1.Picasso介绍
Picasso开源地址：https://github.com/square/picasso
Picasso是Square公司开源的一个Android平台上的图片加载框架，简单易用，可以实现图片下载和缓存功能。
2.Picasso的使用方法
（1）gradle的配置
compile 'com.squareup.picasso:picasso:2.5.2' （2）普通图片加载方式
path 为加载目标(图片)地址
Picasso.with(context).load("path").into(imageView); （3）转换图片以适应布局大小并减少内存占用
Picasso.with(context)
.load(url)
.resize(50, 50)
.centerCrop()
.into(imageView); （4）进行展示空白或者错误占位图片
Picasso.with(context)
.load(url)
.placeholder(R.drawable.placeholder_null)
.error(R.drawable.placeholder_error)
.into(imageView); （5）其他各种不同资源的加载方式
Picasso.with(context).load(R.drawable.landing_screen).into(imageview);
Picasso.with(context).load(new File(...)).into(imageview);
Picasso.with(context).load("file:///android_asset/ic_uncher.png").into(imageview);
3.流程与原理的分析
//主流程1
public static Picasso with(Context context) {
if (singleton == null) {
synchronized (Picasso.class) {
if (singleton == null) {
//指向分流程1
singleton = new Builder(context).build();
}
}
}
return singleton;
}通过上面的代码我们可以分析，首先进行一个单例模式进行初始化了一个Picasso对象
//分流程1
public Picasso build() {
Context context = this.context;
if (downloader == null) {
//创建下载器
downloader = Utils.createDefaultDownloader(context);
}
if (cache == null) {
//创建LruCache缓存
cache = new LruCache(context);
}
if (service == null) {
//创建执行任务
service = new PicassoExecutorService();
}
if (transformer == null) {
transformer = RequestTransformer.IDENTITY;
}
Stats stats = new Stats(cache);
//创建任务调度器
Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);
return new Picasso(context, dispatcher, cache, listener, transformer, requestHandlers, stats,
defaultBitmapConfig, indicatorsEnabled, loggingEnabled);
}
}通过build()方法可知，在创建Picasso对象过程中，进行了一系列初始化，缓存，下载器，任务调度器等。
//主流程2
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.");
}
//指向分流程2
return load(Uri.parse(path));
}
//分流程2
public RequestCreator load(Uri uri) {
//指向分流程3
return new RequestCreator(this, uri, 0);
}
//分流程3
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;
//指向分流程4
this.data = new Request.Builder(uri, resourceId, picasso.defaultBitmapConfig);
}
//分流程4
Builder(Uri uri, int resourceId, Bitmap.Config bitmapConfig) {
this.uri = uri;
this.resourceId = resourceId;
this.config = bitmapConfig;
} 进行请求的数据构造(如图片的地址，图片的配置信息)。
public void into(ImageView target) {
//指向分流程5
into(target, null);
}
//分流程5
public void into(ImageView target, Callback callback) {
long started = System.nanoTime();
//检查当前操作是否在UI线程，如果不在，抛出异常
checkMain();
//检查当前要显示的视图控件是否为null
if (target == null) {
throw new IllegalArgumentException("Target must not be null.");
}
//如果当前的视图控件没有设置图片资源
if (!data.hasImage()) {
picasso.cancelRequest(target);
//如果设置了占位图，加载占位图
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
return;
}
//如果当前的RequestCreator对于图片还有配置
if (deferred) {
//如果用户已经设置了图片大小(fit属性和resize不可以同时使用)
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());
}
//该方法最终会向Map<ImageView, DeferredRequestCreator>中进行添加操作
picasso.defer(target, new DeferredRequestCreator(this, target, callback));
return;
}
//重新设置图片大小
data.resize(width, height);
}
//根据前面的一些配置信息构建出Request对象
Request request = createRequest(started);
//根据请求生成一个带有请求配置信息的一个key
String requestKey = createKey(request);
//根据缓存策略，如果内存中存在
if (shouldReadFromMemoryCache(memoryPolicy)) {
//根据请求的key从内存中获取Bitmap
Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
if (bitmap != null) {
//取消请求
picasso.cancelRequest(target);
//设置Bitmap到控件上 指向分流程6
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;
}
}
//如果设置了占位图
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
Action action =
new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
errorDrawable, requestKey, tag, callback, noFade);
//指向分流程7
picasso.enqueueAndSubmit(action);
}
//分流程6
static void setBitmap(ImageView target, Context context, Bitmap bitmap,
Picasso.LoadedFrom loadedFrom, boolean noFade, boolean debugging) {
Drawable placeholder = target.getDrawable();
//如果当前的控件有动画，停止播放动画
if (placeholder instanceof AnimationDrawable) {
((AnimationDrawable) placeholder).stop();
}
//构建PicassoDrawable对象
PicassoDrawable drawable =
new PicassoDrawable(context, bitmap, placeholder, loadedFrom, noFade, debugging);
target.setImageDrawable(drawable);
}
//分流程7
void enqueueAndSubmit(Action action) {
//获取控件对象
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);
