AsyncTask学习（下）

1 网上图片加载的案例

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="10dp">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        />


    <ProgressBar
        android:id="@+id/progressBar"
        android:visibility="gone"
        android:layout_centerInParent="true"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</RelativeLayout>

2。类的实现

public class ImageAsyncTaskActivity extends AppCompatActivity {

    private ImageView imageView;
    private ProgressBar progressBar;
    private static String imageUrl = "http://img.hb.aicdn.com/761f1bce319b745e663fed957606b4b5d167b9bff70a-nfBc9N_fw580";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_image_asynctask);

        imageView = (ImageView) findViewById(R.id.imageView);
        progressBar = (ProgressBar) findViewById(R.id.progressBar);

        //开启异步线程操作
        new MyAsyncTask().execute(imageUrl);
    }

    class MyAsyncTask extends AsyncTask<String, Void, Bitmap>{

        /**
         * 初始化操作
         */
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            progressBar.setVisibility(View.VISIBLE);

        }

        @Override
        protected Bitmap doInBackground(String... params) {
            String url = params[0];//这里只有一个参数，直接获取
            Bitmap bitmap = null;
            URLConnection connection ;
            InputStream is;
            try {
                connection = new URL(url).openConnection();
                is = connection.getInputStream();
                BufferedInputStream bis = new BufferedInputStream(is);
                //休眠3秒防止加载太快，看不到加载效果
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                bitmap = BitmapFactory.decodeStream(bis);
                bis.close();
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return bitmap;
        }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            super.onPostExecute(bitmap);
            progressBar.setVisibility(View.GONE);
            if(bitmap!=null){
                imageView.setImageBitmap(bitmap);
            }
        }
    }
}

新闻的加载，也是最好的一个，我一直没有仔细的研究，特地粘出来进行改进。
3，配置文件的使用

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <ListView
        android:id="@+id/listView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:dividerHeight="0.5dp"
        android:divider="#eee"/>


</LinearLayout>

4，中间使用的第二个界面的文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="10dp"
    android:orientation="horizontal">

    <ImageView
        android:id="@+id/image_iv"
        android:layout_width="64dp"
        android:layout_height="64dp"
        android:src="@mipmap/ic_launcher"/>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="10dp"
        android:orientation="vertical">
        <TextView
            android:id="@+id/title_tv"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textSize="18sp"
            android:text="title"/>

        <TextView
            android:id="@+id/content_tv"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textSize="16sp"
            android:maxLines="3"
            android:ellipsize="end"
            android:text="content"/>
        </LinearLayout>

</LinearLayout>

6。主函数的使用

public class NewsListActivity extends AppCompatActivity {

    private ListView mListView;
    private String mUrl = "http://www.imooc.com/api/teacher?type=4&num=30";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_news_list);

        mListView = (ListView) findViewById(R.id.listView);
        new NewsAsyncTask().execute(mUrl);

    }

    /**
     * 实现网络的异步访问
     */
    class NewsAsyncTask extends AsyncTask<String, Void, List<News>>{

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

        @Override
        protected void onPostExecute(List<News> list) {
            super.onPostExecute(list);
            NewsListAdapter adapter = new NewsListAdapter(NewsListActivity.this, list, mListView);
            mListView.setAdapter(adapter);
        }
    }

    private List<News> getJsonData(String url) {
        List<News> list = new ArrayList<>();
        try {
            String jsonString = readStream(new URL(url).openStream());
            Log.e("tag", "jsonString:"+jsonString);
            JSONObject jsonObject;
            News news;
            try {
                jsonObject = new JSONObject(jsonString);
                JSONArray jsonArray = jsonObject.getJSONArray("data");
                for (int i = 0; i < jsonArray.length(); i++) {
                    jsonObject = jsonArray.getJSONObject(i);
                    news = new News();
                    news.newsIconUrl = jsonObject.getString("picSmall");
                    news.newsTitle = jsonObject.getString("name");
                    news.newsContent = jsonObject.getString("description");
                    list.add(news);

