使用Picasso进行网络图片的加载

来源：互联网发布：知乎的提问不能删除编辑：程序博客网时间：2024/05/20 01:13

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.com/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 = newSquaredImageView(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();
    }
    returnresult;
  }
  @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(newFile(...)).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 = newLinkedHashMap<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)) {
                    thrownew 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) {
            thrownew AssertionError(String.format(
                    "Attempted to complete action with no result!\n%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;
                returnbitmap;
            }
        }
                                                                                                                                                        
        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);
            }
        }
                                                                                                                                                        
        returnbitmap;
    }

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 = newBuilder(context).build();
        }
        returnsingleton;
    }
                                                                                                                   
    public Picasso build() {
            Context context = this.context;
                                                                                                               
            if(downloader == null) {
                downloader = Utils.createDefaultDownloader(context);
            }
            if(cache == null) {
                cache = newLruCache(context);
            }
            if(service == null) {
                service = newPicassoExecutorService();
            }
            if(transformer == null) {
                transformer = RequestTransformer.IDENTITY;
            }
                                                                                                               
            Stats stats = newStats(cache);
                                                                                                               
            Dispatcher dispatcher = newDispatcher(context, service, HANDLER,
                    downloader, cache, stats);
                                                                                                               
            returnnew 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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