picasso是Square公司开源的一个Android图形缓存库
主要有以下一些特性：
在adapter中回收和取消当前的下载；
使用最少的内存完成复杂的图形转换操作；
自动的内存和硬盘缓存；
图形转换操作，如变换大小，旋转等，提供了接口来让用户可以自定义转换操作；
加载载网络或本地资源；
Picasso.class
他有一个内部类，一般是通过他来创建实例的：
downloader(Downloader downloader) ：
容许使用自定义的下载器，可以用okhttp或者volley，必须实现Downloader接口。
executor(ExecutorService executorService):
容许使用自己的线程池来进行下载
memoryCache(Cache memoryCache)：
容许使用自己的缓存类，必须实现Cache接口。
requestTransformer(RequestTransformer transformer)：
listener(Listener listener)：
addRequestHandler(RequestHandler requestHandler)：
indicatorsEnabled(boolean enabled)：设置图片来源的指示器。
loggingEnabled(boolean enabled)：
再来看build()方法：
/** 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, indicatorsEnabled, loggingEnabled);
}这里会使用默认的下载器，缓存类，线程池，并将这些对象封装到了分发器Dispatcher里。然后在返回一个Picasso对象。
一般情况下，如果不需要自定义bulid里的这些方法，可以使用Picasso.with(context)默认方法来获得单例对象：
public static Picasso with(Context context) {
if (singleton == null) {
synchronized (Picasso.class) {
if (singleton == null) {
singleton = new Builder(context).build();
}
}
}
return singleton;
}如果需要自定义一些对象：
public class SamplePicassoFactory {
private static Picasso sPicasso;
public static Picasso getPicasso(Context context) {
if (sPicasso == null) {
sPicasso = new Picasso.Builder(context)
.downloader(new OkHttpDownloader(context, ConfigConstants.MAX_DISK_CACHE_SIZE))
.memoryCache(new LruCache(ConfigConstants.MAX_MEMORY_CACHE_SIZE))
.build();
}
return sPicasso;
}
}通过上面的方法，获得sPicasso对象后就设置成了单例，但是最好设置成双重校验锁模式。
如果通过以上方法获得对象后，还可以通过Picasso.setSingletonInstance(Picasso picasso)方法设置对象到Picasso中，这样以后还是通过Picasso.with(context)来调用。
接着就可以通过以下方式设置加载源：
可以是uri地址，file文件，res资源drawable。
最终都是通过以下方法来创建一个RequestCreator对象：
public RequestCreator load(Uri uri) {
return new RequestCreator(this, uri, 0);
}
RequestCreator(Picasso picasso, Uri uri, int resourceId) {
if (picasso.shutdown) {
throw new IllegalStateException(
"Picasso instance already shut down. Cannot submit new requests.");
}
this.picasso = picasso;
this.data = new Request.Builder(uri, resourceId);
}RequestCreator对象是用来设置一系列属性的，如：
noPlaceholder()：设置没有加载等待图片
placeholder(int placeholderResId)：设置加载等待图片
placeholder(Drawable placeholderDrawable)：设置加载等待图片
error(int errorResId)：设置加载出错的图片。
error(Drawable errorDrawable)：设置加载出错的图片。
tag(Object tag)：设置标记
fit():自适应，下载的图片有多少像素就显示多少像素
resizeDimen(int targetWidthResId, int targetHeightResId)：设置图片显示的像素。
resize(int targetWidth, int targetHeight)：设置图片显示的像素。
centerCrop()：设置ImageView的ScaleType属性.
centerInside():设置ImageView的ScaleType属性.
