我一直在研究Android SDK平台,有点不清楚如何保存应用程序的状态。因此,鉴于“你好,Android”示例的这个小工具改造:
package com.android.hello; import android.app.Activity; import android.os.Bundle; import android.widget.TextView; public class HelloAndroid extends Activity { private TextView mTextView = null; /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); mTextView = new TextView(this); if (savedInstanceState == null) { mTextView.setText("Welcome to HelloAndroid!"); } else { mTextView.setText("Welcome back."); } setContentView(mTextView); } }
我认为这对于最简单的情况就足够了,但无论我如何离开应用程序,它总是会响应第一条消息。
我确信解决方案就像覆盖onPause或类似的东西一样简单,但我已经在文档中戳了 30 分钟左右,没有发现任何明显的东西。
onPause
您需要覆盖onSaveInstanceState(Bundle savedInstanceState)并将要更改的应用程序状态值写入Bundle参数,如下所示:
onSaveInstanceState(Bundle savedInstanceState)
Bundle
@Override public void onSaveInstanceState(Bundle savedInstanceState) { super.onSaveInstanceState(savedInstanceState); // Save UI state changes to the savedInstanceState. // This bundle will be passed to onCreate if the process is // killed and restarted. savedInstanceState.putBoolean("MyBoolean", true); savedInstanceState.putDouble("myDouble", 1.9); savedInstanceState.putInt("MyInt", 1); savedInstanceState.putString("MyString", "Welcome back to Android"); // etc. }
Bundle 本质上是一种存储 NVP(“名称-值对”)映射的方式,它将被传递到onCreate()您onRestoreInstanceState()将从活动中提取值的位置,如下所示:
onCreate()
onRestoreInstanceState()
@Override public void onRestoreInstanceState(Bundle savedInstanceState) { super.onRestoreInstanceState(savedInstanceState); // Restore UI state from the savedInstanceState. // This bundle has also been passed to onCreate. boolean myBoolean = savedInstanceState.getBoolean("MyBoolean"); double myDouble = savedInstanceState.getDouble("myDouble"); int myInt = savedInstanceState.getInt("MyInt"); String myString = savedInstanceState.getString("MyString"); }
或者来自一个片段。
@Override public void onViewStateRestored(@Nullable Bundle savedInstanceState) { super.onViewStateRestored(savedInstanceState); // Restore UI state from the savedInstanceState. // This bundle has also been passed to onCreate. boolean myBoolean = savedInstanceState.getBoolean("MyBoolean"); double myDouble = savedInstanceState.getDouble("myDouble"); int myInt = savedInstanceState.getInt("MyInt"); String myString = savedInstanceState.getString("MyString"); }
您通常会使用这种技术来存储应用程序的实例值(选择、未保存的文本等)。