android - onSaveInstanceState(Bundle) 方法未定义

Question

我目前正在做一个关于 eclipse android 开发的教程系列，我正在尝试复制代码以帮助记住所有功能，但我收到 onSaveInstanceState(Bundle) 的错误，它说“onSaveInstanceState(Bundle) 方法是对象类型未定义”。我已经多次检查以确保一切准确无误，但没有发现任何错误。

protected void onSaveInstanceState(Bundle outState)
     {
        super.onSaveInstanceState(outState);

        outState.putDouble(TOTAL_BILL, finalBill);
        outState.putDouble(CURRENT_TIP, tipAmount);
        outState.putDouble(BILL_WITHOUT_TIP, finalBill);
     }

Answer 1

您需要使用@Override 并确保您的类扩展Activity。只要具备这两个条件，您就应该做得很好。

class MyActivity extends Activity {

    @Override // May be public depending on the class you are extending
    protected void onSaveInstanceState(Bundle outState) {

        outState.putDouble(TOTAL_BILL, finalBill);
        outState.putDouble(CURRENT_TIP, tipAmount);
        outState.putDouble(BILL_WITHOUT_TIP, finalBill);

        // Wait till after you've added your items to pass the bundle
        super.onSaveInstanceState(outState);
     }
}

根据Activity文档页面其他扩展 Activity 的类是 AccountAuthenticatorActivity , ActivityGroup , AliasActivity , ExpandableListActivity , FragmentActivity , ListActivity , NativeActivity , ActionBarActivity , LauncherActivity , PreferenceActivity和 TabActivity .

如果您想弄清楚某些东西是如何工作的，那么 Android 源代码也是一个不错的地方。很多时候，这些注释更多地描述了事情是如何实现的以及它们依赖什么。

引用:core/java/android/app/Activity.java

    /**
     * Called to retrieve per-instance state from an activity before being killed
     * so that the state can be restored in {@link #onCreate} or
     * {@link #onRestoreInstanceState} (the {@link Bundle} populated by this method
     * will be passed to both).
     *
     * <p>This method is called before an activity may be killed so that when it
     * comes back some time in the future it can restore its state.  For example,
     * if activity B is launched in front of activity A, and at some point activity
     * A is killed to reclaim resources, activity A will have a chance to save the
     * current state of its user interface via this method so that when the user
     * returns to activity A, the state of the user interface can be restored
     * via {@link #onCreate} or {@link #onRestoreInstanceState}.
     *
     * <p>Do not confuse this method with activity lifecycle callbacks such as
     * {@link #onPause}, which is always called when an activity is being placed
     * in the background or on its way to destruction, or {@link #onStop} which
     * is called before destruction.  One example of when {@link #onPause} and
     * {@link #onStop} is called and not this method is when a user navigates back
     * from activity B to activity A: there is no need to call {@link #onSaveInstanceState}
     * on B because that particular instance will never be restored, so the
     * system avoids calling it.  An example when {@link #onPause} is called and
     * not {@link #onSaveInstanceState} is when activity B is launched in front of activity A:
     * the system may avoid calling {@link #onSaveInstanceState} on activity A if it isn't
     * killed during the lifetime of B since the state of the user interface of
     * A will stay intact.
     *
     * <p>The default implementation takes care of most of the UI per-instance
     * state for you by calling {@link android.view.View#onSaveInstanceState()} on each
     * view in the hierarchy that has an id, and by saving the id of the currently
     * focused view (all of which is restored by the default implementation of
     * {@link #onRestoreInstanceState}).  If you override this method to save additional
     * information not captured by each individual view, you will likely want to
     * call through to the default implementation, otherwise be prepared to save
     * all of the state of each view yourself.
     *
     * <p>If called, this method will occur before {@link #onStop}.  There are
     * no guarantees about whether it will occur before or after {@link #onPause}.
     * 
     * @param outState Bundle in which to place your saved state.
     * 
     * @see #onCreate
     * @see #onRestoreInstanceState
     * @see #onPause
     */
    protected void onSaveInstanceState(Bundle outState) {
        outState.putBundle(WINDOW_HIERARCHY_TAG, mWindow.saveHierarchyState());
        Parcelable p = mFragments.saveAllState();
        if (p != null) {
            outState.putParcelable(FRAGMENTS_TAG, p);
        }
        getApplication().dispatchActivitySaveInstanceState(this, outState);
    }