rotate(float degrees):设置旋转角度。
rotate(float degrees, float pivotX, float pivotY):设置以某个中心点设置某个旋转角度。
config(Bitmap.Config config)：设置Bitmap的Config属性
priority(Priority priority)：设置请求的优先级。
transform(Transformation transformation)：
skipMemoryCache()：跳过内存缓存
memoryPolicy(MemoryPolicy policy, MemoryPolicy... additional)：
networkPolicy(NetworkPolicy policy, NetworkPolicy... additional)：
noFade()：没有淡入淡出效果
get()：获得bitmap对象
fetch()：
设置完以上一系列属性之后，最关键的就是into方法，现在以into(ImageView target)举例：
public void into(ImageView target) {
into(target, null);
}他实际调用的是：
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()) {
picasso.cancelRequest(target);
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
return;
}
if (deferred) {
if (data.hasSize()) {
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)) {
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) {
setPlaceholder(target, getPlaceholderDrawable());
}
Action action =
new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
errorDrawable, requestKey, tag, callback, noFade);
picasso.enqueueAndSubmit(action);
}
checkMain();
首先检查是否是主线程
if (target == null) {
throw new IllegalArgumentException("Target must not be null.");
}检查目标view是否存在
if (!data.hasImage()) {
picasso.cancelRequest(target);
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
return;
}如果没有设置uri或者resDrawable,就停止请求，如果设置了加载等待的图片就设置，然后就return
if (deferred) {
if (data.hasSize()) {
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);
}如果设置fit自适应：如果已经设置了图片像素大小就抛异常，如果目标view的长宽等于0，就在设置等待图片后延期处理，如果不等于0就设置size到Request.Builder的data里。
Request request = createRequest(started);
String requestKey = createKey(request);接着就创建Request，并生成一个String类型的requestKey。
/** Create the request optionally passing it through the request transformer. */
private Request createRequest(long started) {
int id = nextId.getAndIncrement();
Request request = data.build();
request.id = id;
request.started = started;
boolean loggingEnabled = picasso.loggingEnabled;
if (loggingEnabled) {
log(OWNER_MAIN, VERB_CREATED, request.plainId(), request.toString());
}
Request transformed = picasso.transformRequest(request);
if (transformed != request) {
// If the request was changed, copy over the id and timestamp from the original.
transformed.id = id;
transformed.started = started;
if (loggingEnabled) {
log(OWNER_MAIN, VERB_CHANGED, transformed.logId(), "into " + transformed);
}
}
return transformed;
}
在以上代码可以看出，这里调用picasso.transformRequest(request);来给使用者提供一个可以更改request的机会，他最终调用的是Picasso.Build里面的通过requestTransformer(RequestTransformer transformer)方法传进去的RequestTransformer 接口：
public interface RequestTransformer {
/**
* Transform a request before it is submitted to be processed.
*
* @return The original request or a new request to replace it. Must not be null.
*/
Request transformRequest(Request request);
/** A {@link RequestTransformer} which returns the original request. */
RequestTransformer IDENTITY = new RequestTransformer() {
@Override public Request transformRequest(Request request) {
return request;
}
};
}默认使用的是IDENTITY。这里没有做任何的修改，如果有需要可以自己设置接口以达到修改Request的目的。
if (shouldReadFromMemoryCache(memoryPolicy)) {
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;
}
}
接着要判断是否需要从缓存中读取图片，如果需要，就要根据requestKey从缓存中读取：
Bitmap quickMemoryCacheCheck(String key) {
Bitmap cached = cache.get(key);
if (cached != null) {
stats.dispatchCacheHit();
} else {
stats.dispatchCacheMiss();
}
return cached;
}这里的cache用的是LruCache内存缓存类，这个内存缓存类，实现了Cache接口，最后调用的是：
@Override public Bitmap get(String key) {
if (key == null) {
throw new NullPointerException("key == null");
}
Bitmap mapValue;
synchronized (this) {
mapValue = map.get(key);
if (mapValue != null) {
hitCount++;
return mapValue;
}
missCount++;
}
return null;
}这里的map是LinkedHashMap<String, Bitmap>：
this.map = new LinkedHashMap<String, Bitmap>(0, 0.75f, true);缓存的策略稍后分析，现在先回到前面，如果从缓存中拿到的bitmap不等于null,就调用picasso.cancelRequest(target)来删除请求，然后通过
setBitmap(target, picasso.context, bitmap, MEMORY, noFade, picasso.indicatorsEnabled);设置图片，然后如果设置了callback回调，再回掉callback.onSuccess();方法，然后就return。
如果没有设置内存缓存，或者缓存中的图片已经不存在：
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
Action action =
new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
errorDrawable, requestKey, tag, callback, noFade);
picasso.enqueueAndSubmit(action);
接着就设置等待加载图片，然后封装一个action,然后将action加入到分发器中：
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);
