Android--图片的三级缓存策略

三级缓存
缓存(内存)--->本地磁盘---->网络

1、首先看一下图片存储到本地磁盘

public class FileUtils {

    String path;//文件存储的地方
    public FileUtils(Context context,String dirName){//文件夹的名称

        if(Environment.getExternalStorageState() == Environment.MEDIA_MOUNTED){//SD卡就绪
            //文件可以放在SD卡中
            path = Environment.getExternalStorageDirectory().getAbsolutePath()+"/"+dirName;
        }else{
            //文件放在内部存储器里
            path = context.getCacheDir().getAbsolutePath()+"/"+dirName;
        }

        new File(path).mkdirs();
    }

    //读取和写入
    public void saveToSDCard(String key, Bitmap bmp){
        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream(new File(path,key));

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        //保存固定格式图片，不压缩
        bmp.compress(Bitmap.CompressFormat.PNG,100,fos);

        try {
            fos.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    //读取
    public Bitmap readFromSDCard(String key){
        return BitmapFactory.decodeFile(new File(path,key).getAbsolutePath());
    }
}

 2、图片加载，先加载缓存图片，缓存没有再到本地加载，本地没有再到网络加载　

public class ImageLoader {
    /**
     * 缓存类
     * LruCache原理：Cache保存一个强引用来限制内容数量。每当Item被访问的时候，此Item就会移动到队列的头部。
     * 当cache已满的时候加入新的item时，在队列尾部的item会被回收。
     * 解释：当超出指定内存值则移除最近最少用的图片内存
     */
    private static LruCache<String,Bitmap> cache;
    private FileUtils fileUtils;
    //使用线程池
    ExecutorService threadLooper;

    public ImageLoader(Context context,String dirName){
        //获取系统分配的最大内存
        cache = new LruCache<String,Bitmap>((int) (Runtime.getRuntime().maxMemory()/8)){
           //判断每一个键对应得值得大小
            //自动释放使用频率低的文件
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getByteCount();//也可以value.getWidth()*value.getHeight()
            }
        };
        fileUtils = new FileUtils(context,dirName);
        threadLooper = Executors.newFixedThreadPool(5);

    }

    //存储到缓存
    private void saveToCache(String key, Bitmap bmp){
        cache.put(key,bmp);
    }

    //从缓存中读取
    private Bitmap readFromCache(String key){
        return cache.get(key);
    }

    public void loadImage(final String url, @NonNull final ImageLoadListener listener){

        final String key = url.replaceAll("[\W]","");//非单次类型用空替换
        //第一级，缓存
        if(readFromCache(key)!=null){
            //缓存已存在，直接拿出来，但是，不是直接返回，因为后面还有请求网络，并不能确定什么时候返回。这里使用回调
            listener.loadImageFinish(readFromCache(key));
            Log.e("TAG","从缓存中加载");
        }else {
            //第二级，本地磁盘
            Bitmap bitmap = fileUtils.readFromSDCard(key);
            if(bitmap!=null){
                //存储到缓存
                saveToCache(key,bitmap);
                //返回
                listener.loadImageFinish(fileUtils.readFromSDCard(key));
                Log.e("TAG","从SDCard中加载");
            }else{

                Log.e("TAG","从网络下载");
                final Handler handler = new Handler(){
                    @Override
                    public void handleMessage(Message msg) {
                        super.handleMessage(msg);
                        listener.loadImageFinish((Bitmap) msg.obj);
                    }
                };

                //下载，使用线程池
                threadLooper.execute(new Runnable() {
                    @Override
                    public void run() {
                        //开始下载，子线程
                        try {

                            Bitmap bitmap1 = BitmapFactory.decodeStream(new URL(url).openStream());
                            //保存到本地，保存到缓存
                            fileUtils.saveToSDCard(key,bitmap1);
                            saveToCache(key,bitmap1);

                            //使用异步任务通知主线程修改UI
                            Message msg = handler.obtainMessage();
                            msg.obj = bitmap1;
                            handler.sendMessage(msg);

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

                    }
                });

            }
        }
    }

    public void cancelDowload(){
        threadLooper.shutdown();
    }

    public interface ImageLoadListener{
        void loadImageFinish(Bitmap bitmap);
    }
}

  3、NetworkImageView自定义View，继承自ImageView

res/values/创建network_attrs.xml

<resources>
    <declare-styleable name="NetworkImageView">
        <attr name="url" format="string"/>
    </declare-styleable>
</resources>

public class NetworkImageView extends ImageView{

    String url;
    ImageLoader loader;

    public NetworkImageView(Context context) {
        super(context);
    }

    public NetworkImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
        //获取自定义属性
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.NetworkImageView);
        url = a.getString(R.styleable.NetworkImageView_url);
        a.recycle();
        loader = new ImageLoader(context,"network");
        if(url!=null){
            setUrl(url);
        }
    }

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
        //设置到图片上去
        loader.loadImage(url, new ImageLoader.ImageLoadListener() {
            @Override
            public void loadImageFinish(Bitmap bitmap) {
                setImageBitmap(bitmap);
            }
        });
    }
}

4、测试

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context="com.example.lesson19_threecache.MainActivity">

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="start"
        android:text="加载图片" />

    <ImageView
        android:id="@+id/iv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@mipmap/ic_launcher" />

    <com.example.mylibrary.widget.NetworkImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:url="http://ent.chinadaily.com.cn/img/site1/20161117/448a5bd66b8519976e9933.jpeg"/>
</LinearLayout>

public class MainActivity extends AppCompatActivity {

    ImageView iv;
    ImageLoader loader;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        iv = (ImageView) findViewById(R.id.iv);
        loader = new ImageLoader(this,"three_cache");
    }

    public void start(View v){

        loader.loadImage("http://image.cqcb.com/d/file/personage/2016-11-17/9b05cf89592d3e3eb2027519fbb98214.jpg", new ImageLoader.ImageLoadListener() {
            @Override
            public void loadImageFinish(Bitmap bitmap) {
                iv.setImageBitmap(bitmap);
            }
        });
    }