Picasso，看的版本是v.2.5.2

• 使用方法，大概這么幾種加載資源的形式

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

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

Picasso.with(context).load("file:///android_asset/DvpvklR.png").into(imageView2);

Picasso.with(context).load(newFile(...)).into(imageView3);

還可以對(duì)圖片進(jìn)行一些操作：設(shè)置大小谭网、裁剪、加載中&加載錯(cuò)誤時(shí)顯示的圖片等逻族，參考Picasso官網(wǎng)蜻底。

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

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

• 類關(guān)系圖

  
  
  

  Picasso-classes-relation
• 開(kāi)始捋源碼
  從外部調(diào)用看，first聘鳞，初始化方法：Picasso.java #with(context)：?jiǎn)卫Ｊ?double-check + 內(nèi)部靜態(tài)類Builder薄辅，在Picasso.java#Builder().build(){}方法中使用默認(rèn)或自定義配置初始化Picasso，返回給外部調(diào)用抠璃。Builder 中初始化了以下配置：

public Picasso build() {
      Context context = this.context;

      if (downloader == null) {  //1
        downloader = Utils.createDefaultDownloader(context);
      }
      if (cache == null) {  //2
        cache = new LruCache(context);
      }
      if (service == null) {  //3
        service = new PicassoExecutorService();
      }
      if (transformer == null) {  //4
        transformer = RequestTransformer.IDENTITY;
      }

      Stats stats = new Stats(cache);  //5

      Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);  //6

      return new Picasso(context, dispatcher, cache, listener, transformer, requestHandlers, stats,
          defaultBitmapConfig, indicatorsEnabled, loggingEnabled);
    }
  }```
  *  downloader
createDefaultDownloader()里站楚，API 9以上使用 OkHttp (最低支持API 9)、以下使用HttpURLConnection 的 Http 的本地緩存搏嗡。

  *  cache
  Picasso自定義了一個(gè) LruCache窿春，與 Google 的實(shí)現(xiàn)類似，但是是針對(duì) value 為 Bitmap 的精簡(jiǎn)實(shí)現(xiàn)采盒。應(yīng)該單獨(dú)寫(xiě)一篇來(lái)回顧下[LRU緩存策略]()旧乞。
  *  service 
創(chuàng)建線程池，默認(rèn)是3個(gè)執(zhí)行線程磅氨，會(huì)根據(jù)網(wǎng)絡(luò)狀況再切換線程數(shù)尺栖。

  * transformer
默認(rèn)配置是返回原始的 Request，Picasso 文檔對(duì)于 RequestTransformer 這個(gè)接口的解釋是
> A transformer that is called immediately before every request is submitted. This can be used to modify any information about a request.
For example, if you use a CDN you can change the hostname for the image based on the current location of the user in order to get faster download speeds.

    如果使用CDN烦租，則可以根據(jù)用戶的當(dāng)前位置更改映像的主機(jī)名延赌，以獲得更快的下載速度。

  * stats 
用于統(tǒng)計(jì)下載和緩存的狀況叉橱，比如總/平均下載數(shù)挫以、緩存命中率/未命中率、下載圖片的總/平均大小等等

 * dispatcher
用以上的1&2&3&5&new Handler(Looper.getMainLooper())窃祝，構(gòu)造了dispatcher掐松，用于調(diào)度任務(wù)，handler與主線程進(jìn)行交互。

  最后用 //6 dispatcher 和一些其他的參數(shù)大磺，構(gòu)造 Picasso返回給外部調(diào)用泻仙。構(gòu)造函數(shù)如下：
          Picasso(Context context, //1
              Dispatcher dispatcher, //2
              Cache cache, //3
              Listener listener, //4
              RequestTransformer requestTransformer, //5
              List<RequestHandler> extraRequestHandlers, //6
              Stats stats, //7
              Bitmap.Config defaultBitmapConfig, //8
              boolean indicatorsEnabled, //9
              boolean loggingEnabled //10) {
              ...
              ...
              int builtInHandlers = 7; // Adjust this as internal handlers are added or removed.
              int extraCount = (extraRequestHandlers != null ? extraRequestHandlers.size() : 0);
              List<RequestHandler> allRequestHandlers =
              new ArrayList<RequestHandler>(builtInHandlers + extraCount);
              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);
              ...
              ...
           }
  
