以下是picasso调用的图解：
1.当我们在调用Picasso.with()方法的时候，会生成一个单例的Picasso
public static Picasso with(Context context) {
if (singleton == null) {
synchronized (Picasso.class) {
if (singleton == null) {
singleton = new Builder(context).build();
}
}
}
return singleton;
}首先是全局的一个单例对象，其次是通过Builder模式生成对应的Picasso对象。
2.我们在调用完上面的方法之后，会继续调用load方法：
public RequestCreator load(String path) {
if (path == null) {
return new RequestCreator(this, null, 0);
}
if (path.trim().length() == 0) {
throw new IllegalArgumentException("Path must not be empty.");
}
return load(Uri.parse(path));
}支持loadFile，Uri，path等，在这里会其实最终都是调用load(uri)的方法，生成RequestCreator，接下来的placeHolder，errImage，设置图片等，通过链式调用实现。
3.在创建完成RequestCreator之后，我们会执行RequestCreator的into方法：
public void into(ImageView target, Callback callback) {
long started = System.nanoTime();
checkMain();
if (target == null) {
throw new IllegalArgumentException("Target must not be null.");
}
if (!data.hasImage()) {//1
picasso.cancelRequest(target);
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
return;
}
if (deferred) {
if (data.hasSize()) {//2
throw new IllegalStateException("Fit cannot be used with resize.");
}
int width = target.getWidth();
int height = target.getHeight();
if (width == 0 || height == 0) {
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
picasso.defer(target, new DeferredRequestCreator(this, target, callback));
return;
}
data.resize(width, height);
}
Request request = createRequest(started);
String requestKey = createKey(request);
if (shouldReadFromMemoryCache(memoryPolicy)) {//3
Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
if (bitmap != null) {
picasso.cancelRequest(target);
setBitmap(target, picasso.context, bitmap, MEMORY, noFade, picasso.indicatorsEnabled);
if (picasso.loggingEnabled) {
log(OWNER_MAIN, VERB_COMPLETED, request.plainId(), "from " + MEMORY);
}
if (callback != null) {
callback.onSuccess();
}
return;
}
}
if (setPlaceholder) {//4
setPlaceholder(target, getPlaceholderDrawable());
}
Action action =
new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
errorDrawable, requestKey, tag, callback, noFade);
picasso.enqueueAndSubmit(action);//5
}在这里主要就是执行图片的获取，转换大小以及图片的显示的操作。
3.1 上面的第一步主要是判断有没有资源文件设置或者本地路径等，如果有那么直接显示，这里主要是变相地实现了Picasso的本地图片的加载的实现。
3.2 第二步设置要显示的ImageView控件的大小，以便计算图片和控件的大小比，以进行加载时候内存的优化
3.3 第三步会从内存中去读取图片，读取到图片增加命中的计算器
3.4 第四步是设置空白图片
3.5 第五步主要就是执行图片的获取操作，picasso.enqueueAndSubmit(Action)方法：
4.picasso.enqueueAndSubmit(Action)方法：
void enqueueAndSubmit(Action action) {
Object target = action.getTarget();//因为在上面的我们初始化的ImageView的Action，所以target是要加载的ImageView。
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);
}在这里我们会调用submit方法，该方法主要是将action时间分发给对应的Dispatcher，dispatcher.dispatchSubmit(action);
void dispatchSubmit(Action action) {
handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));
}对应的会通过Handler发送一个REQUEST_SUBMIT的消息，这个Handler是DispatcherHandler的实例对象，执行对应的HandleMessage方法，
case REQUEST_SUBMIT: {
Action action = (Action) msg.obj;
dispatcher.performSubmit(action);
break;
}5.我们来看看Dispatcher类的performSubmit方法：
void performSubmit(Action action, boolean dismissFailed) {
if (pausedTags.contains(action.getTag())) {
pausedActions.put(action.getTarget(), action);