                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

        return list;
    }

    private String readStream(InputStream is){
        InputStreamReader isr;
        String result = "";
        try {
            String line = "";
            isr = new InputStreamReader(is, "utf-8");//字节流转化为字符流
            BufferedReader br = new BufferedReader(isr);
            while((line = br.readLine())!=null){
                result += line;
            }

        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return result;

    }
}

中间用到的那个类
7，用到的这个newsadapter 这个类

public class NewsListAdapter extends BaseAdapter implements AbsListView.OnScrollListener{

    private Context context;
    private List<News> list;
    private LayoutInflater mInflater;

    private ImageLoader mImageLoader;

    private int mStart, mEnd;
    public static String [] URLS;

    private boolean mFirstIn;

    public NewsListAdapter(Context context, List<News> list, ListView listView) {
        this.context = context;
        this.list = list;
        mInflater = LayoutInflater.from(context);
        mImageLoader = new ImageLoader(listView);
        URLS = new String[list.size()];
        for (int i = 0; i < list.size(); i++) {
            URLS[i] = list.get(i).newsIconUrl;        }
        listView.setOnScrollListener(this);//绑定监听事件
        mFirstIn = true;
    }

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

    @Override
    public Object getItem(int position) {
        return list.get(position);
    }

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

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder viewHolder=null;
        if(convertView==null){
            viewHolder = new ViewHolder();
            convertView = mInflater.inflate(R.layout.news_list_item, null);
            viewHolder.image_iv = (ImageView) convertView.findViewById(R.id.image_iv);
            viewHolder.title_tv = (TextView) convertView.findViewById(R.id.title_tv);
            viewHolder.content_tv = (TextView) convertView.findViewById(R.id.content_tv);
            convertView.setTag(viewHolder);
        }else{
            viewHolder = (ViewHolder) convertView.getTag();
        }

        //viewHolder.image_iv.setImageResource(R.mipmap.ic_launcher);
        String url = list.get(position).newsIconUrl;
        viewHolder.image_iv.setTag(url);//设置tag，防止图片错乱
        //mImageLoader.showImageByThread(viewHolder.image_iv, url);
        mImageLoader.showImageByAsyncTask(viewHolder.image_iv, url);
        viewHolder.title_tv.setText(list.get(position).newsTitle);
        viewHolder.content_tv.setText(list.get(position).newsContent);

        return convertView;
    }

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
        if(scrollState == SCROLL_STATE_IDLE){//滑动停止状态，加载可见项
            mImageLoader.loadImage(mStart, mEnd);
        }else{//停止加载可见项
            mImageLoader.cancelAllTasks();
        }
    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
        mStart = firstVisibleItem;
        mEnd = firstVisibleItem + visibleItemCount;
        //第一次显示的时候调用
        if(mFirstIn && totalItemCount>0){
            mFirstIn = false;
            mImageLoader.loadImage(mStart, mEnd);
        }

    }

    class ViewHolder{
        TextView title_tv, content_tv;
        ImageView image_iv;

    }
}

8 用到的News 这个类

public class News {

    public String newsIconUrl;
    public String newsTitle;
    public String newsContent;
}

9 使用到的图片加载的那个类

public class ImageLoader {

    private ImageView mImageView;
    private String mUrl;

    private LruCache<String, Bitmap> mCaches;//一级缓存保存在内存中
    private ListView mListView;
    private Set<NewsAsyncTask> mTasks;

    public ImageLoader(ListView listView){
        this.mListView = listView;
        mTasks = new HashSet<>();
        //获取最大可用内存
        int maxMemory = (int) Runtime.getRuntime().maxMemory();
        int cacheSize = maxMemory/4;//设置实际缓存大小
        mCaches = new LruCache<String, Bitmap>(cacheSize){
            /**
             * 在每次存入缓存的时候调用
             * @param key
             * @param value
             * @return
             */
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getByteCount();
            }
        };

