picasso 源码解析

picasso的使用

//加载一张图片
Picasso.with(this).load("url").placeholder(R.mipmap.ic_default).into(imageView);

//加载一张图片并设置一个回调接口
Picasso.with(this).load("url").placeholder(R.mipmap.ic_default).into(imageView, new Callback() {
    @Override
    public void onSuccess() {

    }

    @Override
    public void onError() {

    }
});

//预加载一张图片
Picasso.with(this).load("url").fetch();

//同步加载一张图片,注意只能在子线程中调用并且Bitmap不会被缓存到内存里.
new Thread() {
    @Override
    public void run() {
        try {
            final Bitmap bitmap = Picasso.with(getApplicationContext()).load("url").get();
            mHandler.post(new Runnable() {
                @Override
                public void run() {
                    imageView.setImageBitmap(bitmap);
                }
            });
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}.start();

//加载一张图片并自适应imageView的大小,如果imageView设置了wrap_content,会显示不出来.直到该ImageView的
//LayoutParams被设置而且调用了该View的ViewTreeObserver.OnPreDrawListener回调接口后才会显示.
Picasso.with(this).load("url").priority(Picasso.Priority.HIGH).fit().into(imageView);

//加载一张图片并按照指定尺寸以centerCrop()的形式缩放.
Picasso.with(this).load("url").resize(200,200).centerCrop().into(imageView);

//加载一张图片并按照指定尺寸以centerInside()的形式缩放.并设置加载的优先级为高.注意centerInside()或centerCrop()
//只能同时使用一种,而且必须指定resize()或者resizeDimen();
Picasso.with(this).load("url").resize(400,400).centerInside().priority(Picasso.Priority.HIGH).into(imageView);

//加载一张图片旋转并且添加一个Transformation,可以对图片进行各种变化处理,例如圆形头像.
Picasso.with(this).load("url").rotate(10).transform(new Transformation() {
    @Override
    public Bitmap transform(Bitmap source) {
        //处理Bitmap
        return null;
    }

    @Override
    public String key() {
        return null;
    }
}).into(imageView);

//加载一张图片并设置tag,可以通过tag来暂定或者继续加载,可以用于当ListView滚动是暂定加载.停止滚动恢复加载.
Picasso.with(this).load("url").tag(mContext).into(imageView);
Picasso.with(this).pauseTag(mContext);
Picasso.with(this).resumeTag(mContxt);

一张图片加载的过程

创建->入队->执行->解码->变换->批处理->完成->分发->显示(可选)

源码分析

从最简单的加载一张图片开始看起 Picasso.with(this).load(url).into(imageView)；让我们来看看Picasso.with(context)做了什么，

 static volatile Picasso singleton = null;

 public static Picasso with(Context context) {
    if (singleton == null) {
      synchronized (Picasso.class) {
        if (singleton == null) {
          singleton = new Builder(context).build();
        }
      }
    }
    return singleton;
  }
显然是由new Builder(context).build()创建的单例。builder是picasso的静态内部类，其中的build()方法定义：

    /** Create the {@link Picasso} instance. */
    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, requestHandlers, stats,
          defaultBitmapConfig, indicatorsEnabled, loggingEnabled);
    }

让我们来看看picasso的构造函数里做了些什么

Picasso(Context context, Dispatcher dispatcher, Cache cache, Listener listener,
      RequestTransformer requestTransformer, List extraRequestHandlers, Stats stats,
      Bitmap.Config defaultBitmapConfig, boolean indicatorsEnabled, boolean loggingEnabled) {
    this.context = context;
    this.dispatcher = dispatcher;
    this.cache = cache;
    this.listener = listener;
    this.requestTransformer = requestTransformer;
    this.defaultBitmapConfig = defaultBitmapConfig;

    int builtInHandlers = 7; // Adjust this as internal handlers are added or removed.
    int extraCount = (extraRequestHandlers != null ? extraRequestHandlers.size() : 0);
    List allRequestHandlers =
        new ArrayList(builtInHandlers + extraCount);

    // ResourceRequestHandler needs to be the first in the list to avoid
    // forcing other RequestHandlers to perform null checks on request.uri
    // to cover the (request.resourceId != 0) case.
    allRequestHandlers.add(new ResourceRequestHandler(context));
    if (extraRequestHandlers != null) {
      allRequestHandlers.addAll(extraRequestHandlers);
    }
    allRequestHandlers.add(new ContactsPhotoRequestHandler(context));
    allRequestHandlers.add(new MediaStoreRequestHandler(context));
    allRequestHandlers.add(new ContentStreamRequestHandler(context));
    allRequestHandlers.add(new AssetRequestHandler(context));
    allRequestHandlers.add(new FileRequestHandler(context));
    allRequestHandlers.add(new NetworkRequestHandler(dispatcher.downloader, stats));
    requestHandlers = Collections.unmodifiableList(allRequestHandlers);

    this.stats = stats;
    this.targetToAction = new WeakHashMap();
    this.targetToDeferredRequestCreator = new WeakHashMap();
    this.indicatorsEnabled = indicatorsEnabled;
    this.loggingEnabled = loggingEnabled;
    this.referenceQueue = new ReferenceQueue