圖片加載是Android開發(fā)中的常見情景，在日常項目中却妨，也一直在用Picasso來處理各種加載需求饵逐。用的多了，也就自然想了解下其實(shí)現(xiàn)過程彪标。這里倍权，將深入源碼，學(xué)習(xí)Picasso框架的實(shí)現(xiàn)過程和設(shè)計思想捞烟。計劃用三篇文章來對Picasso做一個全面的分析薄声，而這第一篇的目標(biāo)則是梳理一下Picasso加載圖片的整體思路和核心流程。這樣有了主干题画，學(xué)習(xí)起來才能全而不亂默辨。
一、Picasso簡介
Picasso是Square公司推出的開源圖片加載庫苍息。體量輕缩幸、功能完善、使用簡單竞思，配合OkHttp使用效果極佳表谊。
官方鏈接：http://square.github.io/picasso/
若想在項目中使用Picasso，只需在gradle中添加依賴即可：
//2.5.0為版本號
dependencies {
compile 'com.squareup.picasso:picasso:2.5.0'
}
本文所涉及到的源碼均為Picasso 2.5.0版本
二盖喷、源碼分析
在分析源碼之前爆办，我們先看下Picasso是如何使用的：
Picasso.with(context).load(url).into(imageView);
給定一個url和ImageView，就可以將圖片加載到ImageView中课梳，十分簡單距辆。
接下來余佃，我們就一步步的看下，Picasso是如何通過一行代碼實(shí)現(xiàn)圖片加載的跨算。
1. 初始化Picasso實(shí)例
static Picasso singleton = null;
Picasso是一個單例類咙冗，可通過Picasso.with()靜態(tài)方法對Picasso進(jìn)行初始化。
/**
* Lru memory cache: 默認(rèn)緩存大小為程序運(yùn)行內(nèi)存的15%
* Disk cache: 默認(rèn)緩存大小為5MB ~ 50MB
*/
public static Picasso with(Context context) {
if (singleton == null) {
synchronized (Picasso.class) {
if (singleton == null) {
singleton = new Builder(context).build();
}
}
}
return singleton;
}
從源碼可以看到漂彤，Picasso.with()方法做的事情非常簡單，就是使用雙重校驗鎖的方式灾搏，調(diào)用Picasso.Builder.build()構(gòu)造Picasso的單例對象挫望。
既然如此，那么Builder就一定保存了初始化Picasso所需的參數(shù)：
public static class Builder {
//上下文
private final Context context;
//從外部資源(網(wǎng)絡(luò)狂窑、磁盤等)下載圖片的下載器
private Downloader downloader;
//異步加載圖片的線程池
private ExecutorService service;
//內(nèi)存緩存
private Cache cache;
//監(jiān)聽圖片加載失敗的回調(diào)
private Listener listener;
//request的修改器
private RequestTransformer transformer;
//自定義的圖片獲取方式
private List<RequestHandler> requestHandlers;
//圖片配置
private Bitmap.Config defaultBitmapConfig;
//是否顯示debug indicators
private boolean indicatorsEnabled;
//是否允許debug logging
private boolean loggingEnabled;
}
所有變量均提供了set方法供用戶從外部進(jìn)行設(shè)置媳板。若不設(shè)置，則Builder.build()方法也為其提供了默認(rèn)實(shí)現(xiàn)泉哈。
public Picasso build() {
Context context = this.context;
if (downloader == null) {
//若能反射找到com.squareup.okhttp.OkHttpClient,則downloader為OkHttpDownloader
//否則downloader為UrlConnectionDownloader
downloader = Utils.createDefaultDownloader(context);
}
if (cache == null) {
cache = new LruCache(context);
}
if (service == null) {
//核心線程數(shù)和最大線程數(shù)均為3的線程池
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);
}
可以看到蛉幸，Builder.build()方法做的事情就是為各成員變量賦值，并調(diào)用new Picasso()來構(gòu)建Picasso的實(shí)例丛晦。
Picasso(Context context, Dispatcher dispatcher, Cache cache, Listener listener,
RequestTransformer requestTransformer, List<RequestHandler> extraRequestHandlers, Stats stats,
Bitmap.Config defaultBitmapConfig, boolean indicatorsEnabled, boolean loggingEnabled) {
//省略其他代碼
//分發(fā)器
this.dispatcher = dispatcher;
//請求處理器