if (action.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_PAUSED, action.request.logId(),
"because tag '" + action.getTag() + "' is paused");
}
return;
}
BitmapHunter hunter = hunterMap.get(action.getKey());
if (hunter != null) {
hunter.attach(action);
return;
}
if (service.isShutdown()) {
if (action.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_IGNORED, action.request.logId(), "because shut down");
}
return;
}
hunter = forRequest(action.getPicasso(), this, cache, stats, action);//1
hunter.future = service.submit(hunter);
hunterMap.put(action.getKey(), hunter);
if (dismissFailed) {
failedActions.remove(action.getTarget());
}
if (action.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_ENQUEUED, action.request.logId());
}
}注释1中会一次调用各种RequestHandler的canHandleRequest方法，来选择能处理的RequestHandler
piassco在初始化的时候，默认会添加如下的RequestHandler
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)); 6.上述找到对应能处理的RequestHandler之后，创建对应的bitmapHunter，BitmapHunder其实是Runnable的实现。
@Override
public Future submit(Runnable task) {
PicassoFutureTask ftask = new PicassoFutureTask((BitmapHunter) task);
execute(ftask);
return ftask;
}创建对应的Runnable完成之后，加入线程池中执行
@Override public void run() {
try {
updateThreadName(data);
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_EXECUTING, getLogIdsForHunter(this));
}
result = hunt();//1
if (result == null) {
dispatcher.dispatchFailed(this);
} else {
dispatcher.dispatchComplete(this);
}
} catch (Downloader.ResponseException e) {
if (!e.localCacheOnly || e.responseCode != 504) {
exception = e;
}
dispatcher.dispatchFailed(this);
} catch (NetworkRequestHandler.ContentLengthException e) {
exception = e;
dispatcher.dispatchRetry(this);
} catch (IOException e) {
exception = e;
dispatcher.dispatchRetry(this);
} catch (OutOfMemoryError e) {
StringWriter writer = new StringWriter();
stats.createSnapshot().dump(new PrintWriter(writer));
exception = new RuntimeException(writer.toString(), e);
dispatcher.dispatchFailed(this);
} catch (Exception e) {
exception = e;
dispatcher.dispatchFailed(this);
} finally {
Thread.currentThread().setName(Utils.THREAD_IDLE_NAME);
}
}注释1是获取图片的逻辑
Bitmap hunt() throws IOException {
Bitmap bitmap = null;
if (shouldReadFromMemoryCache(memoryPolicy)) {//1
bitmap = cache.get(key);
if (bitmap != null) {
stats.dispatchCacheHit();
loadedFrom = MEMORY;
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_DECODED, data.logId(), "from cache");
}
return bitmap;
}
}
data.networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;
RequestHandler.Result result = requestHandler.load(data, networkPolicy);
if (result != null) {//2
loadedFrom = result.getLoadedFrom();
exifRotation = 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();
try {
bitmap = decodeStream(is, data);
} finally {
Utils.closeQuietly(is);
}
}
}
if (bitmap != null) {//3
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_DECODED, data.logId());
}
stats.dispatchBitmapDecoded(bitmap);
if (data.needsTransformation() || exifRotation != 0) {
synchronized (DECODE_LOCK) {
if (data.needsMatrixTransform() || exifRotation != 0) {
bitmap = transformResult(data, bitmap, exifRotation);
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_TRANSFORMED, data.logId());
}
}
if (data.hasCustomTransformations()) {
