使用'onStart'方法時如何聲明savedInstanceState

Question

是否可以在片段的onStart方法中使用if (savedInstanceState == null) {...} ？ 每當我在onCreate方法中使用它時，我的片段就會顯示為空白。

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

否。如果需要訪問已保存的數據，則需要從onCreate的savedInstanceState捆綁包中提取數據並將其另存為Activity中的字段。 如果您只需要檢測是否重新創建了此Activity（與通常情況下是否檢查saveInstanceState為null相同），則可以在Activity中聲明一個布爾字段，並在saveInstanceState在范圍內時在onCreate中進行設置，然后在其中訪問它。在onStart：

private boolean activityRecreated;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    activityRecreated = savedInstanceState != null;
}

@Override
public void onStart() {
    if (activityRecreated) {
       // code here
    }
}

編輯：獲得有關您要保存的狀態的更多信息后，請嘗試以下操作：您應該在Activity中有一個跟蹤ViewPager狀態的字段。 此標記保存用戶所在的選項卡。 它可以是String標記或Id（int）標記。 您應該通過重寫Activity中的onSaveInstanceState來保存它：

private int tabIndex = 0 // NOTE: get this from your ViewPager's PagerAdapter
@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt("tabIndex", tabIndex)
}

然后，在onCreate中，您可以檢索以下值：

@Override
protected 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
}

以這種方式保存選項卡索引將允許在可見的適當ViewPager選項卡的情況下重新創建活動。