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);
//省略其他代碼
}
在這之中奕纫，有兩個很重要角色：Dispatcher和RequestHandler。大家先對此有個印象烫沙，后面用到時會詳細(xì)講匹层。
2. 創(chuàng)建Request
初始化好了Picasso的實(shí)例，接下來就要調(diào)用Picasso.load()方法了锌蓄。
Picasso.load()默認(rèn)提供了四種重載：
// load(String)和load(File)最終都會調(diào)用load(Uri)升筏。
public RequestCreator load(File file) {
if (file == null) {
return new RequestCreator(this, null, 0);
}
return load(Uri.fromFile(file));
}
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));
}
public RequestCreator load(Uri uri) {
return new RequestCreator(this, uri, 0);
}
//直接創(chuàng)建RequestCreator
public RequestCreator load(int resourceId) {
if (resourceId == 0) {
throw new IllegalArgumentException("Resource ID must not be zero.");
}
return new RequestCreator(this, null, resourceId);
}
可以看到，這四個方法內(nèi)部實(shí)現(xiàn)大同小異瘸爽，最終都會創(chuàng)建一個RequestCreator對象您访。
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;
//data即Request.Builder對象，其封裝了真實(shí)的請求內(nèi)容
this.data = new Request.Builder(uri, resourceId, picasso.defaultBitmapConfig);
}
3. 分發(fā)Action
Picasso.load()方法創(chuàng)建好了RequestCreator剪决，也準(zhǔn)備好了url灵汪，接下來就要創(chuàng)建Action了。
//采用異步方式昼捍，將request的結(jié)果填充到ImageView中
public void into(ImageView target) {
into(target, null);
}
// Callback為監(jiān)聽圖片添加結(jié)果（成功/失斒缎椤）的回調(diào)
public void into(ImageView target, Callback callback) {
long started = System.nanoTime();
//檢查是否為主線程調(diào)用
checkMain();
if (target == null) {
throw new IllegalArgumentException("Target must not be null.");
}
//檢查請求中是否設(shè)置了uri或resourceId。若均沒有設(shè)置妒茬，則視其為無效請求担锤，取消這次request
if (!data.hasImage()) {
picasso.cancelRequest(target);
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
return;
}
//是否調(diào)整圖片尺寸
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);
}
//創(chuàng)建真實(shí)請求
Request request = createRequest(started);
String requestKey = createKey(request);
//是否允許從cache中讀取圖片
if (shouldReadFromMemoryCache(memoryPolicy)) {
Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
//若cache中存在要請求的圖片，則取消請求乍钻，直接加載cache中的圖片
if (bitmap != null) {
picasso.cancelRequest(target);
//將cache中的圖片加載到ImageView上
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;
}
}
//是否需要占位圖片
if (setPlaceholder) {
setPlaceholder(target, getPlaceholderDrawable());
}
//執(zhí)行到此肛循，則表示需要從外部資源下載圖片
//創(chuàng)建ImageViewAction铭腕，ImageViewAction內(nèi)部持有ImageView和Request
Action action =
new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
errorDrawable, requestKey, tag, callback, noFade);
//將Action放入隊列并提交
picasso.enqueueAndSubmit(action);
}
RequestCreator.into()方法主要做了一些檢查和準(zhǔn)備工作，包括：
1）確保在主線程進(jìn)行調(diào)用多糠；
2）檢查request的有效性累舷，即uri或resourceId是否為空；
3）是否從cache中獲取圖片夹孔；
4）若request有效且cache中沒有目標(biāo)圖片被盈，則創(chuàng)建ImageViewAction，提交并放入隊列搭伤。
