Android View - 在 Activity 中自动保存和恢复的内容

Question

我是 Android 的初学者。

在Android中，一些通用元素可以在onSaveInstanceState/onRestoreInstanceState中自动保存/恢复。

例如，EditText 保存/恢复Text 属性，RatingBar 保存/恢复Rating属性(property)...

我从一些测试中看到，但我在文档中找不到任何相关内容。

我如何知道在没有我干预的情况下隐式保存/恢复了什么？

例如，我在哪里可以找到自动保存/恢复EditText.Text？

我特别不想测试所有属性。

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编辑自 JRG 答案:

https://developer.android.com/guide/components/activities/activity-lifecycle.html

Save your activity state As your activity begins to stop, the system calls the onSaveInstanceState() method<...> The default implementation of this method saves transient information about the state of the activity's view hierarchy, such as the text in an EditText widget or the scroll position of a ListView widget.

我怎么知道保存/恢复的默认实现是什么？

重读 JRG 答案后的第二次编辑:

By default, the system uses the Bundle instance state to save information about >each View object in your activity layout (such as the text value entered into >an EditText widget).

默认实现保存/恢复元素 View 的所有状态。

Answer 1

解释有关保存状态的 Android 文档和一篇关于在 Activity 和 fragment 中保存状态的非常好的文章。

• Android 文档: https://developer.android.com/guide/components/activities/activity-lifecycle.html
• 附加文章: https://inthecheesefactory.com/blog/fragment-state-saving-best-practices/en

Saving and restoring activity state There are a few scenarios in which your activity is destroyed due to normal app behavior, such as when the user presses the Back button or your activity signals its own destruction by calling the finish() method. The system may also destroy the process containing your activity to recover memory if the activity is in the Stopped state and hasn't been used in a long time, or if the foreground activity requires more resources.

When your activity is destroyed because the user presses "Back" or the activity finishes itself, the system's concept of that Activity instance is gone forever because the behavior indicates the activity is no longer needed. However, if the system destroys the activity due to system constraints (rather than normal app behavior), then although the actual Activity instance is gone, the system remembers that it existed such that if the user navigates back to it, the system creates a new instance of the activity using a set of saved data that describes the state of the activity when it was destroyed. The saved data that the system uses to restore the previous state is called the instance state and is a collection of key-value pairs stored in a Bundle object.

By default, the system uses the Bundle instance state to save information about each View object in your activity layout (such as the text value entered into an EditText widget). So, if your activity instance is destroyed and recreated, the state of the layout is restored to its previous state with no code required by you. However, your activity might have more state information that you'd like to restore, such as member variables that track the user's progress in the activity.

Save your activity stateAs your activity begins to stop, the system calls the onSaveInstanceState() method so your activity can save state information with a collection of key-value pairs. The default implementation of this method saves transient information about the state of the activity's view hierarchy, such as the text in an EditText widget or the scroll position of a ListView widget.

Caution: You must always call the superclass implementation of onSaveInstanceState() so the default implementation can save the state of the view hierarchy.

To save additional state information for your activity, you must override onSaveInstanceState() and add key-value pairs to the Bundle object that is saved in the event that your activity is destroyed unexpectedly. For example:

static final String STATE_SCORE = "playerScore";
static final String STATE_LEVEL = "playerLevel";
...


@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    // Save the user's current game state
    savedInstanceState.putInt(STATE_SCORE, mCurrentScore);
    savedInstanceState.putInt(STATE_LEVEL, mCurrentLevel);


    // Always call the superclass so it can save the view hierarchy state
    super.onSaveInstanceState(savedInstanceState);
}

Note: In order for the Android system to restore the state of the views in your activity, each view must have a unique ID, supplied by the android:id attribute.

To save persistent data, such as user preferences or data for a database, you should take appropriate opportunities when your activity is in the foreground. If no such opportunity arises, you should save such data during the onStop() method.

Restore your activity stateWhen your activity is recreated after it was previously destroyed, you can recover your saved state from the Bundle that the system passes to your activity. Both the onCreate() and onRestoreInstanceState() callback methods receive the same Bundle that contains the instance state information.

Because the onCreate() method is called whether the system is creating a new instance of your activity or recreating a previous one, you must check whether the state Bundle is null before you attempt to read it. If it is null, then the system is creating a new instance of the activity, instead of restoring a previous one that was destroyed.

For example, the following code snippet shows how you can restore some state data in onCreate():

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState); // Always call the superclass first


    // Check whether we're recreating a previously destroyed instance
    if (savedInstanceState != null) {
        // Restore value of members from saved state
        mCurrentScore = savedInstanceState.getInt(STATE_SCORE);
        mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL);
    } else {
        // Probably initialize members with default values for a new instance
    }
    ...
}

Instead of restoring the state during onCreate() you may choose to implement onRestoreInstanceState(), which the system calls after the onStart() method. The system calls onRestoreInstanceState() only if there is a saved state to restore, so you do not need to check whether the Bundle is null:

public void onRestoreInstanceState(Bundle savedInstanceState) {
    // Always call the superclass so it can restore the view hierarchy
    super.onRestoreInstanceState(savedInstanceState);


    // Restore state members from saved instance
    mCurrentScore = savedInstanceState.getInt(STATE_SCORE);
    mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL);
}

Caution: Always call the superclass implementation of onRestoreInstanceState() so the default implementation can restore the state of the view hierarchy.