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());
     }
 }

FileUtils.java

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);
     }
 }

ImageLoader.java

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>

network_attrs.xml

 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);
             }
         });
     }
 }

NetworkImageView.java

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>

activity_main.xml

 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);
             }
         });
     }

MainActivity.java