//加载一张图片
Picasso.with(this).load("url").placeholder(R.mipmap.ic_default).into(imageView);
//加载一张图片并设置一个回调接口
Picasso.with(this).load("url").placeholder(R.mipmap.ic_default).into(imageView, new Callback() {
@Override
public void onSuccess() {
}
@Override
public void onError() {
}
});
//预加载一张图片
Picasso.with(this).load("url").fetch();
//同步加载一张图片,注意只能在子线程中调用并且Bitmap不会被缓存到内存里.
new Thread() {
@Override
public void run() {
try {
final Bitmap bitmap = Picasso.with(getApplicationContext()).load("url").get();
mHandler.post(new Runnable() {
@Override
public void run() {
imageView.setImageBitmap(bitmap);
}
});
} catch (IOException e) {
e.printStackTrace();
}
}
}.start();
//加载一张图片并自适应imageView的大小,如果imageView设置了wrap_content,会显示不出来.直到该ImageView的
//LayoutParams被设置而且调用了该View的ViewTreeObserver.OnPreDrawListener回调接口后才会显示.
Picasso.with(this).load("url").priority(Picasso.Priority.HIGH).fit().into(imageView);
//加载一张图片并按照指定尺寸以centerCrop()的形式缩放.
Picasso.with(this).load("url").resize(200,200).centerCrop().into(imageView);
//加载一张图片并按照指定尺寸以centerInside()的形式缩放.并设置加载的优先级为高.注意centerInside()或centerCrop()
//只能同时使用一种,而且必须指定resize()或者resizeDimen();
Picasso.with(this).load("url").resize(400,400).centerInside().priority(Picasso.Priority.HIGH).into(imageView);
//加载一张图片旋转并且添加一个Transformation,可以对图片进行各种变化处理,例如圆形头像.
Picasso.with(this).load("url").rotate(10).transform(new Transformation() {
@Override
public Bitmap transform(Bitmap source) {
//处理Bitmap
return null;
}
@Override
public String key() {
return null;
}
}).into(imageView);
//加载一张图片并设置tag,可以通过tag来暂定或者继续加载,可以用于当ListView滚动是暂定加载.停止滚动恢复加载.
Picasso.with(this).load("url").tag(mContext).into(imageView);
Picasso.with(this).pauseTag(mContext);
Picasso.with(this).resumeTag(mContxt);创建->入队->执行->解码->变换->批处理->完成->分发->显示(可选)
从最简单的加载一张图片开始看起
Picasso.with(this).load(url).into(imageView)；让我们来看看Picasso.with(context)做了什么，
static volatile Picasso singleton = null;
public static Picasso with(Context context) {
if (singleton == null) {
synchronized (Picasso.class) {
if (singleton == null) {
singleton = new Builder(context).build();
}
}
}
return singleton;
}显然是由new Builder(context).build()创建的单例。builder是picasso的静态内部类，其中的build()方法定义：
/** 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,
defaultBitmapConfig, indicatorsEnabled, loggingEnabled);
}让我们来看看picasso的构造函数里做了些什么
Picasso(Context context, Dispatcher dispatcher, Cache cache, Listener listener,
RequestTransformer requestTransformer, List<RequestHandler> extraRequestHandlers, Stats stats,
Bitmap.Config defaultBitmapConfig, boolean indicatorsEnabled, boolean loggingEnabled) {
this.context = context;
this.dispatcher = dispatcher;
this.cache = cache;
this.listener = listener;
this.requestTransformer = requestTransformer;
this.defaultBitmapConfig = defaultBitmapConfig;
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);
// 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));
requestHandlers = Collections.unmodifiableList(allRequestHandlers);
this.stats = stats;
this.targetToAction = new WeakHashMap<Object, Action>();
this.targetToDeferredRequestCreator = new WeakHashMap<ImageView, DeferredRequestCreator>();
this.indicatorsEnabled = indicatorsEnabled;
this.loggingEnabled = loggingEnabled;
this.referenceQueue = new ReferenceQueue<Object>();
this.cleanupThread = new CleanupThread(referenceQueue, HANDLER);
this.cleanupThread.start();
}基本上就是给内部变量赋值，作用如下：
- 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
接着初始化RequestHandler的一个ArrayList，那么RequestHandler究竟是干啥的呢，官方源码上的注释说的很清楚了，我暂时还没用过，就不做过多了解。
/**
* {@code RequestHandler} allows you to extend Picasso to load images in ways that are not
* supported by default in the library.
* <p>
* <h2>Usage</h2>
* {@code RequestHandler} must be subclassed to be used. You will have to override two methods
* ({@link #canHandleRequest(Request)} and {@link #load(Request, int)}) with your custom logic to
* load images.
* <p>
* You should then register your {@link RequestHandler} using
* {@link Picasso.Builder#addRequestHandler(RequestHandler)}
* <p>
* <b>Note:</b> This is a beta feature. The API is subject to change in a backwards incompatible
* way at any time.
*
* @see Picasso.Builder#addRequestHandler(RequestHandler)
*/接着往下看load方法。
public RequestCreator load(Uri uri) {
return new RequestCreator(this, uri, 0);
}返回了一个新建的RequestCreator对象
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, picasso.defaultBitmapConfig);
}首先是持有一个Picasso的对象,然后构建一个Request的Builder对象,将我们需要加载的图片的信息都保存在data里,在我们通过.centerCrop()或者.transform()等等方法的时候实际上也就是改变data内的对应的变量标识,再到处理的阶段根据这些参数来进行对应的操作,所以在我们调用into()方法之前,所有的操作都是在设定我们需要处理的参数,真正的操作都是有into()方法引起的。
/**
* Asynchronously fulfills the request into the specified {@link ImageView}.
