- 请求分发模块。负责封装请求,对请求进行优先级排序,并按照类型进行分发。
- 缓存模块。通常包括一个二级的缓存，内存缓存、磁盘缓存。并预置多种缓存策略。
- 下载模块。负责下载网络图片。
- 监控模块。负责监控缓存命中率、内存占用、加载图片平均耗时等。
- 图片处理模块。负责对图片进行压缩、变换等处理。
- 本地资源加载模块。负责加载本地资源，如assert、drawable、sdcard等。
7.显示模块。负责将图片输出显示。
图片加载库
Picasso 使用
Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView);
Picasso.with()
Picasso 的入口函数，用于创建全局唯一的Picasso 实例，使用 double-check 单例模式
public static Picasso with(Context context) {
if (singleton == null) {
synchronized (Picasso.class) {
if (singleton == null) {
singleton = new Builder(context).build();
}
}
}
return singleton;
}
with方法内部通过Builder模式创建Picasso 实例
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);
}
使用默认的缓存策略，内存缓存基于LruCache,磁盘缓存基于http缓存,HttpResponseCache
创建默认的下载器
创建默认的线程池(3个worker线程)
创建默认的Transformer,这个Transformer什么事情也不干，只负责转发请求
创建默认的监控器(Stats),用于统计缓存命中率、下载时长等等
创建默认的处理器集合,即RequestHandlers.它们分别会处理不同的加载请求
load(url)
多种重载方法，load()方法用于从不同地方加载图片，比如网络、resource、File等，只是创建了一个RequestCreator
public RequestCreator load(...) {
return new RequestCreator(...);
}
RequestCreator
RequestCreator是一个封装请求的类,请求在Picasso中被抽象成Request。RequestCreator类提供了 诸如placeholder、tag、error、memoryPolicy、networkPolicy等方法.
Request也使用了Builder模式:
RequestCreator(Picasso picasso, Uri uri, int resourceId)
RequestCreator.into()
into方法有多种重载,Picasso不仅仅可以 将图片加载到ImageView上，还可以加载Target或者RemoteView上.
public void into(Target target) {
long started = System.nanoTime();
//检查是否在主线程运行
checkMain();
if (target == null) {
throw new IllegalArgumentException("Target must not be null.");
}
if (deferred) {
throw new IllegalStateException("Fit cannot be used with a Target.");
}
if (!data.hasImage()) {
picasso.cancelRequest(target);
target.onPrepareLoad(setPlaceholder ? getPlaceholderDrawable() : null);
return;
}
//创建request
Request request = createRequest(started);
String requestKey = createKey(request);
if (shouldReadFromMemoryCache(memoryPolicy)) {//是否需要在缓存里面先查找
Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);//使用请求生成的key查找缓存
if (bitmap != null) {
picasso.cancelRequest(target);
target.onBitmapLoaded(bitmap, MEMORY);
return;
}
}
target.onPrepareLoad(setPlaceholder ? getPlaceholderDrawable() : null);
// 将request封装成action
Action action =
new TargetAction(picasso, target, request, memoryPolicy, networkPolicy, errorDrawable,
requestKey, tag, errorResId);
// 提交action
picasso.enqueueAndSubmit(action);
}
Request
关注的是请求本身，比如请求的源、id、开始时间、图片变换配置、优先级等等，而Action则代表的是一个加载任务，所以不仅需要 Request对象的引用，还需要Picasso实例，是否重试加载等等
Action
有个需要关注的点，那就是WeakReference<T> target,它持有的是Target(比如ImageView..)的弱引用，这样可以保证加载时间很长的情况下 也不会影响到Target的回收了.
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);
}
void submit(Action action) {
dispatcher.dispatchSubmit(action);
}
enqueueAndSubmit 它会先从action任务上拿到对应target，也就是imageView，然后从weakHashMap中通过这个imageView索引到对应的action，如果 发现这个action跟传进来的action不一样的话，那就取消掉之前的加载任务。最后将当前加载任务提交.
最终调用的是Dispatcher的dispatchSubmit(action)方法.
Dispatcher即任务分发器，它是在 Picasso实例创建的时候初始化的.。
Dispatcher
Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);
每一个Dispatcher都需要关联线程池(service)、下载器(downloader)、主线程的Handler(HANDLER)、缓存(cache)、 监控器(stats).
