Picasso源码分析

一 介绍

Square公司开源的图片加载库。优点是功能还算完善，能满足基本的图片加载需求，使用简单，体量轻易推倒。
官方链接：http://square.github.io/picasso/
Git: https://github.com/square/picasso

二 分析

1. 简单调用

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

2. 流程图

Paste_Image.png

3. 核心类分析

（主流程顺序，以下贴出的代码都是经过筛选了比较重要的代码）

①Picasso
-- 单例，构造ExecutorService线程池（默认3个线程），Dispatcher，LruCache，Downloader（调用okhttp），其他基本没干什么正事。

//Builder模式构建Picasso实例
public static class Builder {
  private final Context context;  
  private Downloader downloader;  
  private ExecutorService service;  
  private Cache cache;  
  private Listener listener;  
  private RequestTransformer transformer;  
  private List requestHandlers;  
  private Bitmap.Config defaultBitmapConfig;  
  private boolean indicatorsEnabled;  
  private boolean loggingEnabled;  
  /** Start building a new {@link Picasso} instance. */ 
  public Builder(Context context) {   
     if (context == null) {         
        throw new IllegalArgumentException("Context must not be null.");  
     }
     this.context = context.getApplicationContext();  
  }
  ....//省略以下代码
}

②RequestCreator
-- 主流程调用的方法into(), 里面做的最重要的两件事：1.创建Request， 2.把Action加到Dispatcher队列。（Request, Action, Dispatcher几个概念后面会做说明）

public void into(ImageView target, Callback callback) {
   
    Request request = createRequest(started);
    String requestKey = createKey(request);
    //查看内存缓存是否已经有了，有的话直接set，不用再去构造Action获取了
    if (shouldReadFromMemoryCache(memoryPolicy)) {
      Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
      if (bitmap != null) {
        picasso.cancelRequest(target);
        setBitmap(target, picasso.context, bitmap, MEMORY, noFade, picasso.indicatorsEnabled);
        if (callback != null) {
          callback.onSuccess();
        }
        return;
      }
    }
  
    if (setPlaceholder) {
      setPlaceholder(target, getPlaceholderDrawable());
    }
    //构造Action
    Action action =
        new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
            errorDrawable, requestKey, tag, callback, noFade);
    //扔给任务调度器
    picasso.enqueueAndSubmit(action);
  }

③Request
-- 请求model，存储请求相关的数据，包括Transformation list，部分内置的转换属性，Bitmap压缩属性， 任务优先级， 图片资源的相关信息等。

//类结构，原生注释很详细
public final class Request {
  private static final long TOO_LONG_LOG = TimeUnit.SECONDS.toNanos(5);

  /** A unique ID for the request. */
  int id;
  /** The time that the request was first submitted (in nanos). */
  long started;
  /** The {@link NetworkPolicy} to use for this request. */
  int networkPolicy;

  /**
   * The image URI.
   * 

   * This is mutually exclusive with {@link #resourceId}.
   */
  public final Uri uri;
  /**
   * The image resource ID.
   * 

   * This is mutually exclusive with {@link #uri}.
   */
  public final int resourceId;
  /**
   * Optional stable key for this request to be used instead of the URI or resource ID when
   * caching. Two requests with the same value are considered to be for the same resource.
   */
  public final String stableKey;
  /** List of custom transformations to be applied after the built-in transformations. */
  public final List transformations;
  /** Target image width for resizing. */
  public final int targetWidth;
  /** Target image height for resizing. */
  public final int targetHeight;
  /**
   * True if the final image should use the 'centerCrop' scale technique.
   * 

   * This is mutually exclusive with {@link #centerInside}.
   */
  public final boolean centerCrop;
  /**
   * True if the final image should use the 'centerInside' scale technique.
   * 

   * This is mutually exclusive with {@link #centerCrop}.
   */
  public final boolean centerInside;
  public final boolean onlyScaleDown;
  /** Amount to rotate the image in degrees. */
  public final float rotationDegrees;
  /** Rotation pivot on the X axis. */
  public final float rotationPivotX;
  /** Rotation pivot on the Y axis. */
  public final float rotationPivotY;
  /** Whether or not {@link #rotationPivotX} and {@link #rotationPivotY} are set. */
  public final boolean hasRotationPivot;
  /** True if image should be decoded with inPurgeable and inInputShareable. */
  public final boolean purgeable;
  /** Target image config for decoding. */
  public final Bitmap.Config config;
  /** The priority of this request. */
  public final Priority priority;
....//省略以下代码

④Action
-- 可以看做单个图片加载的任务model，抽象类。
实现类可见下图，常用的是ImageViewAction，它会在抽象方法complete真正实现把Bitmap给ImageView的行为。

Paste_Image.png

abstract class Action {
  final Picasso picasso;  //picasso单例
  final Request request;  //当前任务的request
  final WeakReference target;  //目标，imageview
  final boolean noFade;  //是否渐进渐出
  final int memoryPolicy;  //内存缓存策略
  final int networkPolicy;  //网络缓存策略
  final int errorResId;  //错误占位图资源id
  final Drawable errorDrawable;  //错误占位图资源drawable
  final String key; //内存缓存的key
  final Object tag; //dispatcher的任务标识

  boolean willReplay;  //是否再次放到任务队列中
  boolean cancelled;  //是否取消

  //Bitmap加载成功回调
  abstract void complete(Bitmap result, Picasso.LoadedFrom from);
  //加载失败回调
  abstract void error();
....//省略以下代码

⑤Dispatcher
-- 看名字就看出来是个任务分发调度器，

dispatcher方法流程：

Paste_Image.png

class Dispatcher {
  
  final DispatcherThread dispatcherThread;  //dispatcher线程，主要作用是获取looper给handler用
  final Context context;  
  final ExecutorService service;  //picasso创建的线程池，唯一
  final Downloader downloader;  //下载器，唯一
  final Map hunterMap;  //所有的BitmapHunter map
  final Map failedActions;
  final Map pausedActions;
  final Set