    第6個(gè)參數(shù)比較重要，一個(gè)默認(rèn)添加了7個(gè)RequestHandler的List量没，也可以自定義RequestHandler加進(jìn)去，均繼承自RequestHandler突想，再返回一個(gè)只讀的List殴蹄。類名都很見(jiàn)名知意，對(duì)應(yīng)于開(kāi)篇時(shí)的使用方法猾担，不同的RequestHandler去處理不同資源類型的Request袭灯。除了以上10個(gè)從Builder里傳入的參數(shù)外，Picasso的構(gòu)造函數(shù)里還有以下幾個(gè)：
       Picasso(1,2,3,4,5,6,7,8,9,10){
        ...
        this.targetToAction = new WeakHashMap<Object, Action>();  //11
        this.targetToDeferredRequestCreator = new WeakHashMap<ImageView,    DeferredRequestCreator>();  //12
        this.referenceQueue = new ReferenceQueue<Object>();  //13
        this.cleanupThread = new CleanupThread(referenceQueue, HANDLER);  //14
        this.cleanupThread.start();
      }

  Picasso構(gòu)造好之后绑嘹，調(diào)用** load(File | int | String | Uri) **傳入要加載的資源稽荧，不管傳入的什么類型，都是去構(gòu)造一個(gè)RequestCreator工腋，然后返回姨丈。好的，那就接著去看看RequestCreator擅腰。RequestCreator的構(gòu)造函數(shù)很簡(jiǎn)單蟋恬，其中實(shí)例化了這樣一個(gè)變量：
      private final Request.Builder data;
      RequestCreator(Picasso picasso, Uri uri, int resourceId) {
         ...//省略
        this.data = new Request.Builder(uri, resourceId, picasso.defaultBitmapConfig);
      }
進(jìn)而調(diào)用** load() **方法返回 Request.Builder 后可以調(diào)用一些像在前面使用方法里的那些方法，對(duì)圖像做二次處理趁冈，這些方法都是Request.java 里的方法歼争，rotate、resize什么的渗勘，其中有一個(gè)方法靈活性很高沐绒，可以滿足自定義的一些需求，就是 ** transform(Transformation transformation) **旺坠。比如讓圖片模糊顯示（讓我想起了微信某個(gè)版本的臨時(shí)性玩法）乔遮，等等。詳細(xì)用法可見(jiàn) [Picasso — Image Rotation and Transformation](https://futurestud.io/tutorials/picasso-image-rotation-and-transformation)

  接下來(lái)价淌，最常用到的方法就是**into(ImageView)**方法了申眼，一系列check以及初次調(diào)用沒(méi)有緩存時(shí)，重要的是以下幾個(gè)方法：

public void into(ImageView target, Callback callback) {
...//省略
Request request = createRequest(System.nanoTime());
String requestKey = createKey(request);
Action action = new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,errorDrawable, requestKey, tag, callback, noFade);
picasso.enqueueAndSubmit(action);
}

生成一個(gè)唯一標(biāo)識(shí)的 requestKey 來(lái)構(gòu)造 ImageViewAction蝉衣，它繼承自Action括尸，封裝了一些回調(diào)方法：**complete(),error(),cancel()**等。然后作為參數(shù)病毡，調(diào)用**enqueueAndSubmit(Action)**

void enqueueAndSubmit(Action action) {
Object target = action.getTarget();
if (target != null && targetToAction.get(target) != action) {
// This will also check we are on the main thread.
cancelExistingRequest(target);
targetToAction.put(target, action);
}
submit(action);
}