到此為止只怎，還是沒有看到處理url和添加圖片的過程，只能跟著Picasso.enqueueAndSubmit(action)繼續(xù)往下看怜俐。
void enqueueAndSubmit(Action action) {
//對于ImageViewAction來說身堡，target即ImageView
Object target = action.getTarget();
//檢查ImageView是否有其他正在執(zhí)行的action，若有拍鲤，則取消之前的action
if (target != null && targetToAction.get(target) != action) {
cancelExistingRequest(target);
targetToAction.put(target, action);
}
//提交action
submit(action);
}
Picasso.enqueueAndSubmit()方法將action存入WeakHashMap中贴谎，并確保ImageView當(dāng)前只有一個正在執(zhí)行的action。
void submit(Action action) {
dispatcher.dispatchSubmit(action);
}
Picasso.submit()方法也沒有對action進(jìn)行處理季稳，而是直接調(diào)用Dispatcher.dispatchSubmit()繼續(xù)下發(fā)擅这。
Dispatcher(Context context, ExecutorService service, Handler mainThreadHandler,
Downloader downloader, Cache cache, Stats stats) {
//省略其他代碼
// DispatcherHandler繼承自HandlerThread，因此handler為子線程的handler
this.handler = new DispatcherHandler(dispatcherThread.getLooper(), this);
// mainThreadHandler為主線程handler
this.mainThreadHandler = mainThreadHandler;
//省略其他代碼
}
void dispatchSubmit(Action action) {
//將action裝入message绞幌，并由子線程handler進(jìn)行發(fā)送蕾哟，完成線程切換
handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));
}
Dispatcher是分發(fā)器，該類持有兩個Handler：一個是mainThreadHandler莲蜘，另一個是子線程handler谭确。dispatchSubmit()方法即調(diào)用子線程handler的sendMessage()方法，將action發(fā)送到子線程進(jìn)行處理票渠。
值得注意的是逐哈，之前的代碼是在主線程中運(yùn)行的，而在handler.sendMessage()之后问顷，便完成了主線程到子線程的切換昂秃。這也意味著接下來要做耗時操作了。
4. 提交BitmapHunter
在子線程接收到message后杜窄，會調(diào)用Dispatcher.performSubmit()方法肠骆。
@Override
public void handleMessage(final Message msg) {
switch (msg.what) {
case REQUEST_SUBMIT: {
Action action = (Action) msg.obj;
dispatcher.performSubmit(action);
break;
}
//省略其他代碼
}
}
void performSubmit(Action action) {
performSubmit(action, true);
}
void performSubmit(Action action, boolean dismissFailed) {
//如果當(dāng)前action在pausedTags中，則暫停執(zhí)行并將其放入pausedActions
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;
}
//從active hunters中尋找有無action對應(yīng)的BitmapHunter
BitmapHunter hunter = hunterMap.get(action.getKey());
if (hunter != null) {
hunter.attach(action);
return;
}
//若線程池已關(guān)閉塞耕，則退出執(zhí)行
if (service.isShutdown()) {
if (action.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_IGNORED, action.request.logId(), "because shut down");
}
return;
}
//創(chuàng)建hunter對象
hunter = forRequest(action.getPicasso(), this, cache, stats, action);
//執(zhí)行hunter任務(wù)
hunter.future = service.submit(hunter);
//將hunter放入active hunters
hunterMap.put(action.getKey(), hunter);
if (dismissFailed) {
failedActions.remove(action.getTarget());
}
if (action.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_ENQUEUED, action.request.logId());
}
}
