Android Fragment 使用技巧

如果没有无参构造函数，而是像按照普通类来使用，只创建有参构造函数，则会出现 android.support.v4.app.Fragment$InstantiationException 错误。

原因：Fragment 和 Activity 都是生命周期的组件，不能看做一般的类。如果非要使用有参构造函数，可能在使用的时候第一次传参没有问题，但是大概率在后面使用的时候出现问题。因为Fragment的什么周期依附在Activity中，如果Activity为null，那么Fragment肯定不能够正常使用了，比如手机屏幕的横竖屏切换导致Activity重建了。

至于为什么是这样的呢？看下Fragment初始化的源码,有这么一段:

/**
 * Create a new instance of a Fragment with the given class name.  This is
 * the same as calling its empty constructor.
 *
 * @param context The calling context being used to instantiate the fragment.
 * This is currently just used to get its ClassLoader.
 * @param fname The class name of the fragment to instantiate.
 * @param args Bundle of arguments to supply to the fragment, which it
 * can retrieve with {@link #getArguments()}.  May be null.
 * @return Returns a new fragment instance.
 * @throws InstantiationException If there is a failure in instantiating
 * the given fragment class.  This is a runtime exception; it is not
 * normally expected to happen.
 */
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);
            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);
    }
}

整个过程中,Fragment的创建其实也是利用了无参数的构造方法去实例化.但关键的是,它将Bundle传类新建的Fragment,这样旧的Fragment和新的Fragment就能拥有一样的Bundle,从而达到利用Bundle传递参数的目的. 

查看Android的SDK文档，也给出来相关的说法：

2. 给 Fragment 传递参数

一定要使用 Bundle 方式传递参数，而不是通过重载构造函数传递参数。

public static VechileFrag newInstance(Vehicle vehicle, String userId, boolean isAdd) {
    VechileFrag mf = new VechileFrag();
    Bundle args = new Bundle();
    args.putString("userId", userId);
    args.putBoolean("isAdd", isAdd);
    args.putParcelable("vehicle", vehicle);
    mf.setArguments(args);
    return mf;
}


@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Bundle args = getArguments();
    if (args != null) {
        userId = args.getString("userId");
        isAdd = args.getBoolean("isAdd");
        vehicle = args.getParcelable("vehicle");
        if (vehicle == null) {
            vehicle = new Vehicle();
        }
    }
}            

3. Fragment 与 Activity 通信

在 Fragment 中定义一个接口和要回调的方法， Activity实现Fragment接口，出国留学的条件需要时回调 Fragment 方法。

public IVechile mIVechile;
　　public interface IVechile {
　　public void submitCarSuccess(String carId, String plateNo);
　　}
　　@Override
　　public void onAttach(Activity activity) { 
　　　　fueltypes = FuelType.getList(activity);
　　　　try {
　　　　　　mIVechile = (IVechile) activity;
　　　　}
　　　　catch (Exception e) {
　　　　// TODO: handle exception

　　　　}
　　}
}