通过缓存来实现网络图片的下载，通过RecyclerView和ViewPaper来展示出来

  这几天实现了一个网络下载图片的功能，并且在ViewPaper和RecyclerView中展示出来。展示出来的功能在我之前写过的一篇文章中。

android 使用RecyclerView实现首项左右滑动,剩余项图片自适应
http://blog.youkuaiyun.com/wb175208/article/details/53618862

以下我主要把下载的过程记录下来：
1.下载的主要使用okhttp框架，异步下载和同步下载都使用了，注意在引用okhttp-3.1.2.jar包时，同时也要使用okio.jar这个包

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Handler;
import android.os.Message;

import com.test.wb.asynbitmap.listener.IDownLoadCallback;

import java.io.IOException;

import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

/**
 * Author: aaa
 * Date: 2017/1/6 21:14.
 * 图片下载管理
 */
public class ImageDownloader {

    private static final int IS_SUCCESS = 1;
    private static final int IS_FAIL = 0;

    //异步获取图片-直接调用
    public static void asyncGetImage(String url, final IDownLoadCallback callback) {
        OkHttpClient client = new OkHttpClient();
        final Request request = new Request.Builder().get()
                .url(url)
                .build();

        client.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {

            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {

                if (response.isSuccessful()) {
                    byte[] bytes = response.body().bytes();
                    Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);
                    callback.sucess(bitmap);
                } else {
                    handler.sendEmptyMessage(IS_FAIL);
                }
            }
        });
    }

    //同步获取图片 - 需要放在子线程中
    public static void synchronizedGetImage(String url, IDownLoadCallback callback) {
        OkHttpClient client = new OkHttpClient();
        final Request request = new Request.Builder().get()
                .url(url)
                .build();

        try {
            Response response = client.newCall(request).execute();
            byte[] bytes = response.body().bytes();
            Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);
            callback.sucess(bitmap);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case IS_SUCCESS:
                    byte[] bytes = (byte[]) msg.obj;
                    Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);
                    //                    imageView.setImageBitmap(bitmap);
                    break;
                case IS_FAIL:
                    break;
                default:
                    break;
            }
        }
    };
}

2.RecyclerView在展示图片的时候，采用三级缓存的模式：
  a.先检测内存
  b.再检测文件
  c.最后检测网络下载

private void downloadImage(final ImageView imageView, final String url, final int position) {
        final Handler handler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                switch (msg.what) {
                    case 0:
                        Bitmap resBitmap = (Bitmap) msg.obj;
                        imageView.setImageBitmap(resBitmap);
                        break;
                    default:
                        break;
                }
            }
        };

        new Thread(new Runnable() {
            @Override
            public void run() {
                //1.先从内存中获取
                Bitmap bitmap = getBitmapFromCache(url);
                if (bitmap != null) {
                    Log.i("aaa", "item " + position + " : from Cacher");
//                    imageView.setImageBitmap(bitmap);
                    Message message = handler.obtainMessage();
                    message.what = 0;
                    message.obj = bitmap;
                    handler.sendMessage(message);
                    return;
                }
                //2.从文件系统中获取
                bitmap = ImageLocalManager.getInstance().getBitmapFromFile(url);
                if (bitmap != null) {
                    addBitmapToCache(url, bitmap);
                    Log.i("aaa", "item " + position + " : from File");
                    Message message = handler.obtainMessage();
                    message.what = 0;
                    message.obj = bitmap;
                    handler.sendMessage(message);
//                    imageView.setImageBitmap(bitmap);
                    return;
                }
                //3.从网上下载
                ImageDownloader.asyncGetImage(url, new IDownLoadCallback() {
                    @Override
                    public void sucess(final Bitmap bitmap) {
                        Log.i("aaa", "item " + position + " : from DownLoader");
                        addBitmapToCache(url, bitmap);
ImageLocalManager.getInstance().saveImageToFile(url, bitmap);
                        Message message = handler.obtainMessage();
                        message.what = 0;
                        message.obj = bitmap;
                        handler.sendMessage(message);
                    }
                    @Override
                    public void failed(Bitmap bitmap) {

                    }
                });
            }
        }).start();
    }

3.ViewPaper展示图片的方式，也是通过同步下载的方式下载图片，顺便总结一下ViewPaper。使用ViewPaper时，要使用PagerAdapter作为适配器，ViewPager初始化的时候首先调用：

public Object instantiateItem(ViewGroup container, int position)

初始化第0页和第1页。而且ViewPager初始化页面的始终是保持中间和左右三个页面存在，如果是第一次初始化，只保存前两张页面；当滑动到最后一页的时候，只保存最后两张页面。其余的页面会通过以下函数移除掉：

public void destroyItem(ViewGroup container, int position, Object object)

然后在展示图片的时候，重写以上两个方法就可以了

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        // 当前位置的View移除
        container.removeView(mImageViews.get(position).mImageView);
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        container.addView(mImageViews.get(position).mImageView);

        //        return mImageViews.get(position);
        return getView(mImageViews.get(position), position);
    }

    private ImageView getView(final ViewPagerData data, final int position) {
        final Handler handler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                switch (msg.what) {
                    case 0:
                        Log.i("aaa", "page " + position + " : form Download");
                        Bitmap bitmap = (Bitmap) msg.obj;
                        data.mImageView.setImageBitmap(bitmap);
                        break;
                }
            }
        };

        data.mImageView.setImageBitmap(mDefaultBitmap);//设置默认图片
        Bitmap bitmap = mViewPaperCache.get(data.mUrl);
        if (bitmap != null) {
            Log.i("aaa", "page " + position + " : form cache");
            data.mImageView.setImageBitmap(bitmap);
            return data.mImageView;
        }

        bitmap = ImageLocalManager.getInstance().getBitmapFromFile(data.mUrl);
        if (bitmap != null) {
            mViewPaperCache.put(data.mUrl, bitmap);
            Log.i("aaa", "page " + position + " : form file");
            data.mImageView.setImageBitmap(bitmap);
            return data.mImageView;
        }

        new Thread(new Runnable() {
            @Override
            public void run() {
                ImageDownloader.synchronizedGetImage(data.mUrl, new IDownLoadCallback() {
                    @Override
                    public void sucess(Bitmap bitmap) {
                        mViewPaperCache.put(data.mUrl, bitmap);
                        ImageLocalManager.getInstance().saveImageToFile(data.mUrl, bitmap);
                        Message message = Message.obtain();
                        message.what = 0;
                        message.obj = bitmap;
                        handler.sendMessage(message);
                    }

                    @Override
                    public void failed(Bitmap bitmap) {

                    }
                });
            }
        }).start();

        return data.mImageView;
    }

4.缓存成文件的图片，保存到本地磁盘中，通过MD5的方式命名，防止出现图片名字重复
5.在Android6.0以上使用的时候，主要要动态加载权限

Demo下载