PicassoExecutorService
private static final int DEFAULT_THREAD_COUNT = 3;
PicassoExecutorService() {
super(DEFAULT_THREAD_COUNT, DEFAULT_THREAD_COUNT, 0, TimeUnit.MILLISECONDS,
new PriorityBlockingQueue<Runnable>(), new Utils.PicassoThreadFactory());
}
默认线程数量为 3.但是PicassoExecutorService的特性是可以根据网络情况调整线程数量，wifi下是4个线程，而2g网只有一个线程。具体是 通过在Dispatcher中注册了监听网络变化的广播接收者。
DispatcherThread 是 Dispatcher 的内部类
static class DispatcherThread extends HandlerThread
切换到子线程
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);
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());
}
}
创建了一个BitmapHunter实现Runnable接口,可以被线程池调用。然后判断线程池有没有关闭，如果没有的话， 就会将这个bitmapHunter submit线程池里面
forRequest
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);
}
使用 Picasso 构造器添加的requestHnadlers创建 BitmapHunter
BitmapHunter的run 方法会调用 hunt方法，hunt方法先从缓存拿，缓存没有命中的话，再调用requestHandler.load.
hunt 方法最终会返回一个bitmap，在run方法中处理
result = hunt();
if (result == null) {
dispatcher.dispatchFailed(this);
} else {
dispatcher.dispatchComplete(this);
}
dispatcher 使用内部子线程handler处理
void dispatchComplete(BitmapHunter hunter) {
handler.sendMessage(handler.obtainMessage(HUNTER_COMPLETE, hunter));
}
handler调用performComplete
case HUNTER_COMPLETE: {
BitmapHunter hunter = (BitmapHunter) msg.obj;
dispatcher.performComplete(hunter);
break;
}
会根据事先设置的缓存策略决定是否将结果加到内存缓存。然后调用batch方法
void performComplete(BitmapHunter hunter) {
if (shouldWriteToMemoryCache(hunter.getMemoryPolicy())) {
cache.set(hunter.getKey(), hunter.getResult());
}
hunterMap.remove(hunter.getKey());
batch(hunter);
if (hunter.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_BATCHED, getLogIdsForHunter(hunter), "for completion");
}
}
这个方法会把结果暂存， 然后批量处理(等待200ms)，这样做也是为了防止短时间大量任务阻塞消息队列。到时间后，就会执行performBatchComplete
, 此方法会将这个批次的所有结果一次性发给主线程的Handler
void performBatchComplete() {
List<BitmapHunter> copy = new ArrayList<BitmapHunter>(batch);
batch.clear();
mainThreadHandler.sendMessage(mainThreadHandler.obtainMessage(HUNTER_BATCH_COMPLETE, copy));
logBatch(copy);
}
在Picasso初始化的时候创建了一个主线程handler，
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;
}
主线程接到消息。调用complete处理
void complete(BitmapHunter hunter) {
Action single = hunter.getAction();
List<Action> 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);
}
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);
}
}
complete方法会调用deliverAction方法,最终其实调用的是具体 action的complete方法，如果是ImageView的话，那就是ImageViewAction的complete方法
NetworkRequestHandler
@Override public Result load(Request request, int networkPolicy) throws IOException {
Response response = downloader.load(request.uri, request.networkPolicy);
if (response == null) {
return null;
}
Picasso.LoadedFrom loadedFrom = response.cached ? DISK : NETWORK;
Bitmap bitmap = response.getBitmap();
if (bitmap != null) {
return new Result(bitmap, loadedFrom);
}
InputStream is = response.getInputStream();
if (is == null) {
return null;
}
// Sometimes response content length is zero when requests are being replayed. Haven't found
// root cause to this but retrying the request seems safe to do so.
if (loadedFrom == DISK && response.getContentLength() == 0) {
Utils.closeQuietly(is);
throw new ContentLengthException("Received response with 0 content-length header.");
}
if (loadedFrom == NETWORK && response.getContentLength() > 0) {
stats.dispatchDownloadFinished(response.getContentLength());
}
return new Result(is, loadedFrom);
}
这里会创建一个 downloader
static Downloader createDefaultDownloader(Context context) {
if (SDK_INT >= GINGERBREAD) {
try {
Class.forName("com.squareup.okhttp.OkHttpClient");
return OkHttpLoaderCreator.create(context);
} catch (ClassNotFoundException ignored) {
}
}
return new UrlConnectionDownloader(context);
}
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