedInstanceState);<br /> }</p> <p> // Called at the end of the active lifetime.<br /> @Override<br /> public void onPause()<br /> {<br /> // Suspend UI updates, threads, or CPU intensive processes<br /> // that don’t need to be updated when the Activity isn’t<br /> // the active foreground activity.<br /> super.onPause();<br /> }</p> <p> // Called at the end of the visible lifetime.<br /> @Override<br /> public void onStop()<br /> {<br /> // Suspend remaining UI updates, threads, or processing<br /> // that aren’t required when the Activity isn’t visible.<br /> // Persist all edits or state changes<br /> // as after this call the process is likely to be killed.<br /> super.onStop();<br /> }</p> <p> // Called at the end of the full lifetime.<br /> @Override<br /> public void onDestroy()<br /> {<br /> // Clean up any resources including ending threads,<br /> // closing database connections etc.<br /> super.onDestroy();<br /> }</p> <p>}<br />