滚动优化

通过对ListView滚动状态的判断来决定何时进行异步加载图片，进而有效的解决ListView滚动卡顿的问题。

1.修改NewsAdapter类。

通过实现AbsListView.OnScrollListener接口并编写onScrollStateChanged、onScroll两个方法来决定何时加载图片。
并且优化首次启动的时候就要开始预加载图片。
MyApplication/app/src/main/java/com/example/myapplication/NewsAdapter.java

package com.example.myapplication;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.*;

import java.util.List;

public class NewsAdapter extends BaseAdapter implements AbsListView.OnScrollListener {

    private List<NewBean> mList;
    private LayoutInflater mInflater;
    private ImageLoader mImageLoader;
    private int mstart,mend;
    public static String[] URLS;
    //是否第一次启动
    private boolean mFirst;

    public NewsAdapter(Context context, List<NewBean> data, ListView listView){
        //构造方法
        mList = data;
        mInflater = LayoutInflater.from(context);
        mImageLoader = new ImageLoader(listView);
        URLS = new String[data.size()];
        for (int i = 0;i < data.size();i++){
            URLS[i] = data.get(i).getPicUrl();
        }
        //绑定监听事件
        listView.setOnScrollListener(this);
        //初始化值
        mFirst = true;
    }

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

    @Override
    public Object getItem(int position) {
        return mList.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.item_layout, null);
            viewHolder.newPic = convertView.findViewById(R.id.new_pic);
            viewHolder.newTitle = convertView.findViewById(R.id.new_title);
            viewHolder.newTime = convertView.findViewById(R.id.new_time);
            viewHolder.newDesc = convertView.findViewById(R.id.new_desc);
            viewHolder.newContent = convertView.findViewById(R.id.new_content);
            convertView.setTag(viewHolder);
        }else{
            viewHolder = (ViewHolder) convertView.getTag();
        }
        viewHolder.newPic.setImageResource(R.mipmap.ic_launcher);
        String url = mList.get(position).getPicUrl();
        viewHolder.newPic.setTag(url);
        //new ImageLoader().showImagerByThread(viewHolder.newPic, mList.get(position).getPicUrl());
        //new ImageLoader().showImageByAsyncTask(viewHolder.newPic, mList.get(position).getPicUrl());
        mImageLoader.showImageByAsyncTask(viewHolder.newPic, mList.get(position).getPicUrl());
        viewHolder.newTitle.setText(mList.get(position).getTitle());
        viewHolder.newTime.setText(mList.get(position).getTime());
        viewHolder.newDesc.setText(mList.get(position).getDesc());
        viewHolder.newContent.setText(mList.get(position).getContentUrl());
        return convertView;
    }

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
        //滑动状态的时候调用
        if(scrollState == SCROLL_STATE_IDLE){
            //停止滚动的时候
            mImageLoader.loadImages(mstart,mend);
        } else{
            //停止任务
            mImageLoader.cancelAllTasks();
        }
    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
        //滑不滑动都会调用
        mstart = firstVisibleItem;
        mend = firstVisibleItem + visibleItemCount;
        //第一次显示调用
        if(mFirst == true && visibleItemCount > 0){
            mImageLoader.loadImages(mstart,mend);
            mFirst = false;
        }
    }

    class ViewHolder{
        public TextView newTitle, newTime, newDesc, newContent;
        public ImageView newPic;
    }
}

2.编写ImageLoader类

编辑加载的loadImages方法和停止加载的cancelAllTasks方法。
MyApplication/app/src/main/java/com/example/myapplication/ImageLoader.java

package com.example.myapplication;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Handler;
import android.os.Message;
import android.util.LruCache;
import android.widget.ImageView;
import android.widget.ListView;

import javax.net.ssl.HttpsURLConnection;
import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.util.HashSet;
import java.util.Set;

public class ImageLoader {

    //创建Cache
    private LruCache<String, Bitmap> mCaches;
    private ListView mListView;
    private Set<NewsAsyncTask> mTask;

    public ImageLoader(ListView listView){
        //构造方法
        mListView = listView;
        mTask = new HashSet<>();
        //获取最大可用内存
        int  maxMemory = (int) Runtime.getRuntime().maxMemory();
        //缓存为内存的四分之一
        int cacheSize = maxMemory / 4;
        mCaches = new LruCache<String, Bitmap>(cacheSize){
            @Override
            protected int sizeOf(String key, Bitmap value) {
                //每次加入内存缓存的时候进行调用,把value的实际大小放进去
                return value.getByteCount();
            }
        };

