Question

我在调用ListView的ListView.getChildAt()之后尝试运行setAdapter()方法，但它给了我NullPointerException。似乎设置适配器不会导致创建子视图。如下面的方法所示，我正在尝试获取子视图，以便我可以更改其背景颜色。我该如何解决这个问题？

private void showAnswers(int questionLocation)
{
    List<Answer> answers = getAnswersByQuestionLocation(questionLocation);

    ArrayAdapter<String> answerAdapter = new ArrayAdapter<String>(this,
                                    android.R.layout.simple_list_item_activated_1);

    for (int i = 0; i < answers.size(); i++)
    {
        answerAdapter.add(mOptionLetters[i] +". "+ answers.get(i).getAnswerText());
    }

    mAnswerList.setAdapter(answerAdapter);

    if (mAnswerLocationByQuestionLocation.indexOfKey(questionLocation) > -1)
    {
        Log.v("Child Count",String.valueOf(mAnswerList.getChildCount()));

            //mAnswerList.getChildAt(
            //       mAnswerLocationByQuestionLocation.get(questionLocation))
            //      .setSelected(true);
    }
}

Answer 1

如果您只需更改背景，这是怎么回事：

    ArrayAdapter<String> answerAdapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_activated_1) {
        @Override
        public View getView(int position, View convertView,
                ViewGroup parent) {
            View v =super.getView(position, convertView, parent);;
            v.setBackgroundColor(Color.RED);
            return v;
        }

    };

因为您在UI线程上运行，所以listView将需要足够的时间在线程的下一个循环中创建视图，但您无法猜测何时。所以最好的方法是覆盖适配器中的getView，因为适配器将listView与子节点一起提供：）

class MyModel {

           public MyMode(String txt){
             this.txt = txt
            }

        public String txt;
        public boolean isSelected;

        @Override
        public String toString() {
            return txt;
        }
    }

ArrayAdapter<MyModel> answerAdapter = new ArrayAdapter<MyModel>(this,
        android.R.layout.simple_list_item_activated_1) {
    @Override
    public View getView(int position, View convertView,
            ViewGroup parent) {
        View v =super.getView(position, convertView, parent);;
        MyModel  model = getItem(position);
        if(model.isSelected){
           v.setBackgroundColor(Color.RED);} 
         else{
           v.setBackgroundColor(Color.WHITE);}
        return v;
    }

};

answerAdapter.add(new MyModel(mOptionLetters[i] +". "+ answers.get(i).getAnswerText()));

if (mAnswerLocationByQuestionLocation.indexOfKey(questionLocation) > -1)
    {
        MyModel model = (MyModel)mAnswerList.getItemAtPosition(questionLocation);
    model.isSelected= true;
    answerAdapter.notifyDataSetChanged();

    }
}

无论如何，您需要阅读有关ListViews和Adapters的本教程 http://www.vogella.com/articles/AndroidListView/article.html

设置Adapter后调用ListView的getChildAt（）方法

1 个答案: