高效 list adapter

Android api demo里面有一个编写高效list adapter的demo，里面写了建议的两条高效原则

1. 在getView方法中，重复利用 convertView，convertView是旧的View，建议先判断是否为空，如果不为空，可以修改其内容来显示新的row。

public View getView(int position, View convertView, ViewGroup parent) {
            SpeechView sv;
            if (convertView == null) {
                sv = new SpeechView(mContext, mTitles[position],
                        mDialogue[position]);
            } else {
                sv = (SpeechView) convertView;
                sv.setTitle(mTitles[position]);
                sv.setDialogue(mDialogue[position]);
            }

            return sv;
        }

2. 在getView方法中，利用ViewHolder来保存与convertView相关联的子View，避免调用 findViewById方法，以提高效率

public View getView(int position, View convertView, ViewGroup parent) {
            // A ViewHolder keeps references to children views to avoid unneccessary calls
            // to findViewById() on each row.
            ViewHolder holder;

            // When convertView is not null, we can reuse it directly, there is no need
            // to reinflate it. We only inflate a new View when the convertView supplied
            // by ListView is null.
            if (convertView == null) {
                convertView = mInflater.inflate(R.layout.list_item_icon_text, null);

                // Creates a ViewHolder and store references to the two children views
                // we want to bind data to.
                holder = new ViewHolder();
                holder.text = (TextView) convertView.findViewById(R.id.text);
                holder.icon = (ImageView) convertView.findViewById(R.id.icon);

                convertView.setTag(holder);
            } else {
                // Get the ViewHolder back to get fast access to the TextView
                // and the ImageView.
                holder = (ViewHolder) convertView.getTag();
            }

            holder.text.setText(DATA[position]);
            holder.icon.setImageBitmap((position & 1) == 1 ? mIcon1 : mIcon2);

            return convertView;
        }