android ViewHolder的相关知识

listView的优化方式

重用convertView

viewHolder

static class viewHolder

在列表里面有图片的情况下,监听滑动不加载图片

多个不同布局，可以创建不同的viewHolder和convertView进行重用

ViewHolder不是Android的开发API，而是一种设计方法，就是设计个静态类，缓存一下，省得Listview更新的时候，还要重新操作。

1、正常ViewHolder写法

public class MyAdapter extends BaseAdapter {
    private ArrayList<FileInfo > listFileInfo;
    private Context context;

    public MyAdapter(ArrayList<FileInfo> listFileInfo, Context context) {
        this.listFileInfo = listFileInfo;
        this.context = context;
    }

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

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

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


    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder viewHolder=null;

        if (convertView==null){
            convertView= LayoutInflater.from(context).inflate(R.layout.list_item,null);
            viewHolder=new ViewHolder();
            viewHolder.btnStart=convertView.findViewById(R.id.btnStart);
            viewHolder.btnStop=convertView.findViewById(R.id.btnStop);
            viewHolder.tvFileName=convertView.findViewById(R.id.tvFileName);
            viewHolder.progressBar=convertView.findViewById(R.id.progressBar);

            convertView.setTag(viewHolder);
        }else{
            viewHolder= (ViewHolder) convertView.getTag();
        }
        FileInfo fileInfo=listFileInfo.get(position);

        viewHolder.tvFileName.setText(fileInfo.getFileName());
        viewHolder.btnStart.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

            }
        });
        viewHolder.btnStop.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

            }
        });

        return convertView;
    }

    static class ViewHolder{
        private TextView tvFileName;
        private Button btnStart, btnStop;
        private ProgressBar progressBar;
    }

2、使用万能ViewHolder

**
 * 万能ViewHolder
 */

public class CommonViewHolder {
    /**
     * @param view 所有缓存View的根View
     * @param id   缓存View的唯一标识
     * @return
     */
    public static <T extends View> T get(View view, int id) {

        //SparseArray(稀疏数组).他是Android内部特有的api,标准的jdk是没有这个
        // 类的.在Android内部用来替代HashMap<Integer,E>这种形式,使用SparseArray
        // 更加节省内存空间的使用,SparseArray也是以key和value对数据进行保存的.
        // 使用的时候只需要指定value的类型即可.并且key不需要封装成对象类型.
        SparseArray<View> viewHolder = (SparseArray<View>) view.getTag();
        //如果根view没有用来缓存View的集合
        if (viewHolder == null) {
            viewHolder = new SparseArray<View>();
            view.setTag(viewHolder);//创建集合和根View关联
        }
        View chidlView = viewHolder.get(id);//获取根View储存在集合中的孩纸
        if (chidlView == null) {//如果没有改孩纸
            //找到该孩纸
            chidlView = view.findViewById(id);
            viewHolder.put(id, chidlView);//保存到集合
        }
        return (T) chidlView;
    }
}

public class MyAdapter extends BaseAdapter {
    private ArrayList<FileInfo > listFileInfo;
    private Context context;

    public MyAdapter(ArrayList<FileInfo> listFileInfo, Context context) {
        this.listFileInfo = listFileInfo;
        this.context = context;
    }

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

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

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


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

        if (convertView==null) {
            convertView = LayoutInflater.from(context).inflate(R.layout.list_item, null);
        }

        Button btnStart= CommonViewHolder.get(convertView,R.id.btnStart);
        Button btnStop= CommonViewHolder.get(convertView,R.id.btnStop);
        TextView tvFileName= CommonViewHolder.get(convertView,R.id.tvFileName);
        ProgressBar progressBar= CommonViewHolder.get(convertView,R.id.progressBar);

        FileInfo fileInfo=listFileInfo.get(position);

        tvFileName.setText(fileInfo.getFileName());
        btnStart.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

            }
        });
        btnStop.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

            }
        });

        return convertView;
    }