• picasso-强大的Android图片下载缓存库


      picasso是Square公司开源的一个Android图形缓存库,地址http://square.github.io/picasso/,可以实现图片下载和缓存功能。仅仅只需要一行代码就能完全实现图片的异步加载:

     

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    Picasso.with(context).load("http://i.imgur.co/DvpvklR.png").into(imageView);

      Api看起来非常独特,是吧。

    Sample application screenshot.

        Picasso不仅实现了图片异步加载的功能,还解决了android中加载图片时需要解决的一些常见问题:

       1.在adapter中需要取消已经不在视野范围的ImageView图片资源的加载,否则会导致图片错位,Picasso已经解决了这个问题。

       2.使用复杂的图片压缩转换来尽可能的减少内存消耗

       3.自带内存和硬盘二级缓存功能

     特性以及示例代码:

            ADAPTER 中的下载:Adapter的重用会被自动检测到,Picasso会取消上次的加载

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    @Override public void getView(int position, View convertView, ViewGroup parent) {
      SquaredImageView view = (SquaredImageView) convertView;
      if (view == null) {
        view = new SquaredImageView(context);
      }
      String url = getItem(position);
      Picasso.with(context).load(url).into(view);
    }

       图片转换:转换图片以适应布局大小并减少内存占用

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    Picasso.with(context)
      .load(url)
      .resize(50, 50)
      .centerCrop()
      .into(imageView);

       你还可以自定义转换:

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    public class CropSquareTransformation implements Transformation {
      @Override public Bitmap transform(Bitmap source) {
        int size = Math.min(source.getWidth(), source.getHeight());
        int x = (source.getWidth() - size) / 2;
        int y = (source.getHeight() - size) / 2;
        Bitmap result = Bitmap.createBitmap(source, x, y, size, size);
        if (result != source) {
          source.recycle();
        }
        return result;
      }
      @Override public String key() { return "square()"; }
    }

       将CropSquareTransformation 的对象传递给transform 方法即可。

     

     

       Place holders-空白或者错误占位图片:picasso提供了两种占位图片,未加载完成或者加载发生错误的时需要一张图片作为提示。

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    Picasso.with(context)
        .load(url)
        .placeholder(R.drawable.user_placeholder)
        .error(R.drawable.user_placeholder_error)
    .into(imageView);

       如果加载发生错误会重复三次请求,三次都失败才会显示erro Place holder

       资源文件的加载:除了加载网络图片picasso还支持加载Resources, assets, files, content providers中的资源文件。

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    Picasso.with(context).load(R.drawable.landing_screen).into(imageView1);
    Picasso.with(context).load(new File(...)).into(imageView2);

     

    下面是picasso源码的解析(不看不影响使用)

    Cache,缓存类

     
    Lrucacha,主要是get和set方法,存储的结构采用了LinkedHashMap,这种map内部实现了lru算法(Least Recently Used 近期最少使用算法)。
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    this.map = new LinkedHashMap<String, Bitmap>(0, 0.75f, true);
    最后一个参数的解释:
    true if the ordering should be done based on the last access (from least-recently accessed to most-recently accessed), and false if the ordering should be the order in which the entries were inserted.
    因为可能会涉及多线程,所以在存取的时候都会加锁。而且每次set操作后都会判断当前缓存区是否已满,如果满了就清掉最少使用的图形。代码如下
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    private void trimToSize(int maxSize) {
            while (true) {
                String key;
                Bitmap value;
                synchronized (this) {
                    if (size < 0 || (map.isEmpty() && size != 0)) {
                        throw new IllegalStateException(getClass().getName()
                                + ".sizeOf() is reporting inconsistent results!");
                    }
                                                                                                                                                                                                                                                     
                    if (size <= maxSize || map.isEmpty()) {
                        break;
                    }
                                                                                                                                                                                                                                                     
                    Map.Entry<String, Bitmap> toEvict = map.entrySet().iterator()
                            .next();
                    key = toEvict.getKey();
                    value = toEvict.getValue();
                    map.remove(key);
                    size -= Utils.getBitmapBytes(value);
                    evictionCount++;
                }
            }
    }

    Request,操作封装类

     
    所有对图形的操作都会记录在这里,供之后图形的创建使用,如重新计算大小,旋转角度,也可以自定义变换,只需要实现Transformation,一个bitmap转换的接口。
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    public interface Transformation {
      /**
       * Transform the source bitmap into a new bitmap. If you create a new bitmap instance, you must
       * call {@link android.graphics.Bitmap#recycle()} on {@code source}. You may return the original
       * if no transformation is required.
       */
      Bitmap transform(Bitmap source);
                                                                                                                                                                                                              
      /**
       * Returns a unique key for the transformation, used for caching purposes. If the transformation
       * has parameters (e.g. size, scale factor, etc) then these should be part of the key.
       */
      String key();
    }

    当操作封装好以后,会将Request传到另一个结构中Action。

    Action

     

    Action代表了一个具体的加载任务,主要用于图片加载后的结果回调,有两个抽象方法,complete和error,也就是当图片解析为bitmap后用户希望做什么。最简单的就是将bitmap设置给imageview,失败了就将错误通过回调通知到上层。
     
    ImageViewAction实现了Action,在complete中将bitmap和imageview组成了一个PicassoDrawable,里面会实现淡出的动画效果。

     