    }

    /**
     * 增加到缓存
     * @param url
     * @param bitmap
     */
    public void addBitmapToCache(String url, Bitmap bitmap){
        //判断缓存中是否有这个值
        if(getBitmapFromCache(url) == null){
            mCaches.put(url, bitmap);
        }
    }

    /**
     * 从缓存中读取数据
     * @param url
     * @return
     */
    public Bitmap getBitmapFromCache(String url){
        //获取缓存的对象
        return mCaches.get(url);
    }

    /**
     * 根据url获取图片的Bitmap对象
     * @param urlString
     * @return
     */
    public Bitmap getBitmapFromURL(String urlString){
        Bitmap bitmap;
        InputStream is = null;
        try {
            URL url = new URL(urlString);
            HttpsURLConnection connection = (HttpsURLConnection) url.openConnection();
            is = new BufferedInputStream(connection.getInputStream());
            bitmap = BitmapFactory.decodeStream(is);
            connection.disconnect();
            return bitmap;
        } catch (java.io.IOException e) {
            e.printStackTrace();
        } finally {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    public void showImageByAsyncTask(ImageView imageView,String url){
        //判断缓存中是否有
        Bitmap bitmap = getBitmapFromCache(url);
        if(bitmap == null){
            //缓存中没有就去下载
            //new NewsAsyncTask(imageView, url).execute(url);

            //没有就给默认的
            imageView.setImageResource(R.mipmap.ic_launcher);
        }else{
            imageView.setImageBitmap(bitmap);
        }

    }

    /**
     * 停止运行的任务
     */
    public void cancelAllTasks(){
        if(mTask != null){
            for (NewsAsyncTask task : mTask){
                task.cancel(false);
            }
        }
    }

    public void loadImages(int start, int end){
        for (int i = start; i < end; i++) {
            String url = NewsAdapter.URLS[i];
            //判断缓存中是否有
            Bitmap bitmap = getBitmapFromCache(url);
            if(bitmap == null){
                //缓存中没有就去下载
                //new NewsAsyncTask(imageView, url).execute(url);
                NewsAsyncTask task = new NewsAsyncTask(url);
                task.execute(url);
                mTask.add(task);
            }else{
                //imageView.setImageBitmap(bitmap);
                ImageView imageView = mListView.findViewWithTag(url);
                imageView.setImageBitmap(bitmap);
            }
        }
    }

    /**
     * AsyncTask的方式把图片放入view
     */
    private class NewsAsyncTask extends AsyncTask<String, Void, Bitmap>{

        //private ImageView mImageView;
        private String murl;

        public NewsAsyncTask(String url){
            //mImageView = imageView;
            murl = url;
        }

        @Override
        protected Bitmap doInBackground(String... strings) {
            String url = strings[0];
            //从网络中获取图片
            Bitmap bitmap = getBitmapFromURL(url);
            if(bitmap != null){
                //增加到缓存
                addBitmapToCache(url, bitmap);
            }
            return bitmap;
        }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            super.onPostExecute(bitmap);
            //防止图片重复判断唯一性
//            if(mImageView.getTag().equals(murl)) {
//                mImageView.setImageBitmap(bitmap);
//            }
            ImageView imageView = mListView.findViewWithTag(murl);
            imageView.setImageBitmap(bitmap);
            if(imageView != null && bitmap != null){
                imageView.setImageBitmap(bitmap);
            }
            mTask.remove(this);
        }
    }
}

3.修改MainActivity的调用NewsAdapter类

传参多传一个mListView。
MyApplication/app/src/main/java/com/example/myapplication/MainActivity.java

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

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

    @Override
    protected void onPostExecute(List<NewBean> newBeans) {
        super.onPostExecute(newBeans);
        NewsAdapter adapter = new NewsAdapter(MainActivity.this, newBeans, mListView);
        mListView.setAdapter(adapter);
    }
}