Android实现异步加载

效果图

代码

public class ImageLoader {

private InputStream inputStream;
private HttpURLConnection conn;
private ImageView mImageview;
private LruCache<String,Bitmap> mLrucache;
private ListView mlistview;
private String threadUrl;
private Set<NewsTask> tasks;
private Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if(mImageview.getTag().equals(threadUrl)){
mImageview.setImageBitmap((Bitmap) msg.obj);
}
}
};

public ImageLoader(ListView listView){
mlistview = listView;
tasks = new HashSet<>();
int maxMemory = (int) Runtime.getRuntime().maxMemory();
int cacheSize = maxMemory/8;
mLrucache = new LruCache<String,Bitmap>(cacheSize){
@Override
protected int sizeOf(String key, Bitmap bitmap) {
return bitmap.getByteCount();
}
};
}

public void addBitmapToCache(String url,Bitmap bitmap){
if(getBitmapFromCache(url)==null){
mLrucache.put(url,bitmap);
}
}

public Bitmap getBitmapFromCache(String mUrl){
return mLrucache.get(mUrl);
}

public void showImageByThread(ImageView imageView, final String url){
mImageview = imageView;
threadUrl = url;
new Thread(){
@Override
public void run() {
super.run();
Bitmap bitmap = getBitmapFromUrl(url);
Message message = Message.obtain();
message.obj = bitmap;
mHandler.sendMessage(message);
}
}.start();
}
public Bitmap getBitmapFromUrl(String urlString){
Bitmap bitmap = null;
try {
URL url = new URL(urlString);
conn = (HttpURLConnection) url.openConnection();
conn.setConnectTimeout(2000);
conn.setReadTimeout(2000);
if(conn.getResponseCode()==200){
inputStream = new BufferedInputStream(conn.getInputStream());
bitmap = BitmapFactory.decodeStream(inputStream);
return bitmap;
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}finally {
conn.disconnect();
try {
inputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
return null;
}
public void showImageByAsyncTask(ImageView imageView,String url){
Bitmap bitmap = getBitmapFromCache(url);
if(bitmap != null){
imageView.setImageBitmap(bitmap);
}else{
//getView的时候不去下载
//            new NewsTask(url).execute(url);
imageView.setImageResource(R.mipmap.ic_launcher);
}
}
private class NewsTask extends AsyncTask<String,Void,Bitmap>{
private String mUrl;
public NewsTask(String url){
mUrl = url;
}
@Override
protected Bitmap doInBackground(String... params) {
String url = params[0];
Bitmap bitmap = getBitmapFromUrl(url);
if(bitmap!=null){
addBitmapToCache(url,bitmap);
}
return bitmap;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
super.onPostExecute(bitmap);
//            if(mImageview.getTag().equals(mUrl)){
//                mImageview.setImageBitmap(bitmap);
//            }
ImageView imageView = (ImageView) mlistview.findViewWithTag(mUrl);
if(imageView!=null&&bitmap!=null){
imageView.setImageBitmap(bitmap);
}
tasks.remove(this);
}
}

public void loadImages(int start,int end){
for (int i = start; i < end; i++) {
String url = NewsAdapter.URLS[i];
Bitmap bitmap = getBitmapFromCache(url);
if(bitmap != null){
ImageView imageView = (ImageView) mlistview.findViewWithTag(url);
imageView.setImageBitmap(bitmap);
}else{
NewsTask task = new NewsTask(url);
task.execute(url);
tasks.add(task);
}
}
}
public void canCellAllTask(){
for (NewsTask task:tasks) {
task.cancel(false);
}
}
}

public class NewsAdapter extends BaseAdapter implements AbsListView.OnScrollListener{
private Context context;
private List<MewsBean.DataBean> listDataBeen;
private LayoutInflater mInflate;
private String url;
private ImageLoader mImageLoader;
private int mStart;
private int mEnd;
public static String[] URLS;
private ListView mListview;
private boolean isFirstShow = true;
public NewsAdapter(Context context, List<MewsBean.DataBean> dataBeen,ListView listView){
this.context = context;
this.listDataBeen = dataBeen;
mInflate = LayoutInflater.from(context);
mListview = listView;
mImageLoader = new ImageLoader(mListview);
URLS = new String[dataBeen.size()];
for(int i = 0;i< dataBeen.size();i++){
URLS[i] = dataBeen.get(i).picBig;
}
listView.setOnScrollListener(this);
}
@Override
public int getCount() {
return listDataBeen.size();
}