判定不為空&&取消當(dāng)前target已有請(qǐng)求后濒翻，把當(dāng)前的target和action放入targetToAction中，就是上面構(gòu)造Picasso時(shí)的第11個(gè)變量，**cancelExistingRequest(target)**其實(shí)就是從這個(gè)WeakHashMap里移除有送。重點(diǎn)的方法來(lái)了**submit(action)**：

void submit(Action action) {
dispatcher.dispatchSubmit(action);
}

這個(gè)dispatcher就是前面builder里的第6個(gè)變量淌喻，再來(lái)回顧一下它的構(gòu)造：

Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);

看了dispatcher手里的牌，接下來(lái)要干什么是不是有點(diǎn)不言而喻了雀摘？

void dispatchSubmit(Action action) {
handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));
}

@Override public void handleMessage(final Message msg) {
switch (msg.what) {
case REQUEST_SUBMIT: {
Action action = (Action) msg.obj;
dispatcher.performSubmit(action);
break;
}
...
}

簡(jiǎn)化一下performSubmit：

void performSubmit(Action action, boolean dismissFailed) {
BitmapHunter hunter = hunterMap.get(action.getKey());
//1
if (hunter != null) {
hunter.attach(action);
return;
}
...
//2
hunter = forRequest(action.getPicasso(), this, cache, stats, action);
hunter.future = service.submit(hunter);
hunterMap.put(action.getKey(), hunter);
...
}

截了兩段我覺(jué)得比較重要的片段裸删。
* 第1段，沒(méi)明白attach之后是要干什么阵赠，因?yàn)橹苯泳蛂eturn了涯塔，按照 **hunter.attach(action)**里的邏輯看是把相同key的action添加到了BitmapHunter內(nèi)部維護(hù)的一個(gè)List<Action>中，但是目前沒(méi)有找到對(duì)于List<Action>的主動(dòng)操作清蚀，只是cancel匕荸、detach用到了。
* 第2段枷邪，先是構(gòu)造了BitmapHunter

static BitmapHunter forRequest(Picasso picasso, Dispatcher dispatcher, Cache cache, Stats stats,
Action action) {
Request request = action.getRequest();
List<RequestHandler> requestHandlers = picasso.getRequestHandlers();

  // Index-based loop to avoid allocating an iterator.
  //noinspection ForLoopReplaceableByForEach
  for (int i = 0, count = requestHandlers.size(); i < count; i++) {
    RequestHandler requestHandler = requestHandlers.get(i);
    if (requestHandler.canHandleRequest(request)) {
      return new BitmapHunter(picasso, dispatcher, cache, stats, action, requestHandler);
    }
  }

  return new BitmapHunter(picasso, dispatcher, cache, stats, action, ERRORING_HANDLER);
}

通過(guò)責(zé)任鏈模式榛搔，匹配之前添加的那些RequestHandler誰(shuí)能處理當(dāng)前發(fā)送過(guò)來(lái)的Request（匹配request的資源的Uri的scheme等判斷邏輯），**requestHandler.canHandleRequest(request)**為true就用這個(gè)匹配的requestHandler作為參數(shù)構(gòu)造一個(gè)BitmapHunter返回。BitmapHunter是實(shí)現(xiàn)了Runnable接口的，再貼一下上面的代碼：

hunter.future = service.submit(hunter);