bitmap = applyCustomTransformations(data.transformations, bitmap);
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_TRANSFORMED, data.logId(), "from custom transformations");
}
}
}
if (bitmap != null) {
stats.dispatchBitmapTransformed(bitmap);
}
}
}
return bitmap;
}上述注释1就是从内存中获取图片，如果取不到图片注释2由RequestHandler.load拿到图片数据的流的结果对象，然后解析成图片，注释3在拿到图片之后，对图片根据控件的大小进行转换。拿到图片之后，发送消息code为HUNTER_COMPLETE的消息到MessageQueue中，
if (result == null) {
dispatcher.dispatchFailed(this);
} else {
dispatcher.dispatchComplete(this);
}接着dispatcherHandler执行
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);//1
if (hunter.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_BATCHED, getLogIdsForHunter(hunter), "for completion");
}
}进行内存中保存。完成图片的内存缓存之后，就是对图片设置到ImageView上，Dispatch的batch方法
private void batch(BitmapHunter hunter) {
if (hunter.isCancelled()) {
return;
}
batch.add(hunter);
if (!handler.hasMessages(HUNTER_DELAY_NEXT_BATCH)) {
handler.sendEmptyMessageDelayed(HUNTER_DELAY_NEXT_BATCH, BATCH_DELAY);
}
} case HUNTER_DELAY_NEXT_BATCH: {
dispatcher.performBatchComplete();
break;
}void performBatchComplete() {
List copy = new ArrayList(batch);
batch.clear();
mainThreadHandler.sendMessage(mainThreadHandler.obtainMessage(HUNTER_BATCH_COMPLETE, copy));
logBatch(copy);
} 其实上面的操作是将io线程切换到主线程上来，到绑定主线程Looper的handler接收到消息，会执行
case HUNTER_BATCH_COMPLETE: {
@SuppressWarnings("unchecked") List batch = (List) msg.obj;
//noinspection ForLoopReplaceableByForEach
for (int i = 0, n = batch.size(); i < n; i++) {
BitmapHunter hunter = batch.get(i);
hunter.picasso.complete(hunter);
}
break;
} void complete(BitmapHunter hunter) {
Action single = hunter.getAction();
List joined = hunter.getActions();
boolean hasMultiple = joined != null && !joined.isEmpty();
boolean shouldDeliver = single != null || hasMultiple;
if (!shouldDeliver) {
return;
}
Uri uri = hunter.getData().uri;
Exception exception = hunter.getException();
Bitmap result = hunter.getResult();
LoadedFrom from = hunter.getLoadedFrom();
if (single != null) {
deliverAction(result, from, single);//1
}
if (hasMultiple) {
//noinspection ForLoopReplaceableByForEach
for (int i = 0, n = joined.size(); i < n; i++) {
Action join = joined.get(i);
deliverAction(result, from, join);
}
}
if (listener != null && exception != null) {
listener.onImageLoadFailed(this, uri, exception);
}
} 注释1就是将图片设置到Image的操作。
private void deliverAction(Bitmap result, LoadedFrom from, Action action) {
if (action.isCancelled()) {
return;
}
if (!action.willReplay()) {
targetToAction.remove(action.getTarget());
}
if (result != null) {
if (from == null) {
throw new AssertionError("LoadedFrom cannot be null.");
}
action.complete(result, from);
if (loggingEnabled) {
log(OWNER_MAIN, VERB_COMPLETED, action.request.logId(), "from " + from);
}
} else {
action.error();
if (loggingEnabled) {
log(OWNER_MAIN, VERB_ERRORED, action.request.logId());
}
}通过ImageViewAction执行界面的设置操作。
总结
上面其实只是一个源码的阅读流程记录而已，其实其中的更多的是设计思想的理解，设计模式的学习等，比如这里的Action接口，多个子类实现了该接口，都会执行compelete方法，这就是策略模式的模型，会根据你传入的是ImageViewAction还是GetAction来执行对应的complete，这样就需要判断是ImageViewAction还是GetAction来执行对应的操作；还有在初始化的时候，会在list中增加各种RequestHandler，然后在根据你传入的uri，找到对应的RequestHandler这种设计，个人觉得这种设计在解决一个问题有多种串行的尝试的时候，这个还是很有用。
如果你们觉得文章对你有启示作用，希望你们帮忙点个赞或者关注下，谢谢。