* <p>
* <em>Note:</em> This method keeps a weak reference to the {@link ImageView} instance and will
* automatically support object recycling.
*/
public void into(ImageView target) {
into(target, null);
}
/**
* Asynchronously fulfills the request into the specified {@link ImageView} and invokes the
* target {@link Callback} if it's not {@code null}.
* <p>
* <em>Note:</em> The {@link Callback} param is a strong reference and will prevent your
* {@link android.app.Activity} or {@link android.app.Fragment} from being garbage collected. If
* you use this method, it is <b>strongly</b> recommended you invoke an adjacent
* {@link Picasso#cancelRequest(android.widget.ImageView)} call to prevent temporary leaking.
*/
public void into(ImageView target, Callback callback) {
long started = System.nanoTime();
//check that method call happens from the main thread
checkMain();
if (target == null) {
throw new IllegalArgumentException("Target must not be null.");
}
//如果没有设置需要加载的uri,或者resourceId
if (!data.hasImage()) {
picasso.cancelRequest(target);
//如果设置占位图片,直接加载并返回
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
return;
}
//如果是延时加载,也就是选择了fit()模式
if (deferred) {
//fit()模式是适应target的宽高加载,所以并不能手动设置resize,如果设置就抛出异常
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 || target.isLayoutRequested()) {
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
//监听ImageView的ViewTreeObserver.OnPreDrawListener接口,一旦ImageView
//的宽高被赋值,就按照ImageView的宽高继续加载.
picasso.defer(target, new DeferredRequestCreator(this, target, callback));
return;
}
data.resize(width, height);
}
Request request = createRequest(started);
String requestKey = createKey(request);
if (shouldReadFromMemoryCache(memoryPolicy)) {
//通过LruCache来读取内存里的缓存图片
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);
}
//如果设置了回调接口就回调接口的方法.
if (callback != null) {
callback.onSuccess();
}
return;
}
}
//如果缓存里没读到,先根据是否设置了占位图并设置占位
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
//构建一个Action对象,由于我们是往ImageView里加载图片,所以这里创建的是一个ImageViewAction对象
Action action =
new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
errorDrawable, requestKey, tag, callback, noFade);
//将Action对象入列提交
picasso.enqueueAndSubmit(action);
}整个流程看下来应该是比较清晰的,最后是创建了一个ImageViewAction对象并通过picasso提交,这里简要说明一下ImageViewAction,实际上Picasso会根据我们调用的不同方式来实例化不同的Action对象,当我们需要往ImageView里加载图片的时候会创建ImageViewAction对象,如果是往实现了Target接口的对象里加载图片是则会创建TargetAction对象,这些Action类的实现类不仅保存了这次加载需要的所有信息,还提供了加载完成后的回调方法.也是由子类实现并用来完成不同的调用的。然后让我们继续去看picasso.enqueueAndSubmit(action)方法:
void enqueueAndSubmit(Action action) {
Object target = action.getTarget();
//取消这个target已经有的action.
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);
}
//调用dispatcher来派发action
void submit(Action action) {
dispatcher.dispatchSubmit(action);
}
void dispatchSubmit(Action action) {
handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));
}看到通过一个handler对象发送了一个REQUEST_SUBMIT的消息,那么这个handler是存在与哪个线程的呢?
Dispatcher(Context context, ExecutorService service, Handler mainThreadHandler,
Downloader downloader, Cache cache, Stats stats) {
this.dispatcherThread = new DispatcherThread();
this.dispatcherThread.start();
this.handler = new DispatcherHandler(dispatcherThread.getLooper(), this);
this.mainThreadHandler = mainThreadHandler;
}
static class DispatcherThread extends HandlerThread {
DispatcherThread() {
super(Utils.THREAD_PREFIX + DISPATCHER_THREAD_NAME, THREAD_PRIORITY_BACKGROUND);
}
}上面是Dispatcher的构造方法(省略了部分代码),可以看到先是创建了一个HandlerThread对象,然后创建了一个DispatcherHandler对象,这个handler就是刚刚用来发送REQUEST_SUBMIT消息的handler,这里我们就明白了原来是通过Dispatcher类里的一个子线程里的handler不断的派发我们的消息,这里是用来派发我们的REQUEST_SUBMIT消息,而且最终是调用了 dispatcher.performSubmit(action);方法:
void performSubmit(Action action) {
performSubmit(action, true);
}
void performSubmit(Action action, boolean dismissFailed) {
//是否该tag的请求被暂停
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;
}
//通过action的key来在hunterMap查找是否有相同的hunter,这个key里保存的是我们
//的uri或者resourceId和一些参数,如果都是一样就将这些action合并到一个
//BitmapHunter里去.
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;
}
//创建BitmapHunter对象
hunter = forRequest(action.getPicasso(), this, cache, stats, action);
//通过service执行hunter并返回一个future对象
hunter.future = service.submit(hunter);
//将hunter添加到hunterMap中
hunterMap.put(action.getKey(), hunter);
if (dismissFailed) {
failedActions.remove(action.getTarget());
}
if (action.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_ENQUEUED, action.request.logId());
}
}这里我们再分析一下forRequest()是如何实现的:
未完待续