值得注意的是這里hunter內(nèi)部持有了一個(gè)future對(duì)象肥照，是Future類型欢际，它代表一個(gè)異步任務(wù)的計(jì)算結(jié)果。把**service.submit(Runnable)**的結(jié)果賦值給了future，這是為了拿到這個(gè)返回結(jié)果可以做后續(xù)的**cancel()**操作。如果不需要這個(gè)結(jié)果，一般調(diào)用的是**service.execute(Runnable)**穷娱。stackoverflow上有一段關(guān)于這兩個(gè)方法的選擇問(wèn)答，說(shuō)的還蠻清楚的运沦。[Choose between ExecutorService's submit and ExecutorService's execute](http://stackoverflow.com/questions/3929342/choose-between-executorservices-submit-and-executorservices-execute)
然后去看看實(shí)現(xiàn)了Runnable接口的BitmapHunter內(nèi)部的**run()**方法（簡(jiǎn)化后的）：

@Override
public void run() {
...
result = hunt();
if (result == null) {
dispatcher.dispatchFailed(this);
} else {
dispatcher.dispatchComplete(this);
}
...
}

Bitmap hunt() throws IOException {
Bitmap bitmap = null;

if (shouldReadFromMemoryCache(memoryPolicy)) {
  bitmap = cache.get(key);
  if (bitmap != null) {
    stats.dispatchCacheHit();
    loadedFrom = MEMORY;
    }
    return bitmap;
  }
}

data.networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;
RequestHandler.Result result = requestHandler.load(data, networkPolicy);
if (result != null) {
  loadedFrom = result.getLoadedFrom();
  exifOrientation = result.getExifOrientation();
  bitmap = result.getBitmap();

  // If there was no Bitmap then we need to decode it from the stream.
  if (bitmap == null) {
    InputStream is = result.getStream();
      bitmap = decodeStream(is, data);
  }
}

if (bitmap != null) {
  stats.dispatchBitmapDecoded(bitmap);
  if (data.needsTransformation() || exifOrientation != 0) {
    synchronized (DECODE_LOCK) {
      if (data.needsMatrixTransform() || exifOrientation != 0) {
        bitmap = transformResult(data, bitmap, exifOrientation);
      }
      if (data.hasCustomTransformations()) {
        bitmap = applyCustomTransformations(data.transformations, bitmap);  
      }
    }
    if (bitmap != null) {
      stats.dispatchBitmapTransformed(bitmap);
    }
  }
}
return bitmap;

}

先根據(jù)緩存策略判斷是否允許從緩存中讀取泵额，允許的話就從cache中g(shù)et，get到的話在stats中記錄命中了緩存并返回命中的bitmap携添，不允許或未get到就去從之前匹配到的**RequestHandler.load()**嫁盲，得到**load()**的結(jié)果再判斷是否有對(duì)結(jié)果的二次處理什么的，就不再贅述烈掠。主要分析下load()跟進(jìn)去的方法羞秤，比如此次請(qǐng)求的是個(gè)url資源，匹配的是**NetworkRequestHandler**左敌，去看下它的**load()**方法（簡(jiǎn)化后的）：

@Override @Nullable public Result load(Request request, int networkPolicy) throws IOException {
Response response = downloader.load(request.uri, request.networkPolicy);
Picasso.LoadedFrom loadedFrom = response.cached ? DISK : NETWORK;
InputStream is = response.getInputStream();
return new Result(is, loadedFrom);
}

先調(diào)用downloader的load方法瘾蛋，去獲得請(qǐng)求的Response，我開(kāi)始看的是grep code上的源碼2.5.2版矫限，http請(qǐng)求時(shí)的downloader還像前文分析的那樣分為OkHttpDownloader（API 9以上）和UrlConnectionDownloader（API 9以下）哺哼，寫(xiě)這篇時(shí)再看github上的源碼由于OkHttp更新到OkHttp3佩抹，只有OkHttp3Downloader這一個(gè)用于url請(qǐng)求時(shí)的Downloader了。無(wú)論哪種其內(nèi)部邏輯都是請(qǐng)求網(wǎng)絡(luò)返回Response取董，然后根據(jù)Picasso的緩存策略利用的http的本地緩存棍苹。
一大圈的調(diào)用，順利地在BitmapHunter的**run()**方法中拿到result且不為空的話茵汰，就調(diào)用了**dispatcher.dispatchComplete(this)**枢里，這個(gè)方法又調(diào)用了：

