Android基礎--ListView的複用機制

RecycleBin 機制

RecycleBin 緩存機制如圖：

RecycleBin 代碼實現：其實就是BaseAdapter中的getView方法的第二個參數convertView複用代碼如下：這是兩種樣式的item的複用

@Override
    public View getView(int position, View convertView, ViewGroup viewGroup) {
        LayoutInflater inflater = LayoutInflater.from(viewGroup.getContext());
        if (getItemViewType(position) == ViewType.TEXT){
            if (convertView == null){
                convertView = inflater.inflate(R.layout.text_message_item,viewGroup,false);
            }
            TextView textView = convertView.findViewById(R.id.message_content);
            TextMessage textMessage = (TextMessage)messageList.get(position);
            textView.setText(textMessage.getText());
        }else {
            if (convertView == null){
                convertView = inflater.inflate(R.layout.image_message_item,viewGroup,false);
            }
            ImageView imageView = convertView.findViewById(R.id.image_head);
            ImageMessage imageMessage = (ImageMessage)messageList.get(position);
            imageView.setImageResource(imageMessage.getImage());
        }
        return convertView;
    }

ViewHolder機制

如上面代碼中每次都會去findViewById這項操作，findViewById其實是很耗時的一個操作，ViewHolder機制就是把findViewById的結果緩存起來，例子中有兩ViewHolder : ViewHolderText 與 ViewHolderImage對應兩種item

public class MyBaseAdapter extends BaseAdapter {

    private static final int ViewTypeCount = 2;
    private interface ViewType {
        int TEXT = 0;
        int IMAGE = 1;
    }


    private List<Object> messageList;

    public MyBaseAdapter(List<Object> messageList) {
        this.messageList = messageList;
    }

    @Override
    public int getCount() {
        return messageList.size();
    }

    @Override
    public Object getItem(int position) {
        return messageList.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup viewGroup) {
        int type = getItemViewType(position);
        ViewHolderText viewHolderText = null;
        ViewHolderImage viewHolderImage = null;
        LayoutInflater inflater = LayoutInflater.from(viewGroup.getContext());
        if (type == ViewType.TEXT){
            if (convertView == null){
                convertView = inflater.inflate(R.layout.text_message_item,viewGroup,false);
                viewHolderText = new ViewHolderText();
                viewHolderText.textMessage = convertView.findViewById(R.id.message_content);
                convertView.setTag(viewHolderText);
            }
            TextMessage textMessage = (TextMessage)messageList.get(position);
            viewHolderText = (ViewHolderText)convertView.getTag();
            viewHolderText.textMessage.setText(textMessage.getText());
        }else {
            if (convertView == null){
                convertView = inflater.inflate(R.layout.image_message_item,viewGroup,false);
                viewHolderImage = new ViewHolderImage();
                viewHolderImage.imageView = convertView.findViewById(R.id.image_head);
                convertView.setTag(viewHolderImage);
            }
            ImageMessage imageMessage = (ImageMessage)messageList.get(position);
            viewHolderImage = (ViewHolderImage)convertView.getTag();
            viewHolderImage.imageView.setImageResource(imageMessage.getImage());
        }
        return convertView;
    }

    @Override
    public int getItemViewType(int position) {
        if (getItem(position) instanceof TextMessage){
            return ViewType.TEXT;
        }else {
            return ViewType.IMAGE;
        }
    }

    @Override
    public int getViewTypeCount() {
        return ViewTypeCount;
    }

    class ViewHolderText{
        private TextView textMessage;
    }

    class ViewHolderImage{
        private ImageView imageView;
    }
}