ListView的优化

ListView的优化

在没有优化之前，每当显示一个屏幕的ListView的Item时都会执行Adapter的getView方法几次，反复被调用，导致越划越卡

• 优化每次都加载布局文件（对视图的优化）
• 优化每次都findViewbyid()找组件
• 让每个ListView里的Item长的不相同

1. 在getView方法中第二个参数convertView就是用来缓存布局文件的，当显示完一屏的时候，在显示下一个ITem时，第一个Item离开屏幕，并缓存给要显示的Item。所以我们可以如下方法来加载需要的布局：

   if (convertView == null) {
   convertView=View.inflate(MainActivity.this,
   R.layout.myitem_adpter, null); }

2. 以上虽然对视图进行了优化，不用每次都重新加载，但是每次都得findviewById。我们先需要定义内部static类

   static class ViewHolder {
   TextView textView;
   }

定义静态可以保证只加载一次，并且优先加载

然后通过如下代码放到加载布局文件的那个if语句中

  viewHolder = new ViewHolder();
  viewHolder.textView = (TextView) convertView
                    .findViewById(R.id.iv_item);
    convertView.setTag(viewHolder);  

然后在这个else代码中设置

 else { viewHolder = (ViewHolder) convertView.getTag(); }  

自定义ListView的万能Adapter

定义ViewHolder类

public class ViewHolder {

private Context mContext;
private int mPosition;
private View mconvertView;
private SparseArray mViews = new SparseArray<>();

public ViewHolder(int position, View convertView, ViewGroup parent,
        int resId, Context context) {
    convertView = LayoutInflater.from(context)
            .inflate(resId, parent, false);
    mContext = context;
    mPosition = position;
    mconvertView = convertView;
    convertView.setTag(this);
}

public static ViewHolder get(int position, View convertView,
        ViewGroup parent, int resId, Context context) {

    if (convertView == null) {
        return new ViewHolder(position, convertView, parent, resId, context);
    } else {
        ViewHolder viewHolder = (ViewHolder) convertView.getTag();
        return viewHolder;
    }
}

public  T getView(int resId) {

    View view = mViews.get(resId);
    if (view == null) {
        view = mconvertView.findViewById(resId);
        mViews.put(resId, view);
    }

    return (T) view;
}

public View getmConvertView() {
    return mconvertView;
}
}  

在自己写个类继承BaseAdapter

public abstract class CommonAdapter extends BaseAdapter {
protected List mDatas;
protected Context mContext;
protected int mResId;

public CommonAdapter(Context context, List data, int ResId) {
    mDatas = data;
    mContext = context;
    mResId = ResId;
}

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

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

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

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

    ViewHolder holde = ViewHolder.get(position, convertView, parent,
            mResId, mContext);
    current(holde, getItem(position));
    return holde.getmConvertView();

}

public abstract void current(ViewHolder holder, Object object);  
}  

自己写的类只需要继承CommonAdapter就行重写current方法

public class OmnipotentAdapter extends CommonAdapter {

public OmnipotentAdapter(Context context, List data, int ResId) {
    super(context, data, ResId);
}

@Override
public void current(ViewHolder holder, Object object) {

    Bean bean = (Bean) object;
    ((TextView) holder.getView(R.id.tv_title)).setText(bean.getTitle());
    ((TextView) holder.getView(R.id.tv_desc)).setText(bean.getDesc());
    ((TextView) holder.getView(R.id.tv_time)).setText(bean.getTime());
    ((TextView) holder.getView(R.id.tv_num)).setText(bean.getPhone());
}

}