Dispatcher.performSubmit()方法是Dispatcher中非常重要的方法蚀腿，所有的請求都會經(jīng)此方法進(jìn)行提交。該方法主要干了幾件事：
1）從action中取出對應(yīng)的tag，并檢查其是否在pausedTags中莉钙。如果存在廓脆，則暫停執(zhí)行該action并將其放入pausedActions；
2）從active hunters中尋找有無action對應(yīng)的BitmapHunter磁玉。如果有停忿，則直接取出對應(yīng)的BitmapHunter，并將action attach上去蚊伞；
3）若以上條件均不滿足席赂，則為當(dāng)前action創(chuàng)建新的BitmapHunter，并提交給線程池執(zhí)行时迫。
5. hunt目標(biāo)圖片
至此氧枣，我們可以大膽猜測，這個BitmapHunter應(yīng)該就是一個Runnable别垮，圖片下載過程應(yīng)該就在其run()方法中。
class BitmapHunter implements Runnable {
static BitmapHunter forRequest(Picasso picasso, Dispatcher dispatcher, Cache cache, Stats stats, Action action) {
Request request = action.getRequest();
List<RequestHandler> requestHandlers = picasso.getRequestHandlers();
//遍歷Picasso.getRequestHandlers()扎谎，尋找合適的請求處理器
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);
}
@Override
public void run() {
try {
updateThreadName(data);
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_EXECUTING, getLogIdsForHunter(this));
}
//調(diào)用hunt()方法獲取結(jié)果
// result是一個Bitmap變量
result = hunt();
//如果bitmap為null碳想，則圖片獲取失敗，調(diào)用dispatchFailed()方法分發(fā)失敗消息
//否則毁靶，調(diào)用dispatchComplete()方法分發(fā)成功消息
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);
}
}
}
BitmapHunter有一個名為forRequest()的靜態(tài)方法胧奔，用于BitmapHunter的實(shí)例化。在該方法內(nèi)部预吆，會遍歷Picasso.getRequestHandlers()列表龙填，尋找合適的RequestHandler。而這個RequestHandlers列表便是在Picasso.new()方法中創(chuàng)建的拐叉，參考2.1節(jié)岩遗。
得到BitmapHunter后，便可將其交給線程池來執(zhí)行了凤瘦。run()方法主要做了兩件事：
1）調(diào)用hunt()方法得到Bitmap結(jié)果宿礁；
2）若成功獲取圖片，則調(diào)用dispatchComplete()方法分發(fā)成功消息蔬芥，否則梆靖，調(diào)用dispatchFailed()方法分發(fā)失敗消息。
接著看BitmapHunter.hunt()方法：
Bitmap hunt() throws IOException {
Bitmap bitmap = null;
//從cache中獲取目標(biāo)圖片
if (shouldReadFromMemoryCache(memoryPolicy)) {
bitmap = cache.get(key);
if (bitmap != null) {
//若找到目標(biāo)圖標(biāo)笔诵，則更新stats狀態(tài)并返回bitmap
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;
//調(diào)用requestHandler.load()方法獲取目標(biāo)圖片
//目標(biāo)圖片會以Bitmap或Stream的形式存在result中
RequestHandler.Result result = requestHandler.load(data, networkPolicy);
if (result != null) {
loadedFrom = result.getLoadedFrom();
exifRotation = result.getExifOrientation();
bitmap = result.getBitmap();
//如果目標(biāo)圖片是Stream形式返吻，則需decodeStream()
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);
//是否需要對圖片進(jìn)行變換
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;
}
可以看到，圖片獲取過程是由requestHandler.load()完成的乎婿。而這個RequestHandler就是我們在forRequest()方法中找的請求處理器测僵。
6. 下載目標(biāo)圖片
假設(shè)我們要從網(wǎng)絡(luò)上獲取圖片，那么forRequest()中找的的目標(biāo)RequestHandler應(yīng)該就是NetworkRequestHandler次酌。load()方法自然就是發(fā)送網(wǎng)絡(luò)請求恨课，返回Result的過程舆乔。
@Override
public Result load(Request request, int networkPolicy) throws IOException {