}
//指向分流程8
submit(action);
}
//分流程8
void submit(Action action) {
//指向分流程9
dispatcher.dispatchSubmit(action);
}
//分流程9
void dispatchSubmit(Action action) {
//通过handler发送一条提交请求的消息 分流程10会进行处理
handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));
}
//分流程10
@Override public void handleMessage(final Message msg) {
switch (msg.what) {
case REQUEST_SUBMIT: {
//获取Action
Action action = (Action) msg.obj;
//指向分流程11
dispatcher.performSubmit(action);
break;
}
//分流程11
void performSubmit(Action action) {
//指向分流程12
performSubmit(action, true);
}
//分流程12
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);
//执行BitmapHunter的run方法 指向分流程13
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());
}
}BtimapHunter 是一个Runnable 负责解析字节流，生成Bitmap
//分流程13
@Override public void run() {
try {
//更新线程名字
updateThreadName(data);
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_EXECUTING, getLogIdsForHunter(this));
}
//指向分流程14，该方法本质就是获取一个Bitmap
result = hunt();
//根据结果，dispatcure任务调度器进行分发
if (result == null) {
dispatcher.dispatchFailed(this);
} else {
//内部是通过handler发送一个消息，hanlder是通过Picasso初始化传过来的，下面看一下Picasso类里面handleMessage的处理方法，执行分流程15
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);
}
}
//分流程14
Bitmap hunt() throws IOException {
Bitmap bitmap = null;
//根据缓存策略，如果从缓存中读取
if (shouldReadFromMemoryCache(memoryPolicy)) {
//根据key找出内存中的bitmap
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;
}
}
//----下面是请求网络，获取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();
//判断如果bitmap为null,通过流的方式进行强转bitmap
if (bitmap == null) {
InputStream is = result.getStream();
try {
bitmap = decodeStream(is, data);
} finally {
Utils.closeQuietly(is);
}
}
}
//下方主要是根据图片的一些配置信息，进行相应的处理，最终返回bitmap
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;
} //分流程15
static final Handler HANDLER = new Handler(Looper.getMainLooper()) {
@Override public void handleMessage(Message msg) {
switch (msg.what) {
//处理图片加载完成的逻辑
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);
//指向分流程16
hunter.picasso.complete(hunter);
}
break;
}
case REQUEST_GCED: {
Action action = (Action) msg.obj;
if (action.getPicasso().loggingEnabled) {
log(OWNER_MAIN, VERB_CANCELED, action.request.logId(), "target got garbage collected");
}
action.picasso.cancelExistingRequest(action.getTarget());
break;
}
case REQUEST_BATCH_RESUME:
@SuppressWarnings("unchecked") List<Action> batch = (List<Action>) msg.obj;
//noinspection ForLoopReplaceableByForEach
for (int i = 0, n = batch.size(); i < n; i++) {
Action action = batch.get(i);
action.picasso.resumeAction(action);
}
break;
default:
throw new AssertionError("Unknown handler message received: " + msg.what);
}
}
}; //分流程16
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();
//获取Bitmap对象从BitmapHunter中
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);
//进行Action里面的方法分发 指向分流程17
deliverAction(result, from, join);
}
}
if (listener != null && exception != null) {
listener.onImageLoadFailed(this, uri, exception);
}
} //分流程17
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.");
}
//回调complete接口 指向分流程18
action.complete(result, from);
if (loggingEnabled) {
log(OWNER_MAIN, VERB_COMPLETED, action.request.logId(), "from " + from);
}
} else {
//回调error接口
action.error();
if (loggingEnabled) {
log(OWNER_MAIN, VERB_ERRORED, action.request.logId());
}
}
} //分流程18
@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));
}
//获取目标控件
ImageView target = this.target.get();
if (target == null) {
return;
}
Context context = picasso.context;
boolean indicatorsEnabled = picasso.indicatorsEnabled;
//执行分流程19
PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);
if (callback != null) {
callback.onSuccess();
}
} //分流程19
static void setBitmap(ImageView target, Context context, Bitmap bitmap,
Picasso.LoadedFrom loadedFrom, boolean noFade, boolean debugging) {
Drawable placeholder = target.getDrawable();
if (placeholder instanceof AnimationDrawable) {
((AnimationDrawable) placeholder).stop();
}
//创建PicassoDrawable对象
PicassoDrawable drawable =
new PicassoDrawable(context, bitmap, placeholder, loadedFrom, noFade, debugging);
//加载drawable到控件上
target.setImageDrawable(drawable);
}原理的分析参考一篇博客，博客的路径是：http://blog.csdn.net/a1002450926/article/details/50430019
希望大家一起学习。