@Override
public MewsBean.DataBean getItem(int position) {
return listDataBeen.get(position);
}

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

@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHodler hodler;
if(convertView == null){
convertView = mInflate.inflate(R.layout.item_layout,null);
hodler = new ViewHodler();
hodler.imageView = (ImageView) convertView.findViewById(R.id.iv_icon);
hodler.tvTitle = (TextView) convertView.findViewById(R.id.tv_title);
hodler.tvContent = (TextView) convertView.findViewById(R.id.tv_content);
convertView.setTag(hodler);
}else{
hodler = (ViewHodler) convertView.getTag();
}
MewsBean.DataBean dataBean = listDataBeen.get(position);
hodler.imageView.setImageResource(R.mipmap.ic_launcher);
url = dataBean.picBig;
hodler.imageView.setTag(url);
//        new ImageLoader().showImageByThread(hodler.imageView,url);
//        new ImageLoader().showImageByAsyncTask(hodler.imageView,url);
//        mImageLoader.showImageByAsyncTask(hodler.imageView,url);
hodler.tvTitle.setText(dataBean.name);
hodler.tvContent.setText(dataBean.description);
return convertView;
}

@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
if(scrollState == SCROLL_STATE_IDLE){
mImageLoader.loadImages(mStart,mEnd);
}else{
//停止所有加载
mImageLoader.canCellAllTask();
}
}
//第一次进来的时候此方法会调用
@Override
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
mStart = firstVisibleItem;
mEnd = firstVisibleItem + visibleItemCount;
//初始化调用的时候visibleItemCount=0,此时item还没有加载
if(isFirstShow&&visibleItemCount>0){
mImageLoader.loadImages(mStart,mEnd);
isFirstShow = false;
}
}

public static class ViewHodler{
public ImageView imageView;
public TextView tvTitle;
public TextView tvContent;
}
}

public class MainActivity extends Activity{
ListView listview;
private ListAdapter adapter;
//这里必须带http的前缀
public static final String URL = "http://www.imooc.com/api/teacher?type=4&num=30";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
listview = (ListView) findViewById(R.id.listview);
new NewsTask().execute(URL);
}
class NewsTask extends AsyncTask<String,Void,List<MewsBean.DataBean>>{

@Override
protected List<MewsBean.DataBean> doInBackground(String... params) {
return getJsonData(params[0]);
}

@Override
protected void onPostExecute(List<MewsBean.DataBean> dataBeen) {
super.onPostExecute(dataBeen);
NewsAdapter adapter = new NewsAdapter(MainActivity.this,dataBeen,listview);
listview.setAdapter(adapter);
}
}
public List<MewsBean.DataBean> getJsonData(String url){
List<MewsBean.DataBean> list = new ArrayList<>();
try {
//此句功能与url.openConnection().openInputStream功能相同
String jsonString = getJsonString(new URL(url).openStream());
Gson gson = new Gson();
MewsBean mewsBean = gson.fromJson(jsonString, MewsBean.class);
for (int i = 0; i < mewsBean.data.size(); i++) {
list.add(mewsBean.data.get(i));
}
} catch (IOException e) {
e.printStackTrace();
}
return list;
}

public String getJsonString(InputStream inputStream){
String result = "";
InputStreamReader isr = null;
try {
String line = "";
isr = new InputStreamReader(inputStream,"utf-8");
BufferedReader reader = new BufferedReader(isr);
while((line = reader.readLine())!=null){
result+=line;
}
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return result;
}
}