使用“onStart”方法时如何声明savedInstanceState

Question

可以if (savedInstanceState == null) {...}在onStart我的片段的方法中使用吗？每当我在onCreate方法中使用它时，我的片段都显示为空白。

public class MyActivity extends AppCompatActivity {

    private static final String TAG = MyActivity.class.getSimpleName();

    private Boolean mCurrentValue;

    @Override

    public void onCreate(Bundle savedInstanceState) {

        Log.d(TAG, "onCreate:::: retrieving preferences");

        SharedPreferences mSharedPreferences = PreferenceManager.getDefaultSharedPreferences(this);

        mCurrentValue = mSharedPreferences.getBoolean("my_preference", false);

        Log.d(TAG, "onCreate:::: my_preference and mCurrentValue=" + mCurrentValue);

        if (mCurrentValue) {

            setTheme(R.style.MyDarkAppCompatTheme);

            Log.d(TAG, "onCreate:::: setTheme:MyDarkAppCompatNoActionBarTheme");

        } else {

            setTheme(R.style.MyLightTheme);

            Log.d(TAG, "onCreate:::: setTheme:MyLightAppCompatNoActionBarTheme");

        }

        super.onCreate(savedInstanceState);

        setContentView(R.layout.my_activity);

        if (savedInstanceState == null) {

        }

    }

    // in order to recreate Activity we must check value here because when we come back from another Activity onCreate doesn't called again

    @Override

    protected void onStart() {

        super.onStart();

        setContentView(R.layout.my_activity);

        SharedPreferences mSharedPreferences = PreferenceManager.getDefaultSharedPreferences(this);

        Boolean mNewValue = mSharedPreferences.getBoolean("my_preference", false);

        // if value differs from previous Theme, we recreate Activity

        if(mCurrentValue != mNewValue){

            recreate();

        }

        MyFragment newFragment = new MyFragment();

        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

        transaction.replace(R.id.detail_container, newFragment);

        transaction.commit();

    }

}

Answer 1

不可以。如果您需要访问保存的数据片段，您需要从 onCreate 中的 savedInstanceState Bundle 中提取它们，并将它们保存为您的 Activity 中的字段。如果您只需要检测此 Activity 是否已重新创建（与检查 savedInstanceState 是否为 null 一样常见），您可以在 Activity 中声明一个布尔字段，并在savedInstanceState 在范围内时将其设置在 onCreate 中，然后在开始：private boolean activityRecreated;@Overridepublic void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    activityRecreated = savedInstanceState != null;}@Overridepublic void onStart() {    if (activityRecreated) {       // code here    }}编辑：获得有关您要保存的状态的更多信息后，请尝试以下操作：您的活动中应该有一个字段来跟踪 ViewPager 的状态。此标签保存用户所在的选项卡。它可以是 String 标签或 Id (int) 标签。您应该通过在您的活动中覆盖 onSaveInstanceState 来保存它：private int tabIndex = 0 // NOTE: get this from your ViewPager's PagerAdapter@Overrideprotected void onSaveInstanceState(Bundle outState) {    super.onSaveInstanceState(outState);    outState.putInt("tabIndex", tabIndex)}然后，在 onCreate 中，您可以检索此值：@Overrideprotected void onCreate(@Nullable Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    if (savedInstanceState != null) {      tabIndex = savedInstanceState.getInt("tabIndex", 0);      // here, make sure your ViewPager's PagerAdapter is created and set it to this tab    }    // rest of your onCreate goes here}以这种方式保存标签索引将允许在正确的 ViewPager 标签可见的情况下重新创建活动。