按变量名称启动片段

Question

我想知道是否可以fragment通过variable名称启动而不是硬编码fragments名称.

请允许我发布样本

这是传统方式启动片段的方式:

FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(R.id.your_placehodler, new YourFragment());
ft.commit();

但是说你试图在fragment不知道它的名字的情况下启动它,或者可能是fragment它的名字.说像listFragment,或者Listview你正在通过一个运行array的Fragment名字.因此你会做这样的事情:

@Override
public void onListItemClick(ListView l, View v, int position, long id) {

     private String[] values = new String[] { "frag1", "frag2", "frag3" };

     String someFragment = values[position];
     String fragName = (someFragment + ".class");

     try {
     FragmentTransaction ft = getFragmentManager().beginTransaction();
     ft.replace(R.id.your_placehodler, new fragName());
     ft.commit();
     } catch (Exception e) {
     //print message
     }

我知道这不正确,但我觉得如果有可能我可能会接近.我搜索了一会儿,但我一无所获.

所以我的问题,这可能吗？如果是这样我将如何实现它？谢谢!

编辑 我尝试使用此代码尝试使用Reflections API

@Override
public void onListItemClick(ListView l, View v, int position, long id) {
    String questions = values[position];

    try {
        Fragment frags = (Fragment) Class.forName("com.example.android." + questions).newInstance();            
        getFragmentManager()
                .beginTransaction()
                .setCustomAnimations(android.R.animator.fade_in,
                        android.R.animator.fade_out)
                .replace(R.id.header_fragment_container, frags).commit();

    }

    catch (Exception e) {

    }

}
}

我收到一条消息说 05-08 04:38:14.124: W/dalvikvm(812): dvmFindClassByName rejecting 'com.android.example.Ovens'

然而,如果在我的代码中我改变了行说 Fragment frags = (Fragment) Class.forName("com.android.example." + "Ovens").newInstance();

有用

变量"questions"是类名的精确副本.我不明白为什么它不起作用.没有任何事情发生,没有任何东西打印到logcat

最终编辑

得到它了!我错过了""标记.这是最终的工作代码,感谢所有的帮助

@Override
public void onListItemClick(ListView l, View v, int position, long id) {
    String questions = values[position];
    try {
        Fragment frags = (Fragment) Class.forName("com.android.example." + "" + questions).newInstance();
        getFragmentManager()
                .beginTransaction()
                .setCustomAnimations(android.R.animator.fade_in,
                        android.R.animator.fade_out)
                .replace(R.id.header_fragment_container, frags).commit();
    }

    catch (Exception e) {

    }
}
}

Answer 1

可能为时已晚,无法回答其他人.而不是使用

 Fragment frags = (Fragment) Class.forName("com.android.example." + "" + questions).newInstance();

使用

Fragment frags = Fragment.instantiate(mContext,"com.android.example." + "" + questions);

如果你想将参数传递给你可以使用的片段

Fragment frags = Fragment.instantiate(mContext,"com.android.example." + "" + questions,bundle);

bundle是包含数据的Bundle.

编辑:

至于为什么你应该更喜欢这个而不是其他:

1. 您可以将Bundle args发送给您片段
2. 使用sClassMap进行缓存
3. 完整性检查初始化的类是否为片段

这是片段中实例化方法的代码

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.newInstance();
            if (args != null) {
                args.setClassLoader(f.getClass().getClassLoader());
                f.mArguments = 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);
        }
    }

其中sClassMap是HashMap

private static final HashMap<String, Class<?>> sClassMap =
        new HashMap<String, Class<?>>();

显然你可以在上面的代码中实现相同的功能.不这样做的理由"干"

Answer 2

实现这一目标的一种方法是通过反射API.

Class.forName("com.example.MyFragment").newInstance();

这可以抛出一大堆异常,所以请注意这一点.

另一种方法是为Fragments创建一个简单的工厂类.

public abstract class MyFragmentFactory {
    private MyFragmentFactory(){}

    public static <T extends Fragment> T getFragment(String name){
        if("MyFragment".equals(name)){
            return new MyFragment();
        }else if("whatever".equals(name)){
            // ...
        }else{
            throw new RuntimeException("unknown fragment "+ name);
        }
    }
}