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    @Override
        public void complete(Bitmap result, Picasso.LoadedFrom from) {
            if (result == null) {
                throw new AssertionError(String.format(
                        "Attempted to complete action with no result! %s", this));
            }
                                                                                                                                                                                   
            ImageView target = this.target.get();
            if (target == null) {
                return;
            }
                                                                                                                                                                                   
            Context context = picasso.context;
            boolean debugging = picasso.debugging;
            PicassoDrawable.setBitmap(target, context, result, from, noFade,
                    debugging);
                                                                                                                                                                                   
            if (callback != null) {
                callback.onSuccess();
            }
        }

    有了加载任务,具体的图片下载与解析是在哪里呢?这些都是耗时的操作,应该放在异步线程中进行,就是下面的BitmapHunter。

    BitmapHunter

     

    BitmapHunter是一个Runnable,其中有一个decode的抽象方法,用于子类实现不同类型资源的解析。

     

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    @Override
        public void run() {
            try {
                Thread.currentThread()
                        .setName(Utils.THREAD_PREFIX + data.getName());
                                                                                                                                                            
                result = hunt();
                                                                                                                                                            
                if (result == null) {
                    dispatcher.dispatchFailed(this);
                } else {
                    dispatcher.dispatchComplete(this);
                }
            } catch (IOException e) {
                exception = e;
                dispatcher.dispatchRetry(this);
            } catch (Exception e) {
                exception = e;
                dispatcher.dispatchFailed(this);
            } finally {
                Thread.currentThread().setName(Utils.THREAD_IDLE_NAME);
            }
        }
                                                                                                                                                            
        abstract Bitmap decode(Request data) throws IOException;
                                                                                                                                                            
        Bitmap hunt() throws IOException {
            Bitmap bitmap;
                                                                                                                                                            
            if (!skipMemoryCache) {
                bitmap = cache.get(key);
                if (bitmap != null) {
                    stats.dispatchCacheHit();
                    loadedFrom = MEMORY;
                    return bitmap;
                }
            }
                                                                                                                                                            
            bitmap = decode(data);
                                                                                                                                                            
            if (bitmap != null) {
                stats.dispatchBitmapDecoded(bitmap);
                if (data.needsTransformation() || exifRotation != 0) {
                    synchronized (DECODE_LOCK) {
                        if (data.needsMatrixTransform() || exifRotation != 0) {
                            bitmap = transformResult(data, bitmap, exifRotation);
                        }
                        if (data.hasCustomTransformations()) {
                            bitmap = applyCustomTransformations(
                                    data.transformations, bitmap);
                        }
                    }
                    stats.dispatchBitmapTransformed(bitmap);
                }
            }
                                                                                                                                                            
            return bitmap;
        }
    可以看到,在decode生成原始bitmap,之后会做需要的转换transformResult和applyCustomTransformations。最后在将最终的结果传递到上层dispatcher.dispatchComplete(this)。
    基本的组成元素有了,那这一切是怎么连接起来运行呢,答案是Dispatcher。

    Dispatcher任务调度器

    在bitmaphunter成功得到bitmap后,就是通过dispatcher将结果传递出去的,当然让bitmaphunter执行也要通过Dispatcher。
     
    Dispatcher内有一个HandlerThread,所有的请求都会通过这个thread转换,也就是请求也是异步的,这样应该是为了Ui线程更加流畅,同时保证请求的顺序,因为handler的消息队列。
    外部调用的是dispatchXXX方法,然后通过handler将请求转换到对应的performXXX方法。
    例如生成Action以后就会调用dispather的dispatchSubmit()来请求执行,
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    void dispatchSubmit(Action action) {
            handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));
        }

    handler接到消息后转换到performSubmit方法

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    void performSubmit(Action action) {
            BitmapHunter hunter = hunterMap.get(action.getKey());
            if (hunter != null) {
                hunter.attach(action);
                return;
            }
                                                                                                                           
            if (service.isShutdown()) {
                return;
            }
                                                                                                                           
            hunter = forRequest(context, action.getPicasso(), this, cache, stats,
                    action, downloader);
            hunter.future = service.submit(hunter);
            hunterMap.put(action.getKey(), hunter);
        }
    这里将通过action得到具体的BitmapHunder,然后交给ExecutorService执行。
    下面是Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView)的过程,
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    public static Picasso with(Context context) {
            if (singleton == null) {
                singleton = new Builder(context).build();
            }
            return singleton;
        }
                                                                                                                       
        public Picasso build() {
                Context context = this.context;
                                                                                                                   
                if (downloader == null) {
                    downloader = Utils.createDefaultDownloader(context);
                }
                if (cache == null) {
                    cache = new LruCache(context);
                }
                if (service == null) {
                    service = new PicassoExecutorService();
                }
                if (transformer == null) {
                    transformer = RequestTransformer.IDENTITY;
                }
                                                                                                                   
                Stats stats = new Stats(cache);
                                                                                                                   
                Dispatcher dispatcher = new Dispatcher(context, service, HANDLER,
                        downloader, cache, stats);
                                                                                                                   
                return new Picasso(context, dispatcher, cache, listener,
                        transformer, stats, debugging);
            }

    在 Picasso.with()的时候会将执行所需的所有必备元素创建出来,如缓存cache、执行executorService、调度dispatch 等,在load()时创建Request,在into()中创建action、bitmapHunter,并最终交给dispatcher执行。

    参考:http://blog.csdn.net/xu_fu/article/details/17043231

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  • 原文地址:https://www.cnblogs.com/jasonxcj/p/4885514.html
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