浅谈Android ListView 异步图片获取

    浅谈Android ListView 异步图片获取

 

 

         Android应用中，从远程获取图片算是经常要用的一个功能，而图片资源往往会消耗比较大的流量，而且如果加载在图片比较多的View中，如ListView GridView WebView中，加载速度会比较缓慢。Android中图片加载速度很慢的话，程序很容易就挂掉，等待的用户用户同样会等到崩溃。

 

我们一般的处理方法是：

 

 

异步下载

 

本地缓存

 

 

-----------------------------------------------------------

先说异步下载

 

 

如一个ListView的异步下载

 

public class AsyncActivity extends Activity  {
    List<data> insList;
    private ListView mListView;
    private ArrayList<data> mListItems;
    private ItemAdapter mAdapter;
    private ImageView mLfteView;
    private ImageView mRightView;



    private static final int REFRESH_LIST = 1;
    Handler mHandler = new Handler() {
        public void handleMessage(android.os.Message msg) {
            int type = msg.what;
            switch (type) {
            case REFRESH_LIST: {

                if (mListItems.size() > 0) {
                    mListView.setVisibility(View.VISIBLE);
                    if (mAdapter== null) {
                        mAdapter = new Adapter(AsyncActivity.this,
                                mListItems);
                        mListView.setAdapter(mAdapter);
                    }
                        mAdapter.notifyDataSetChanged();

                }
                mListView.requestFocus();

                unShowDialogLoading();
                break;
            }


            default:
                break;
            }
        };
    };


    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.insurance_list);
        initViews();
        getList();
    }

    /**
     * 初始化view
     */
    private void initViews(){

        mListView = (ListView)findViewById(R.id.list);
        mListView.setAdapter(mAdapter);
        mListView.setOnItemClickListener(new OnItemClickListener(){
            public void onItemClick(AdapterView<?> arg0, View v, int id,
                    long pos) {
                // 获得被单击的项
                //跳转

            }
        });


        mListItems = new ArrayList<data>();

    }



    private void getList() {
        showDialogLoading();
         //得到列表
        Data ins = new Data();
        insList = ins.getList();
        mListItems.clear();
        mListItems.addAll(insList);
        mHandler.sendEmptyMessage(REFRESH_LIST);


    }


    private ProgressDialog mLoadingDialog;

    private void showDialogLoading() {
        mLoadingDialog = new ProgressDialog(this);
        mLoadingDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
         //spinner  自旋体 像螺旋桨那样
        mLoadingDialog.setMessage("载入中,请稍候...");
        mLoadingDialog.setIndeterminate(false);//设置进度条是否为不明确
        mLoadingDialog.setCancelable(true);//设置进度条是否可以按退回健取消
        mLoadingDialog.show();
    }

    private void unShowDialogLoading() {
        if (mLoadingDialog == null)
            return;
        else
            mLoadingDialog.dismiss();
    }

}

 

 

 它的Adapter是

 

public class ItemAdapter extends BaseAdapter{
    private ArrayList<data> mList;
    private Context mContext;
    // 异步加载图片的线程
    private AsyncImageLoader imageLoader = new AsyncImageLoader();
    //当前的缓存
    private Map<Integer, View> viewMap = new HashMap<Integer, View>();

    public InsuranceItemAdapter(Context context, ArrayList<data> ins) {
        mContext = context;
        mList= ins;
    }

    public InsuranceItemAdapter(Context context, ArrayList<data> ins,
            Handler handler) {
        mContext = context;
        mList= ins;
    }

    public void setInsurance(ArrayList<data> ins) {
        mList= ins;
    }



    public int getCount() {
        return mList.size();
    }

    public Object getItem(int position) {
        try {
            return mList.get(position);
        } catch (Exception ex) {
            return null;
        }
    }

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


    private View newView() {
        InsItemView insView = new InsItemView(mContext);
        return insView;
    }



    public View getView(int position, View convertView, ViewGroup parent) {
        //先从缓存里面读取
        ViewHolder holder = null;
        View view;

         if (viewMap.get(position) == null) {
            view = newView();
            holder = new ViewHolder();
            holder.mTitle = (TextView)view.findViewById(R.id.ins_title);
            holder.mTime = (TextView)view.findViewById(R.id.ins_time);
            holder.mType = (TextView)view.findViewById(R.id.ins_from);
            holder.mPic = (ImageView)view.findViewById(R.id.ins_small_pic);
            final int p = position;
            viewMap.put(position, view);
            view.setTag(holder);
         }else{
             Log.e("MainActivity","position2 = "+position);
             view = viewMap.get(position);
             holder = (ViewHolder)view.getTag();
         }

         data ins = mList.get(position);
         holder.mTitle.setText(ins.getTitle());

        holder.mPic.setVisibility(View.VISIBLE);

        // 异步加载图片
        Drawable cachedImage = imageLoader.loadDrawable(ins.getPic(), holder.mPic,
                new ImageCallback() {
                    public void imageLoaded(Drawable imageDrawable,
                            ImageView imageView, String imageUrl) {
                        imageView.setImageDrawable(imageDrawable);
                    }
                });
        if (cachedImage != null) {
            holder.mPic.setImageDrawable(cachedImage);
        }

         return view;

    }


    static class ViewHolder{
        ImageView mPic;
        TextView mTitle;
        TextView mTime;
        TextView mType;
    }
}

 

 

 

 

 

 

  

原理简单，不罗嗦了

 

本地缓存

  

   就是先读取本地的数据，如果本地没有再从网络上获取

 

WebView中很简单，

//优先缓存 mWebView.getSettings().setCacheMode(WebSettings.LOAD_CACHE_ELSE_NETWORK);

 

 

  就能实现了。

 

 

其他地方我们就得自己写一个异步图片下载了，

 

 

package cn.ethink.activity.net;

import java.lang.ref.SoftReference;
import java.util.HashMap;

import android.graphics.drawable.Drawable;
import android.os.Handler;
import android.os.Message;
import android.widget.ImageView;
import cn.ethink.activity.view.utils.ImageUtil;


public class AsyncImageLoader {

     //SoftReference是软引用，是为了更好的为了系统回收变量
    private static HashMap<String, SoftReference<Drawable>> imageCache;

    static {
        imageCache = new HashMap<String, SoftReference<Drawable>>();
    }


    public AsyncImageLoader() {

    }
    public Drawable loadDrawable(final String imageUrl,final ImageView imageView, final ImageCallback imageCallback){
        if (imageCache.containsKey(imageUrl)) {
            //从缓存中获取
            SoftReference<Drawable> softReference = imageCache.get(imageUrl);
            Drawable drawable = softReference.get();
            if (drawable != null) {
                return drawable;
            }
        }
        final Handler handler = new Handler() {
            public void handleMessage(Message message) {
                imageCallback.imageLoaded((Drawable) message.obj, imageView,imageUrl);
            }
        };
        //建立新一个新的线程下载图片
        new Thread() {
            @Override
            public void run() {
                Drawable drawable = null;
                try {
                    drawable = ImageUtil.geRoundDrawableFromUrl(imageUrl, 20);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
                Message message = handler.obtainMessage(0, drawable);
                handler.sendMessage(message);