浅谈Android ListView 异步图片获取

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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);
            }
        }.start();
        return null;
    }
    //回调接口
    public interface ImageCallback {
        public void imageLoaded(Drawable imageDrawable,ImageView imageView, String imageUrl);
    }
}

当然我们也可以存数据库中，从数据库中优先获取，当没有网络的时候达到离线阅读的功能。