android listView滑動中止後加載圖片

解決思路：添加listView的滾動監聽事件，在listView滾動時使加載圖片線程處於等待狀態。當中止活動後，得到當前界面的listView的開始位置和結束位置。而後開啓加載圖片線程。

代碼片斷：

public AsyncImageLoader imageLoader; //聲明圖片加載類的對象

在getView中添加listView滑動監聽事件

public AbsListView.OnScrollListener onScrollListener = new AbsListView.OnScrollListener() {

public void onScrollStateChanged(AbsListView view, int scrollState) {

switch (scrollState) {

case AbsListView.OnScrollListener.SCROLL_STATE_FLING:

imageLoader.lock();//處於滾動狀態鎖定加載線程

break;

case AbsListView.OnScrollListener.SCROLL_STATE_IDLE:

int start = listView.getFirstVisiblePosition();//獲得listView當前屏的開始位置

int end = listView.getLastVisiblePosition(); //獲得結束位置

if (end >= getCount()) {

end = getCount() - 1;

}

imageLoader.setLoadLimit(start, end);//設置要加載圖片的起始位置和結束位置

imageLoader.unlock();//解除鎖定

break;

case AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL:

imageLoader.lock();

break;

default:

break;

}

}

public void onScroll(AbsListView view, int firstVisibleItem,

int visibleItemCount, int totalItemCount) {

}

};

getView中的圖片加載代碼：

// 異步加載圖片

imageLoader.loadDrawable(position, hotel.getHotelTitlePic(),

new ImageCallback() {

public void imageLoaded(Drawable imageDrawable,

String imageUrl) {

ImageView imageView = (ImageView) listView

.findViewWithTag(imageUrl);

if (imageView != null) {

BitmapDrawable bd = (BitmapDrawable) imageDrawable;

if (bd != null) {

Bitmap bitmap = bd.getBitmap();

BitmapDrawable b = new BitmapDrawable(

toRoundCorner(bitmap, 5));  //將圖片轉換爲圓角，此處函數不便列出。

imageView.setImageDrawable(b);

imageView.setTag("");

if (bitmap != null && !bitmap.isRecycled()) {

bitmap.recycle();

}

}

}

}

});

listView綁定監聽事件：    

listView.setOnScrollListener(onScrollListener);   

加載圖片類：

/**

 * 異步圖片加載，加載item的小圖片

 * @author Administrator

 *

 */

public class AsyncImageLoader {

private static final String TAG = "AsyncImageLoader";

public boolean allow = true;

private Object lock = new Object();

private int mStartLoadLimit;

private int mStopLoadLimit;

private boolean firstLoad = true;

//緩存處理類

FileCache fileCache;

public AsyncImageLoader(Context context){

fileCache = new FileCache(context);

}

public void setLoadLimit(int startLoadLimit,int stopLoadLimit){

if(startLoadLimit > stopLoadLimit){

return;

}

mStartLoadLimit = startLoadLimit;

mStopLoadLimit = stopLoadLimit;

synchronized (lock) {

lock.notifyAll();

}

}

public void lock(){

allow = false;

firstLoad = false;

}

public void setFirst(){

firstLoad = true;

}

public void unlock(){

allow = true;

}

public Drawable loadDrawable(final String imageUrl,final ImageCallback imageCallback){

final Handler handler=new Handler(){

@Override

public void handleMessage(Message msg) {

imageCallback.imageLoaded((Drawable) msg.obj, imageUrl);

}

};

new Thread(){

public void run() {

Drawable drawable=loadImageFromUrl(imageUrl);

if(drawable == null){

// handler.sendMessage(handler.obtainMessage(0, R.drawable.downloadfalse));

return;

}

handler.sendMessage(handler.obtainMessage(0,drawable));

};

}.start();

return null;

}

public Drawable loadDrawable(final int postion, final String imageUrl,final ImageCallback imageCallback){

final Handler handler=new Handler(){

@Override

public void handleMessage(Message msg) {

imageCallback.imageLoaded((Drawable) msg.obj, imageUrl);

}

};

new Thread(){

public void run() {

if(!allow){

synchronized (lock) {

try {

lock.wait();

} catch (InterruptedException e) {

e.printStackTrace();

}

}

}

Drawable drawable= null;

if(firstLoad || (postion>=mStartLoadLimit && postion<=mStopLoadLimit)){ //第一次加載或

//只有在當前屏的item纔會加載

drawable = loadImageFromUrl(imageUrl);

}

if(drawable == null){

return;

}

handler.sendMessage(handler.obtainMessage(0,drawable));

};

}.start();

return null;

}

protected Drawable loadImageFromUrl(String imageurl) {

if(null == imageurl || imageurl.equals("")){

return null;

}

File f = fileCache.getFile(imageurl);

//從SD卡

Drawable d = filetoDrable(f);

if (d != null)

return d;

//從網絡

URL m;  

    InputStream in = null; 

try {

m = new URL(ConstDefinition.HTTPURL+imageurl);  

            in = (InputStream) m.getContent();  

            OutputStream os = new FileOutputStream(f);

            CopyStream(in, os);

            os.close();

            return  filetoDrable(f);

            

} catch (Exception ex){

// ex.printStackTrace();

}

return null;

}

public interface ImageCallback{

public void imageLoaded(Drawable imageDrawable,String imageUrl);

}

private void CopyStream(InputStream is, OutputStream os) {

final int buffer_size = 1024;

try {

byte[] bytes = new byte[buffer_size];

for (;;) {

int count = is.read(bytes, 0, buffer_size);

if (count == -1)

break;

os.write(bytes, 0, count);

}

is.close();

os.close();

} catch (Exception ex) {

}

}

private Drawable filetoDrable(File f)  {

  try {

Drawable drawable = Drawable.createFromStream(new FileInputStream(f) , "src");

Log.i("filetoDrable",f.getName());

return drawable;

} catch (Exception e) {

// TODO Auto-generated catch block

// e.printStackTrace();

// Log.i("filetoDrable",e.toString());

}catch(OutOfMemoryError e1){

// e1.printStackTrace();

}

 return null;

}

}

問題研究：此種方法會很好的提升用戶體驗，但有一個問題，當用戶不但的快速滑動時程序中等待的線程數會愈來愈多，這樣可能會產生問題。但願有更好的解決方案，共同窗習。