解決問題：fragment does not have a view
要點(diǎn)：要在onViewCreated（）中使用viewpager，不能在onCreateView（）中使用。

public class TestDialog extends DialogFragment {
   View view;

   ViewPager vp ;
   MyAdapter myAdapter;

   @Override
   public void onCreate(@Nullable Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setStyle(DialogFragment.STYLE_NO_FRAME,R.style.Mdialog);
   }

   @Override
   public void onStart() {
       super.onStart();

       Window win = getDialog().getWindow();
       win.setBackgroundDrawable( new ColorDrawable(Color.TRANSPARENT));
       DisplayMetrics dm = new DisplayMetrics();
       getActivity().getWindowManager().getDefaultDisplay().getMetrics( dm );
       WindowManager.LayoutParams params = win.getAttributes();
       params.gravity = Gravity.BOTTOM;
       params.width =  ViewGroup.LayoutParams.MATCH_PARENT;
       params.height = ViewGroup.LayoutParams.WRAP_CONTENT;
       win.setAttributes(params);
   }

   @Nullable
   @Override
   public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
       view = inflater.inflate(R.layout.test2, container);
       return view;
   }

   @Override
   public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
       super.onViewCreated(view, savedInstanceState);

       myAdapter = new MyAdapter(getChildFragmentManager());
       vp = (ViewPager) view.findViewById(R.id.vp);
       vp.setAdapter(myAdapter);
   }
}