ListView的优化

ListView是比较常用的组件，基本用法相信大部分同志们都已经非常熟悉熟练的使用了，接下来我来介绍一下简单的优化。

一、convertView结合ViewHolder

首先我们要在自定义adaper中定义一个内部类，代码如下：

//布局的控件资源  
class viewHolder{  
    CheckBox checkBox;  
    TextView textView;  
}  

接下来只要在getView()方法中通过视图缓存机制来重用以缓存，完整的示例代码如下：

class MyAdapter extends BaseAdapter{  
    Context mContext;  
    private List mData;
    private  LayoutInflater inflater;  
  
    public MyAdapter(Context context,List data) {  
        mContext = context;  
        this.mData=data;
        inflater = LayoutInflater.from(mContext);  
    }  
  
    @Override  
    public int getCount() {  
        return mData.size();  
    }  
  

    @Override  
    public Object getItem(int arg0) {  
        return mData.get(arg0);  
    }  
  
    @Override  
    public long getItemId(int position) {  
        return position;  
    }  
  
    @Override  
    public View getView(int position, View convertView, ViewGroup parent) {  
        viewHolder1 holder = null;  
        if(convertView == null)  {   
                convertView = inflater.inflate(R.layout.listitem2, parent, false);
                holder = new viewHolder();  
                holder.textView = (TextView)convertView.findViewById(R.id.textview);  
                holder.imageView = (ImageView)convertView.findViewById(R.id.imageview);  
                convertView.setTag(holder);  
                break;  
           
        }  
        else  {            
                holder = (viewHolder) convertView.getTag();       
        }  
        holder.textView.setText(Integer.toString(position));  
        holder.imageView.setBackgroundResource(R.drawable.icon); 
        return convertView;  
      
}  

class viewHolder{  
    ImageView imageView;  
    TextView textView;  
}  

这样一来 就避免了反复创建大量view的问题了

但是上面的仍然可以再优化。当我们的ListView中填充的item有多种形式时，比如微信聊天中你和好友的聊天记录明显是两种布局，我们需要用到2种item的布局方式
此时如果只是单纯判断convert是否存在 会造成回收的view不符合你当前需要的布局
这里要提到Adapter中的另外2个方法：
public int getItemViewType(int position) {}
public int getViewTypeCount() {}
下面附上一个代码示例：

class MyAdapter extends BaseAdapter{
    private Context mContext;
    private LayoutInflater inflater;
        private List mData;
    TextView tex;
    final int VIEW_TYPE = 2;
    final int TYPE_1 = 0;
    final int TYPE_2 = 1;

    public MyAdapter(Context context) {
        mContext = context;
        inflater = LayoutInflater.from(mContext);
    }

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

    //每个convert view都会调用此方法，获得当前所需要的view样式
    @Override
    public int getItemViewType(int position) {
        int p = position%2;
                String str=mData.get(position);
        if(p == 0)
            return TYPE_1;
        else if
            return TYPE_2;
        
    }

    @Override
    public int getViewTypeCount() {
        return 2;
    }

    @Override
    public Object getItem(int arg0) {
        return mData.get(arg0);
    }

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

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        viewHolder1 holder1 = null;
        viewHolder2 holder2 = null;
        int type = getItemViewType(position);

        //无convertView，需要new出各个控件
        if(convertView == null)
        { 
            //按当前所需的样式，确定new的布局
            switch(type)
            {
                case TYPE_1:
                convertView = inflater.inflate(R.layout.listitem1, parent, false);
                holder1 = new viewHolder1();
                holder1.textView = (TextView)convertView.findViewById(R.id.textview1);
                holder1.checkBox = (CheckBox)convertView.findViewById(R.id.checkbox);
                convertView.setTag(holder1);
                break;
                case TYPE_2:
                convertView = inflater.inflate(R.layout.listitem2, parent, false);
                holder2 = new viewHolder2();
                holder2.textView = (TextView)convertView.findViewById(R.id.textview2);
                holder2.imageView = (ImageView)convertView.findViewById(R.id.imageview);
                convertView.setTag(holder2);
                break;
            }
        }
        else
        {
            //有convertView，按样式，取得不用的布局
            switch(type)
            {
                case TYPE_1:
                holder1 = (viewHolder1) convertView.getTag();
                break;
                case TYPE_2:
                holder2 = (viewHolder2) convertView.getTag();
                break;
            }
                //设置资源
            switch(type)
            {
                case TYPE_1:
                holder1.textView.setText(Integer.toString(position));
                holder1.checkBox.setChecked(true);
                break;
                case TYPE_2:
                holder2.textView.setText(Integer.toString(position));
                holder2.imageView.setBackgroundResource(R.drawable.icon);
                break;
            }
        }
        return convertView;
    }
}
//各个布局的控件资源
class viewHolder1{
    CheckBox checkBox;
    TextView textView;
}
class viewHolder2{
    ImageView imageView;
    TextView textView;
}

当然getItemViewType也可以根据对象类型来设置。像微信聊天界面就是用下面这种方法来判断布局：

getItemViewType

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总结完毕！以上。

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