Android 报错 Activity 状态

我会带着你远行 2022-08-06 03:30 225阅读 0赞

我近期学习了一下android sdk,不太清楚里面的一些原理，如何保存应用程序的状态，看下面hello的小例子：

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     <div> 
      <code>package</code> 
      <code> com.android.hello;</code> 
     </div> 
     <div></div> 
     <div> 
      <code>import</code> 
      <code> android.app.Activity;</code> 
     </div> 
     <div> 
      <code>import</code> 
      <code> android.os.Bundle;</code> 
     </div> 
     <div> 
      <code>import</code> 
      <code> android.widget.TextView;</code> 
     </div> 
     <div></div> 
     <div> 
      <code>public</code> 
      <code> class</code> 
      <code>HelloAndroid </code> 
      <code>extends</code> 
      <code>Activity { </code> 
     </div> 
     <div> 
      <code></code> 
      <code>/** Called when the activity is first created. */</code> 
     </div> 
     <div> 
      <code></code> 
      <code>@Override</code> 
     </div> 
     <div> 
      <code></code> 
      <code>public</code> 
      <code>void</code> 
      <code>onCreate(Bundle savedInstanceState) { </code> 
     </div> 
     <div> 
      <code></code> 
      <code>super</code> 
      <code>.onCreate(savedInstanceState);</code> 
     </div> 
     <div></div> 
     <div> 
      <code></code> 
      <code>mTextView = </code> 
      <code>new</code> 
      <code>TextView(</code> 
      <code>this</code> 
      <code>);</code> 
     </div> 
     <div></div> 
     <div> 
      <code></code> 
      <code>if</code> 
      <code>(savedInstanceState == </code> 
      <code>null</code> 
      <code>) { </code> 
     </div> 
     <div> 
      <code></code> 
      <code>mTextView.setText(</code> 
      <code>"Welcome to HelloAndroid!"</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>} </code> 
      <code>else</code> 
      <code>{ </code> 
     </div> 
     <div> 
      <code></code> 
      <code>mTextView.setText(</code> 
      <code>"Welcome back."</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>}</code> 
     </div> 
     <div></div> 
     <div> 
      <code></code> 
      <code>setContentView(mTextView);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>}</code> 
     </div> 
     <div></div> 
     <div> 
      <code></code> 
      <code>private</code> 
      <code>TextView mTextView = </code> 
      <code>null</code> 
      <code>;</code> 
     </div> 
     <div> 
      <code>}</code> 
     </div> 
    </div> </td> 
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这个android 小例子应该是每个人都开发过的，我每次关闭程序再打开是都是给我显示第一个Welcome to HelloAndroid ，我想在第二次访问的时候能够显示 Welcome back，我该如何处理？

### 处理方法 ###

你需要重写 onSaveInstanceState(Bundle savedInstanceState)，把你想要保存的状态写入到 Bundle 中，如下

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      <code>@Override</code> 
     </div> 
     <div> 
      <code>public</code> 
      <code> void</code> 
      <code>onSaveInstanceState(Bundle savedInstanceState) { </code> 
     </div> 
     <div> 
      <code></code> 
      <code>super</code> 
      <code>.onSaveInstanceState(savedInstanceState);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>// Save UI state changes to the savedInstanceState.</code> 
     </div> 
     <div> 
      <code></code> 
      <code>// This bundle will be passed to onCreate if the process is</code> 
     </div> 
     <div> 
      <code></code> 
      <code>// killed and restarted.</code> 
     </div> 
     <div> 
      <code></code> 
      <code>savedInstanceState.putBoolean(</code> 
      <code>"MyBoolean"</code> 
      <code>, </code> 
      <code>true</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>savedInstanceState.putDouble(</code> 
      <code>"myDouble"</code> 
      <code>, </code> 
      <code>1.9</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>savedInstanceState.putInt(</code> 
      <code>"MyInt"</code> 
      <code>, </code> 
      <code>1</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>savedInstanceState.putString(</code> 
      <code>"MyString"</code> 
      <code>, </code> 
      <code>"Welcome back to Android"</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>// etc.</code> 
     </div> 
     <div> 
      <code>}</code> 
     </div> 
    </div> </td> 
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Bundle 的本质是键值对的方式存储，你可以通过 onCreate()和 onRestoreInstanceState() 或的对应的值

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      <code>@Override</code> 
     </div> 
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      <code>public</code> 
      <code> void</code> 
      <code>onRestoreInstanceState(Bundle savedInstanceState) { </code> 
     </div> 
     <div> 
      <code></code> 
      <code>super</code> 
      <code>.onRestoreInstanceState(savedInstanceState);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>// Restore UI state from the savedInstanceState.</code> 
     </div> 
     <div> 
      <code></code> 
      <code>// This bundle has also been passed to onCreate.</code> 
     </div> 
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      <code></code> 
      <code>boolean</code> 
      <code>myBoolean = savedInstanceState.getBoolean(</code> 
      <code>"MyBoolean"</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>double</code> 
      <code>myDouble = savedInstanceState.getDouble(</code> 
      <code>"myDouble"</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>int</code> 
      <code>myInt = savedInstanceState.getInt(</code> 
      <code>"MyInt"</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code></code> 
      <code>String myString = savedInstanceState.getString(</code> 
      <code>"MyString"</code> 
      <code>);</code> 
     </div> 
     <div> 
      <code>}</code> 
     </div> 
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您通常会使用这项技术来存储实例的值的应用程序（选择，未保存的文本等）。

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