FragmentTransaction的隐藏/显示有时无效

Question

我有一个带有底部导航选项卡的活动，这些选项卡会更改其中的片段。当我在这些选项卡上来回点击时，某个时刻它停止工作了。代码执行得很好，因为我在其中添加了一些日志。但是片段没有被切换。

代码使用 Kotlin 编写，但它非常简单明了。

fun showTabFragment(tag: String) {
        val currentFragment: Fragment? = supportFragmentManager.fragments?.lastOrNull()
        var fragment = supportFragmentManager.findFragmentByTag(tag)
        val fragmentExists = fragment != null
        if (fragment == null) {
            when (tag) {
                TAG_LOGBOOK -> fragment = LogbookFragment()
                TAG_RECIPES -> fragment = RecipesFragment()
                TAG_PROFILE -> fragment = ProfileFragment()
                else -> fragment = MeetingPlacesFragment()
            }
        }

        val transaction = supportFragmentManager.beginTransaction()

        if (currentFragment != null) {
            Log.i("jacek", "hiding " + currentFragment.javaClass.simpleName)
            transaction.hide(currentFragment)
        }

        if (fragmentExists) {
            Log.i("jacek", "showing " + fragment.javaClass.simpleName)
            transaction.show(fragment)
        } else {
            Log.i("jacek", "adding " + fragment.javaClass.simpleName)
            transaction.add(R.id.container, fragment, tag)
        }

        transaction.commit()
    }

这些碎片相当沉重。我会尝试一些轻量级的，但在我看来这不应该是一个问题。还有其他什么可以尝试的吗？

我正在使用最新的支持库-25.2.0。 此外，我不想替换这些碎片，因为重点是在不重新创建它们的情况下添加交叉淡入淡出动画。

- Jacek Kwiecień

---

当您在同一事务中对相同片段调用hide()和show()时，可能会出现问题。就像在这里可能会发生的那样，currentFragment引用与fragment相同的片段。说到这个问题，我认为声称supportFragmentManager.fragments列表中的最后一个片段将是最后显示的片段（而不是最近添加的片段）是错误的。您应该遍历所有fragments并搜索具有isVisible()为true的片段，或者只需存储最后显示的片段标记并稍后查找它。 - Maciej Ciemięga

Answer 1

您需要重复使用要隐藏或显示的片段的同一实例。

private fun replaceFragment(fragment: Fragment) {
    supportFragmentManager.beginTransaction().apply {
        if (fragment.isAdded) {
            show(fragment)
        } else {
            add(R.id.fmFragmentContainer, fragment)
        }

        supportFragmentManager.fragments.forEach {
            if (it != fragment && it.isAdded) {
                hide(it)
            }
        }
    }.commit()
}

- dtunctuncer

Answer 2

@Ali的回答很好，但是想象一下如果你有5个片段。这是另一种显示/隐藏你的片段的方式：

@Ali的回答不���，但如果您有5个片段，那么以下是另一种显示/隐藏您的片段的方法：

    // in BaseFragment
    public abstract String getTAG();

    //in FragmentA, FragmentB and FragmentC
    public String getTAG(){
        return TAG;
    }

    //Activity containing the fragments
    //android.support.v4.app.Fragment;    
    private FragmentA fragmentA; //inherited BaseFragment
    private FragmentB fragmentB; //inherited BaseFragment
    private FragmentC fragmentC; //inherited BaseFragment
    private ConcurrentHashMap<String,BaseFragment> mapOfAddedFragments = new ConcurrentHashMap<>();


    /**
     * Displays fragment A
     */
    private void displayFragmentA() {
        displayFragment(fragmentA)
    }

    /**
     * Displays  fragment B
     */
    private void displayFragmentB() {
       displayFragment(fragmentB)
    }

    /**
     * Displays  fragment C
     */
    private void displayFragmentC() {
        displayFragment(fragmentC)
    }


     /**
     * Loads a fragment using show a fragment
     * @param fragment
     */
    private void displayFragment(BaseFragment fragment){
        if(!mapOfAddedFragments.containsKey(fragment.getTAG()))
            mapOfAddedFragments.put(fragment.getTAG(), fragment);

        showFragment(fragment.getTAG(), R.id.containerBody);
    }

    /**
     * Displays a fragment and hides all the other ones
     * @param fragmentTag is the tag of the fragment we want to display
     */
    private void showFragment(String fragmentTag, @IdRes int containerViewId){
        FragmentTransaction ft = this.getSupportFragmentManager().beginTransaction();
        BaseFragment fragment = null;

        fragment = mapOfAddedFragments.get(fragmentTag);
        if(fragment != null) {
            if (fragment.isAdded())
                ft.show(fragment);
            else { //fragment needs to be added to the frame container
                ft.add(containerViewId, fragment, fragment.getTAG());
            }
        }
        else //the chosen fragment doesn't exist
            return;

