android - 我的自定义 ListView 适配器上的致命异常

Question

我试图在 ParseUser 对象上(粗略地)实现一个搜索界面，但我遇到了问题。

搜索方法如下:

private void QueryUser() {
    FlatEditText gUname;
    gUname = (FlatEditText)findViewById(R.id.search_editText);

    String gUsername = gUname.getText().toString();

    Log.d("singleTune", "Check for "+gUsername);
    ParseQuery<ParseUser> query = ParseUser.getQuery();
    query.whereEqualTo("username", gUsername);
    query.orderByAscending(ParseConstants.KEY_USERNAME);
    query.setLimit(1000);
    query.findInBackground(new FindCallback<ParseUser>() {
        @Override
        public void done(List<ParseUser> users, ParseException e) {
            if(e != null){
                Log.d("singleTune", "Nothing found");
            }else{
                mUsers = users;
                String[] usernames = new String[mUsers.size()];
                int i = 0;
                for(ParseUser user : mUsers){
                    usernames[i] = user.getUsername();
                    i++;
                }
                UserListAdapter adapter = new UserListAdapter(SearchActivity.this, mUsers);
                setListAdapter(adapter);
            }

        }
    });
}

这是我的适配器:

public class UserListAdapter extends ArrayAdapter<ParseUser> {

    protected Context mContext;
    protected List<ParseUser> mParseUsers;

    public UserListAdapter(Context context, List<ParseUser> users){
        super(context, R.layout.user_item, users);
        mContext = context;
        mParseUsers = users;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        View rowView = convertView;
        ViewHolder holder;

        if (rowView == null){
            rowView = LayoutInflater.from(mContext).inflate(R.layout.user_item, null);
            holder = new ViewHolder();
            holder.avatar = (ImageView)convertView.findViewById(R.id.user_imageView);
            holder.usernameLabel = (TextView)convertView.findViewById(R.id.username_textView);
            holder.followButton = (FlatButton)convertView.findViewById(R.id.item_follow_button);
        }
        else{
            holder = (ViewHolder)convertView.getTag();
        }

        ParseUser user = mParseUsers.get(position);
        Log.d("SingleTune", "Username from query is "+user.getUsername());
        holder.avatar.setImageResource(R.drawable.ic_action_picture);
        holder.usernameLabel.setText(user.getString(ParseConstants.KEY_USERNAME));

        return rowView                                                                                                                                                                                                                                                                                                                                                                                                                                                                      ;
    }

    private static class ViewHolder{
        ImageView avatar;
        TextView usernameLabel;
        FlatButton followButton;
    }
}

这是我的日志:

04-30 08:09:04.727 1144-1144/com.pixel.singletune.app E/AndroidRuntime? FATAL EXCEPTION: main java.lang.NullPointerException
at adapters.UserListAdapter.getView(UserListAdapter.java:43)
at android.widget.AbsListView.obtainView(AbsListView.java:2267)
at android.widget.ListView.measureHeightOfChildren(ListView.java:1244)
at android.widget.ListView.onMeasure(ListView.java:1156)
at android.view.View.measure(View.java:15172)
at android.widget.RelativeLayout.measureChild(RelativeLayout.java:602)
at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:415)
at android.view.View.measure(View.java:15172)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4814)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
at android.view.View.measure(View.java:15172)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:833)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:574)
at android.view.View.measure(View.java:15172)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4814)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2148) at android.view.View.measure(View.java:15172)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1848)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1100)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1273)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:998)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4212)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:725)
at android.view.Choreographer.doCallbacks(Choreographer.java:555)
at android.view.Choreographer.doFrame(Choreographer.java:525)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:711)
at android.os.Handler.handleCallback(Handler.java:615)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)

我难住了伙计们。有帮助吗？

Answer 1

考虑 convertView 为 null 的情况。 rowView 将被分配相同的值(null)，因此您将进入此 if block 。

View rowView = convertView;
ViewHolder holder;

if (rowView == null){
    rowView = LayoutInflater.from(mContext).inflate(R.layout.user_item, null);
    holder = new ViewHolder();
    holder.avatar = (ImageView)convertView.findViewById(R.id.user_imageView);
    holder.usernameLabel = (TextView)convertView.findViewById(R.id.username_textView);
    holder.followButton = (FlatButton)convertView.findViewById(R.id.item_follow_button);
}

您为 rowView 分配了一个值，但此时 convertView 仍指向 null。这可能是您遇到异常的地方。您可以在 rowView 上调用 findViewById 来解决这个问题。