//downloader才是request的真正執(zhí)行者
Response response = downloader.load(request.uri, request.networkPolicy);
if (response == null) {
return null;
}
Picasso.LoadedFrom loadedFrom = response.cached ? DISK : NETWORK;
//目標(biāo)圖片以Bitmap的形式返回
Bitmap bitmap = response.getBitmap();
if (bitmap != null) {
return new Result(bitmap, loadedFrom);
}
//目標(biāo)圖片以Stream的形式返回
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);
}
通過源碼發(fā)現(xiàn)，NetworkRequestHandler并非request的真正處理者剂公，其內(nèi)部的downloader才是request的負(fù)責(zé)人希俩。
Downloader是一個接口，Picasso內(nèi)置兩種實(shí)現(xiàn)：OkHttpDownloader和UrlConnectionDownloader纲辽。如果用戶不主動設(shè)置的話颜武，Picasso.Builder.build()方法會根據(jù)規(guī)則兩者選一：若能反射找到com.squareup.okhttp.OkHttpClient，則使用OkHttpDownloader拖吼，否則downloader為UrlConnectionDownloader鳞上。
這里，我們以UrlConnectionDownloader為例吊档，看一下UrlConnectionDownloader.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);
}
//建立HttpURLConnection
HttpURLConnection connection = openConnection(uri);
//使用cache
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));
//返回Response
return new Response(connection.getInputStream(), fromCache, contentLength);
}
基本就是HttpURLConnection發(fā)起網(wǎng)絡(luò)請求的過程篙议。
7. 分發(fā)Response
在UrlConnectionDownloader.load()得到Response后，會一層層地將結(jié)果返回：UrlConnectionDownloader.load() ---> NetworkRequestHandler.load() ---> BitmapHunter.hunt() ---> BitmapHunter.run()怠硼。
在BitmapHunter.run()中鬼贱，根據(jù)目標(biāo)圖片是否為null來決定是發(fā)送成功消息，還是失敗消息香璃。
@Override
public void run() {
//省略其他代碼
result = hunt();
if (result == null) {
dispatcher.dispatchFailed(this);
} else {
dispatcher.dispatchComplete(this);
}
//省略其他代碼
}
這里这难，我們假設(shè)分發(fā)成功消息，即調(diào)用dispatcher.dispatchComplete(this)葡秒。
void dispatchComplete(BitmapHunter hunter) {
handler.sendMessage(handler.obtainMessage(HUNTER_COMPLETE, hunter));
}
@Override
public void handleMessage(final Message msg) {
switch (msg.what) {
case HUNTER_COMPLETE: {
BitmapHunter hunter = (BitmapHunter) msg.obj;
dispatcher.performComplete(hunter);
break;
}
//省略其他代碼
}
}
void performComplete(BitmapHunter hunter) {
//是否將目標(biāo)圖片緩存起來
if (shouldWriteToMemoryCache(hunter.getMemoryPolicy())) {
cache.set(hunter.getKey(), hunter.getResult());
}
//將當(dāng)前的BitmapHunter從active hunters中移除
hunterMap.remove(hunter.getKey());
//批處理
batch(hunter);
if (hunter.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_BATCHED, getLogIdsForHunter(hunter), "for completion");
}
}
在Dispatcher.performComplete()中姻乓，根據(jù)內(nèi)存策略決定是否緩存目標(biāo)圖片并調(diào)用Dispatcher.batch()繼續(xù)批處理。
private void batch(BitmapHunter hunter) {
if (hunter.isCancelled()) {
return;
}
//將hunter添加到List中
batch.add(hunter);
if (!handler.hasMessages(HUNTER_DELAY_NEXT_BATCH)) {
//延遲200ms發(fā)送消息
handler.sendEmptyMessageDelayed(HUNTER_DELAY_NEXT_BATCH, BATCH_DELAY);