        //we hide the other fragments
        for (ConcurrentHashMap.Entry<String, BaseFragment> entry : mapOfAddedFragments.entrySet()){
            if(!entry.getKey().equals(fragmentTag)){
                BaseFragment fragmentTemp = entry.getValue();
                // Hide the other fragments
                if(fragmentTemp != null)
                    if(fragmentTemp.isAdded())
                        ft.hide(fragmentTemp);
            }
        }

        //commit changes
        ft.commit();
    }

要实例化它们，您可以在活动的onCreate()方法中执行以下操作：

//don't forget to get the .TAG elsewhere before using them here
    //never call them directly
    private void instantiateFragments(Bundle inState) {
        if (inState != null) {
            fragmentA = inState.containsKey(FragmentA.TAG) ?
                    (FragmentA) getSupportFragmentManager().getFragment(inState, FragmentA.TAG):
                    FragmentA.newInstance(FragmentA.TAG,"0");

            fragmentB = inState.containsKey(FragmentB.TAG) ?
                    (FragmentB) getSupportFragmentManager().getFragment(inState, FragmentB.TAG):
                    FragmentB.newInstance(FragmentB.TAG,"1");        

            fragmentc = inState.containsKey(FragmentC.TAG) ?
                    (FragmentC) getSupportFragmentManager().getFragment(inState, FragmentC.TAG):
                    FragmentC.newInstance(FragmentC.TAG,"2");         
        }
        else{
            fragmentA = FragmentA.newInstance(FragmentA.TAG,"0");
            fragmentB = FragmentB.newInstance(FragmentB.TAG,"1");
            fragmentc = FragmentC.newInstance(FragmentC.TAG,"2");
        }
    }

根据Shujaat Ali Khan的问题进行编辑：

BaseFragment扩展了support4 fragment：

public abstract class BaseFragment extends Fragment {
    public abstract String getTAG();
    //whatever we can add to be inherited
}

例如，FragmentA：

public class FragmentA extends BaseFragment {
    // Store instance variables
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    private String mParam1;
    private String mParam2;

    public static final String TAG = "FragmentA";

    // newInstance constructor for creating fragment with arguments
    public static FragmentA newInstance(String param1, String param2) {
        FragmentA fragment = new FragmentA();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }

    // Store instance variables based on arguments passed
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }
    }

    // Inflate the view for the fragment based on layout XML
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragmentA, container, false);
        return view;
    }

    //other lifecycle methods

    @Override
    public String getTAG() {
        return TAG;
    }
}

最后，R.id.containerBody 是一个 FrameLayout 的 id，该 FrameLayout 包含在包含这些碎片的活动中。

- Maxime Claude

---

请问您能否展示至少 FragmentA 类的实现？由于我是初学者，所以对 NewInstance() 方法的定义感到困惑。 - Shuji

@ShujaatAliKhan 我刚刚编辑了答案，添加了一个例子。 - Maxime Claude

Answer 3

问题在于，即使您隐藏了“当前”片段，仍会在内存中加载其他片段，这会导致不一致的行为。

您应该能够通过隐藏除要显示的片段之外的所有片段来解决此问题。

感谢这个答案。 在Android中显示/隐藏片段

例如：

private FragmentA fragmentA;
private FragmentB fragmentB;
private FragmentC fragmentC;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    fragmentA = FragmentA.newInstance();
    fragmentB = FragmentB.newInstance();
    fragmentC = FragmentC.newInstance();

}

protected void displayFragmentA() {

    FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
    if (fragmentA.isAdded()) { 
        ft.show(fragmentA);
    } else { 
        ft.add(R.id.fragement_container, fragmentA);
    }

    if (fragmentB.isAdded()) { ft.hide(fragmentB); }

    if (fragmentC.isAdded()) { ft.hide(fragmentC); }

    ft.commit();
}

同样，您需要编写显示Fragment B()和显示Fragment C()的函数。

- Ali