void dispatchComplete(BitmapHunter hunter) {
handler.sendMessage(handler.obtainMessage(HUNTER_COMPLETE, hunter));
}

case HUNTER_COMPLETE: {
BitmapHunter hunter = (BitmapHunter) msg.obj;
dispatcher.performComplete(hunter);
break;
}

void performComplete(BitmapHunter hunter) {
if (shouldWriteToMemoryCache(hunter.getMemoryPolicy())) {
cache.set(hunter.getKey(), hunter.getResult());
}
hunterMap.remove(hunter.getKey());
batch(hunter);
}
}

在這里，根據(jù)緩存策略蹂午，將結(jié)果里的Bitmap加入內(nèi)存Cache中坡垫，也就是前面實(shí)例化的LruCache。然后發(fā)送一個(gè)HUNTER_DELAY_NEXT_BATCH画侣，
handleMessage(Message)中處理：

case HUNTER_DELAY_NEXT_BATCH: {
dispatcher.performBatchComplete();
break;
}

void performBatchComplete() {
List<BitmapHunter> copy = new ArrayList<>(batch);
batch.clear();
mainThreadHandler.sendMessage(mainThreadHandler.obtainMessage( HUNTER_BATCH_COMPLETE, copy));
logBatch(copy);
}

這個(gè)mainThreadHandler也就是最開(kāi)始Picasso的構(gòu)造函數(shù)里Builder里放入去實(shí)例化Dispatcher傳入的那個(gè)HANDLER，再次回到Picasso.java里去看當(dāng)時(shí)HANDLER的初始化：

static final Handler HANDLER = new Handler(Looper.getMainLooper()) {
@Override public void handleMessage(Message msg) {
switch (msg.what) {
case HUNTER_BATCH_COMPLETE: {
@SuppressWarnings("unchecked") List<BitmapHunter> batch = (List<BitmapHunter>) msg.obj;
//noinspection ForLoopReplaceableByForEach
for (int i = 0, n = batch.size(); i < n; i++) {
BitmapHunter hunter = batch.get(i);
hunter.picasso.complete(hunter);
}
break;
}
...
}
};

一切又都回到了當(dāng)初入口類里的**complete(BitmapHunter)** 堡妒，按照源碼邏輯配乱，順利的話接下來(lái)是 **deliverAction(Bitmap result, LoadedFrom from, Action action, Exception e)**，再action.complete(result, from);從上面的分析可知皮迟，最終調(diào)用的是繼承了Action的ImageViewAction的complete方法搬泥，let's go（依然是簡(jiǎn)化版）:

@Override
public void complete(Bitmap result, Picasso.LoadedFrom from) {
ImageView target = this.target.get();
Context context = picasso.context;
boolean indicatorsEnabled = picasso.indicatorsEnabled;
PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);
if (callback != null) {
callback.onSuccess();
}
}

這樣就完成對(duì)控件的圖片設(shè)置。

  源碼分析下來(lái)伏尼，發(fā)現(xiàn)Picasso其實(shí)非常適合作為一個(gè)骨架去自定義重要模塊的實(shí)現(xiàn)忿檩，比如LruCache，比如本地緩存的Downloader爆阶，優(yōu)秀的框架是易于擴(kuò)展的框架燥透。
* 小結(jié)：Picasso的源碼不多，也就30幾個(gè)類辨图，通讀一遍也就倆小時(shí)班套，但是記錄成文字倒是花費(fèi)了很長(zhǎng)時(shí)間。這也是我第一次寫(xiě)這么多東西故河。深知了那些寫(xiě)文章分享技術(shù)的人的不易吱韭。同時(shí)也希望自己可以養(yǎng)成記錄的習(xí)慣，這對(duì)于徹底理解一件事物還是非常有幫助的鱼的。