}然后调用了dispatcher的performSubmit()方法：
void performSubmit(Action action) {
performSubmit(action, true);
}
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());
}
}
这里通过 hunter = forRequest(action.getPicasso(), this, cache, stats, action);获得一个BitmapHunter对象，接着就提交到线程池中 hunter.future = service.submit(hunter);：
接着线程池就会调用hunter 的run方法，因为这实现了Runnable对象：
@Override public void run() {
try {
updateThreadName(data);
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_EXECUTING, getLogIdsForHunter(this));
}
result = hunt();
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);
}
}接着调用hunt()方法：
Bitmap hunt() throws IOException {
Bitmap bitmap = null;
if (shouldReadFromMemoryCache(memoryPolicy)) {
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) {
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) {
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;
}这里如果从内存缓存中渠道bitmap对象，就直接返回了；否则通过requestHandler.load(data, networkPolicy);来发起网络请求(这个是NetworkRequestHandler类型的)：
@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);
}这里调用了downloader.load(request.uri, request.networkPolicy)方法，这是一个UrlConnectionDownloader类型的对象，调用的是UrlConnectionDownloader的load()方法：
@Override public Response load(Uri uri, int networkPolicy) throws IOException {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
installCacheIfNeeded(context);
}
HttpURLConnection connection = openConnection(uri);
connection.setUseCaches(true);
if (networkPolicy != 0) {
String headerValue;
if (NetworkPolicy.isOfflineOnly(networkPolicy)) {
headerValue = FORCE_CACHE;
} else {
StringBuilder builder = CACHE_HEADER_BUILDER.get();
builder.setLength(0);
if (!NetworkPolicy.shouldReadFromDiskCache(networkPolicy)) {
builder.append("no-cache");
}
if (!NetworkPolicy.shouldWriteToDiskCache(networkPolicy)) {
if (builder.length() > 0) {
builder.append(',');
}
builder.append("no-store");
}
headerValue = builder.toString();
}
connection.setRequestProperty("Cache-Control", headerValue);
}
int responseCode = connection.getResponseCode();
if (responseCode >= 300) {
connection.disconnect();
throw new ResponseException(responseCode + " " + connection.getResponseMessage(),
networkPolicy, responseCode);
}
long contentLength = connection.getHeaderFieldInt("Content-Length", -1);
boolean fromCache = parseResponseSourceHeader(connection.getHeaderField(RESPONSE_SOURCE));
return new Response(connection.getInputStream(), fromCache, contentLength);
}
如果系统版本是4.0以上，就使用installCacheIfNeeded(context)来启动data\data\包名\cacha下的某个目录的磁盘缓存：
private static void installCacheIfNeeded(Context context) {
// DCL + volatile should be safe after Java 5.
if (cache == null) {
try {
synchronized (lock) {
if (cache == null) {
cache = ResponseCacheIcs.install(context);
}
}
} catch (IOException ignored) {
}
}
}
private static class ResponseCacheIcs {
static Object install(Context context) throws IOException {
File cacheDir = Utils.createDefaultCacheDir(context);
HttpResponseCache cache = HttpResponseCache.getInstalled();
if (cache == null) {
long maxSize = Utils.calculateDiskCacheSize(cacheDir);
cache = HttpResponseCache.install(cacheDir, maxSize);
}
return cache;
}但是如果要是用这个磁盘缓存，就要在HttpURLConnection的响应头上加上缓存的控制头"Cache-Control"！
最后返回new Response(connection.getInputStream(), fromCache, contentLength)请求结果。
接着回到上面的hunt()方法的流程继续：
if (result != null) {
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) {
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;
然后bitmap返回到run()方法里面的result应用上：
if (result == null) {
dispatcher.dispatchFailed(this);
} else {
dispatcher.dispatchComplete(this);
}如果result有结果就分发完成消息，最后将会调用到Picasso里面：
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;
}
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);
}
}
接着分发action:
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());
}
}
}然后调用的是ImageViewAction里面的action.complete(result, from)方法：
@Override public void complete(Bitmap result, Picasso.LoadedFrom from) {
if (result == null) {
throw new AssertionError(
String.format("Attempted to complete action with no result!\n%s", this));
}
ImageView target = this.target.get();
if (target == null) {
return;
}
Context context = picasso.context;
boolean indicatorsEnabled = picasso.indicatorsEnabled;
PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);
if (callback != null) {
callback.onSuccess();
}
}
最后一步通过PicassoDrawable.setBitmap来设置，就算大功完成了。
附上部分流程图：
