如何使用保存实例状态保存活动状态? [英] How can I save an activity state using the save instance state?

问题描述

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，考虑到对你好，Android"示例的这个小改动:

I've been working on the Android SDK platform, and it is a little unclear how to save an application's state. So given this minor re-tooling of the 'Hello, Android' example:

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，它总是以第一条消息作为响应.

I thought it would be enough for the simplest case, but it always responds with the first message, no matter how I navigate away from the app.

我确信解决方案就像覆盖 onPause 或类似的东西一样简单，但我已经在文档中摸索了 30 分钟左右，但没有发现任何明显的东西.

I'm sure the solution is as simple as overriding onPause or something like that, but I've been poking away in the documentation for 30 minutes or so and haven't found anything obvious.

推荐答案

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:

You need to override onSaveInstanceState(Bundle savedInstanceState) and write the application state values you want to change to the Bundle parameter like this:

@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，它将被传递给 onCreate() 和 onRestoreInstanceState()代码>，然后您将从活动中提取值，如下所示:

The Bundle is essentially a way of storing a NVP ("Name-Value Pair") map, and it will get passed in to onCreate() and also onRestoreInstanceState() where you would then extract the values from activity like this:

@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).

You would usually use this technique to store instance values for your application (selections, unsaved text, etc.).

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