android - fragment 真的需要一个空的构造函数吗？

Question

我有一个带有多个参数的构造函数的 Fragment。我的应用在开发过程中运行良好，但在生产中我的用户有时会看到此崩溃:

android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment 
make sure class name exists, is public, and has an empty constructor that is public

我可以按照此错误消息的提示创建一个空的构造函数，但这对我来说没有意义，因为那时我必须调用一个单独的方法来完成设置 Fragment。

我很好奇为什么这种崩溃只是偶尔发生。也许我使用 ViewPager 不正确？我自己实例化所有 Fragment 并将它们保存在 Activity 内的列表中。我不使用 FragmentManager 事务，因为我看到的 ViewPager 示例不需要它，并且在开发过程中一切似乎都在工作。

Answer 1

是的。

你不应该真的重写构造函数。您应该定义一个 newInstance() 静态方法并通过参数(包)传递任何参数

例如:

public static final MyFragment newInstance(int title, String message) {
    MyFragment f = new MyFragment();
    Bundle bdl = new Bundle(2);
    bdl.putInt(EXTRA_TITLE, title);
    bdl.putString(EXTRA_MESSAGE, message);
    f.setArguments(bdl);
    return f;
}

当然也可以这样获取参数:

@Override
public void onCreate(Bundle savedInstanceState) {
    title = getArguments().getInt(EXTRA_TITLE);
    message = getArguments().getString(EXTRA_MESSAGE);

    //...
    //etc
    //...
}

然后你会像这样从 fragment 管理器中实例化:

@Override
public void onCreate(Bundle savedInstanceState) {
    if (savedInstanceState == null){
        getSupportFragmentManager()
            .beginTransaction()
            .replace(R.id.content, MyFragment.newInstance(
                R.string.alert_title,
                "Oh no, an error occurred!")
            )
            .commit();
    }
}

这样，如果分离并重新附加，对象状态可以通过参数存储。很像附加到 Intent 的 bundle 。

原因 - 额外阅读

我想我会为想知道为什么的人解释原因。

如果您检查:https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/app/Fragment.java

你会看到Fragment类中的instantiate(..)方法调用了newInstance方法:

public static Fragment instantiate(Context context, String fname, @Nullable Bundle args) {
    try {
        Class<?> clazz = sClassMap.get(fname);
        if (clazz == null) {
            // Class not found in the cache, see if it's real, and try to add it
            clazz = context.getClassLoader().loadClass(fname);
            if (!Fragment.class.isAssignableFrom(clazz)) {
                throw new InstantiationException("Trying to instantiate a class " + fname
                        + " that is not a Fragment", new ClassCastException());
            }
            sClassMap.put(fname, clazz);
        }
        Fragment f = (Fragment) clazz.getConstructor().newInstance();
        if (args != null) {
            args.setClassLoader(f.getClass().getClassLoader());
            f.setArguments(args);
        }
        return f;
    } catch (ClassNotFoundException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (java.lang.InstantiationException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (IllegalAccessException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (NoSuchMethodException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": could not find Fragment constructor", e);
    } catch (InvocationTargetException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": calling Fragment constructor caused an exception", e);
    }
}

http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#newInstance()解释为什么在实例化时它会检查访问器是 public 并且该类加载器允许访问它。

总的来说，这是一个非常讨厌的方法，但它允许 FragmentManger 杀死并重新创建带有状态的 Fragments。 (Android 子系统用 Activity 做类似的事情)。

示例类

我被问到很多关于调用 newInstance 的问题。不要将此与类方法混淆。这个整个类的例子应该显示用法。

/**
 * Created by chris on 21/11/2013
 */
public class StationInfoAccessibilityFragment extends BaseFragment implements JourneyProviderListener {

    public static final StationInfoAccessibilityFragment newInstance(String crsCode) {
        StationInfoAccessibilityFragment fragment = new StationInfoAccessibilityFragment();

        final Bundle args = new Bundle(1);
        args.putString(EXTRA_CRS_CODE, crsCode);
        fragment.setArguments(args);

        return fragment;
    }

    // Views
    LinearLayout mLinearLayout;

    /**
     * Layout Inflater
     */
    private LayoutInflater mInflater;
    /**
     * Station Crs Code
     */
    private String mCrsCode;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mCrsCode = getArguments().getString(EXTRA_CRS_CODE);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        mInflater = inflater;
        return inflater.inflate(R.layout.fragment_station_accessibility, container, false);
    }

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        mLinearLayout = (LinearLayout)view.findViewBy(R.id.station_info_accessibility_linear);
        //Do stuff
    }

    @Override
    public void onResume() {
        super.onResume();
        getActivity().getSupportActionBar().setTitle(R.string.station_info_access_mobility_title);
    }

    // Other methods etc...
}