}
}
@Override
public void handleMessage(final Message msg) {
switch (msg.what) {
case HUNTER_DELAY_NEXT_BATCH: {
dispatcher.performBatchComplete();
break;
}
//省略其他代碼
}
}
Dispatcher.batch()方法會將分發(fā)過來的hunter放到batch列表中眯牧，并延遲發(fā)送蹋岩。
void performBatchComplete() {
List<BitmapHunter> copy = new ArrayList<BitmapHunter>(batch);
batch.clear();
//調(diào)用mainThreadHandler發(fā)送消息
mainThreadHandler.sendMessage(mainThreadHandler.obtainMessage(HUNTER_BATCH_COMPLETE, copy));
logBatch(copy);
}
@Override
public void handleMessage(final Message msg) {
switch (msg.what) {
case HUNTER_BATCH_COMPLETE: {
List<BitmapHunter> batch = (List<BitmapHunter>) msg.obj;
//遍歷batch列表，依次處理各個hunter
for (int i = 0, n = batch.size(); i < n; i++) {
BitmapHunter hunter = batch.get(i);
hunter.picasso.complete(hunter);
}
break;
}
//省略其他代碼
}
}
在接收到HUNTER_DELAY_NEXT_BATCH消息后学少，Dispatcher.performBatchComplete()調(diào)用主線程handler繼續(xù)分發(fā)消息星澳，實(shí)現(xiàn)線程切換。
8. 加載圖片
在Picasso.handleMessage()收到批處理消息后旱易，會依次取出各個hunter禁偎，并調(diào)用Picasso.complete()。
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();
//處理action
if (single != null) {
deliverAction(result, from, single);
}
//處理action
if (hasMultiple) {
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);
}
}
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.");
}
//回調(diào)action.complete()
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());
}
}
}
經(jīng)過Picasso.complete() ---> Picasso.deliverAction()阀坏，最終回調(diào)到ImageViewAction.complete()方法如暖。在該方法中，完成了ImageView加載圖片的過程忌堂。
class ImageViewAction extends Action<ImageView> {
@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;
//將圖片加載到ImageView上
PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);
if (callback != null) {
callback.onSuccess();
}
}
}
final class PicassoDrawable extends BitmapDrawable {
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();
}
PicassoDrawable drawable =
new PicassoDrawable(context, bitmap, placeholder, loadedFrom, noFade, debugging);
//為ImageView添加圖片
target.setImageDrawable(drawable);
}
}
三盒至、總結(jié)
經(jīng)過上述源碼分析，想必大家都已了解Picasso是如何從網(wǎng)絡(luò)下載圖片并加載到ImageView的。這里枷遂，直接用流程圖作簡要總結(jié)：
其中樱衷，橙線為發(fā)送過程調(diào)用鏈，綠線為返回過程調(diào)用鏈酒唉。主要步驟如下：
1）在Picasso中創(chuàng)建RequestCreator矩桂；
2）創(chuàng)建并提交Action至Dispatcher；
3）Dispatcher創(chuàng)建BitmapHunter并交給線程池執(zhí)行痪伦；
4）BitmapHunter調(diào)用ResourceHandler及Downloader下載圖片侄榴；
5）圖片下載成功后，返回BitmapHunter并由Dispatcher負(fù)責(zé)分發(fā)网沾；
6）Picasso收到下載成功的消息后癞蚕，回調(diào)Action的complete()方法，將圖片加載到ImageView上辉哥。
最后
本文作為Picasso源碼解析系列的第一篇桦山，目標(biāo)是從源碼入手，梳理Picasso加載圖片的工作流程醋旦，掌握其主要思路度苔。如大家對Picasso框架的實(shí)現(xiàn)細(xì)節(jié)感興趣，歡迎關(guān)注該系列的后續(xù)文章浑度。如對本文有疑問，歡迎留言交流鸦概。如需要轉(zhuǎn)載箩张，則請注明出處。
