ListView優化

在getView()方法中，每次都將佈局從新加載了一遍，當ListView快速滾動的時候，這就會成爲性能的瓶頸。

getView()方法中還有一個convertView參數，這個參數用於將以前加載好的佈局進行緩存，以便以後能夠進行重用。

所以咱們能夠經過他來進行優化：

 @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        Fruit fruit = getItem(position);
        View view;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(resourceId, parent,
                false);
        } else {
            view = convertView;
        }
        ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
        TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
        fruitImage.setImageResource(fruit.getImageId());
        fruitName.setText(fruit.getName());
        return view;
    }

不過，目前咱們的這份代碼仍是能夠繼續優化的，雖然如今已經不會再重複去加載佈局，可是每次在getView()方法中仍是會調用View的findViewById()方法來獲取一次控件的實例。咱們能夠藉助一個ViewHolder來對這部分性能進行優化：

 @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        Fruit fruit = getItem(position);
        View view;
        ViewHolder viewHolder;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(resourceId, parent,
            false);
            viewHolder = new ViewHolder();
            viewHolder.fruitImage = (ImageView) view.findViewById
            (R.id.fruit_image);
            viewHolder.fruitName = (TextView) view.findViewById (R.id.fruit_name);
            view.setTag(viewHolder); // 將ViewHolder存儲在View中
        } else {
            view = convertView;
            viewHolder = (ViewHolder) view.getTag(); // 從新獲取ViewHolder
        }
        viewHolder.fruitImage.setImageResource(fruit.getImageId());
        viewHolder.fruitName.setText(fruit.getName());
        return view;
    }

    class ViewHolder {

        ImageView fruitImage;

        TextView fruitName;

    }