    }

    /**
     * 增加到缓存
     * @param url
     * @param bitmap
     */
    public void addBitmapToCache(String url, Bitmap bitmap){

        if(getBitmapFromCache(url)==null){

            mCaches.put(url, bitmap);
        }
    }

    /**
     * 从缓存中获取数据
     * @param url
     * @return
     */
    public Bitmap getBitmapFromCache(String url){

        return mCaches.get(url);
    }

    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            if(mImageView.getTag().equals(mUrl)){
                mImageView.setImageBitmap((Bitmap) msg.obj);
            }

        }
    };

    public void showImageByThread(ImageView imageView, final String imageUrl){
        mImageView = imageView;
        mUrl = imageUrl;
        new Thread(){
            @Override
            public void run() {
                super.run();
                Bitmap bmp = getBitmapFromUrl(imageUrl);
                Message message = Message.obtain();//使用现有的Messsage，提高Message的效率
                message.obj = bmp;
                handler.sendMessage(message);

            }
        }.start();

    }

    /**
     * 从网络下载图片
     * @param iurl
     * @return
     */
    public Bitmap getBitmapFromUrl(String iurl){

        Bitmap bitmap = null;
        InputStream is = null;
        try {
            URL url = new URL(iurl);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            is = new BufferedInputStream(connection.getInputStream());
            bitmap = BitmapFactory.decodeStream(is);
            connection.disconnect();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return bitmap;
    }

    public void showImageByAsyncTask(ImageView imageView, final String imageUrl){
        //从缓存中获取图片，判断是否存在
        Bitmap bitmap = getBitmapFromCache(imageUrl);
        if(bitmap==null){
            //从网络下载图片
            imageView.setImageResource(R.mipmap.ic_launcher);
            //new NewsAsyncTask(imageView, imageUrl).execute(imageUrl);
        }else{
            //如果缓存中存在，则直接显示图片
            imageView.setImageBitmap(bitmap);
        }


    }

    private class NewsAsyncTask extends AsyncTask<String, Void, Bitmap>{

        //private ImageView mImageView;
        private String mUrl;

        public NewsAsyncTask(String url) {
            //this.mImageView = mImageView;
            this.mUrl = url;
        }

        @Override
        protected Bitmap doInBackground(String... params) {
            //从网络获取图片
            Bitmap bitmap = getBitmapFromUrl(params[0]);
            if(bitmap!=null){
                //将Bitmap保存到缓存
                addBitmapToCache(params[0], bitmap);
            }
            return bitmap;
        }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            super.onPostExecute(bitmap);
            ImageView imageView = (ImageView) mListView.findViewWithTag(mUrl);
            if(imageView!=null && bitmap!=null){
                imageView.setImageBitmap(bitmap);
            }
            mTasks.remove(this);
        }
    }

    /**
     * 用来加载从start到end的数据
     * @param start
     * @param end
     */
    public void loadImage(int start, int end){

        for (int i = start; i < end; i++) {
            String url = NewsListAdapter.URLS[i];
            //从缓存中获取图片，判断是否存在
            Bitmap bitmap = getBitmapFromCache(url);
            if(bitmap==null){
                //从网络下载图片
                NewsAsyncTask task = new NewsAsyncTask(url);
                task.execute(url);
                mTasks.add(task);
            }else{
                //如果缓存中存在，则直接显示图片
                ImageView imageView = (ImageView) mListView.findViewWithTag(url);
                imageView.setImageBitmap(bitmap);
            }
        }

    }

    public void cancelAllTasks(){
        if(mTasks!=null){
            for (NewsAsyncTask task : mTasks){
                task.cancel(true);
            }

        }

    }
}

哈哈，最后程序整理完成，是个很好的学习的项目，可以供大家进行参考