上一系列我们讲述了Glide的简单使用和其源码解析，还没看的同学可以移步去浏览：
Glide4.9图片框架(一)之加载图片的常规使用方式
讲完了Glide及其源码，我们继续看看同样作为图片加载框架的Picasso，内部源码就是怎样的呢，这里就不去分析它的使用了，因为和Glide如出一辙，我们主要还是看内部源码的加载流程，本次源码版本为2.71828。
Picasso的简单使用
Picasso.get()
//相当于glide的with方法，初始化Picasso
.load("url")
//传入图片url，构建requestCreater
.placeholder(R.drawable.error)
//设置占位图
.error(R.drawable.error)
//设置加载错误占位图
.resize(480,800)
//设置加载图片的宽高
.centerCrop()
//设置scyletype
.rotate(90)
//设置旋转90度
.priority(Picasso.Priority.HIGH)
//加载的优先级，不是决定性的因素
.tag("list view")
//给加载添加tag
.memoryPolicy(MemoryPolicy.NO_CACHE)
//设置没有内存缓存
.into(imageview);
Picasso.get()
public static Picasso get() {
if (singleton == null) {
synchronized (Picasso.class) {
if (singleton == null) {
if (PicassoProvider.context == null) {
throw new IllegalStateException("context == null");
}
singleton = new Builder(PicassoProvider.context).build();
}
}
}
return singleton;
}
get方法采用的双重检查锁实现的单例，其中对PicassoProvider.context 判空，这里的context是什么呢，我们跟进去看一下:
public final class PicassoProvider extends ContentProvider {
@SuppressLint("StaticFieldLeak") static Context context;
@Override public boolean onCreate() {
context = getContext();
return true;
}
@Nullable @Override
public Cursor query(@NonNull Uri uri, @Nullable String[] projection, @Nullable String selection,
@Nullable String[] selectionArgs, @Nullable String sortOrder) {
return null;
}
@Nullable @Override public String getType(@NonNull Uri uri) {
return null;
}
@Nullable @Override public Uri insert(@NonNull Uri uri, @Nullable ContentValues values) {
return null;
}
@Override public int delete(@NonNull Uri uri, @Nullable String selection,
@Nullable String[] selectionArgs) {
return 0;
}
@Override
public int update(@NonNull Uri uri, @Nullable ContentValues values, @Nullable String selection,
@Nullable String[] selectionArgs) {
return 0;
}
}
可以看到创建这个contentprovider就是为了获取context，从而不需要从get方法里每次再传入context，那么回到上面的get方法，我们继续看singleton = new Builder(PicassoProvider.context).build()中的Builder构造方法：
public Builder(@NonNull Context context) {
if (context == null) {
throw new IllegalArgumentException("Context must not be null.");
}
this.context = context.getApplicationContext();
}
这里是直接获取ApplicationContext，因此图片的加载是跟application绑定的。继续看build()方法:
public Picasso build() {
Context context = this.context;
//创建下载器，这里直接封装了OkHttp3的请求框架作为下载器
if (downloader == null) {
downloader = new OkHttp3Downloader(context);
}
// 创建LruCache缓存
if (cache == null) {
cache = new LruCache(context);
}
//创建线程池
if (service == null) {
service = new PicassoExecutorService();
}
//创建请求转换器，默认没有添加任何转换则直接请求
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);
}
build()之一 OkHttp3Downloader
public OkHttp3Downloader(final Context context) {
this(Utils.createDefaultCacheDir(context));
}
static File createDefaultCacheDir(Context context) {
File cache = new File(context.getApplicationContext().getCacheDir(), PICASSO_CACHE);
if (!cache.exists()) {
//noinspection ResultOfMethodCallIgnored
cache.mkdirs();
}
return cache;
}
public OkHttp3Downloader(final File cacheDir) {
this(cacheDir, Utils.calculateDiskCacheSize(cacheDir));
}
static long calculateDiskCacheSize(File dir) {
long size = MIN_DISK_CACHE_SIZE;
try {
StatFs statFs = new StatFs(dir.getAbsolutePath());
//noinspection deprecation
long blockCount =
SDK_INT < JELLY_BEAN_MR2 ? (long) statFs.getBlockCount() : statFs.getBlockCountLong();
//noinspection deprecation
long blockSize =
SDK_INT < JELLY_BEAN_MR2 ? (long) statFs.getBlockSize() : statFs.getBlockSizeLong();
long available = blockCount * blockSize;
// Target 2% of the total space.
size = available / 50;
} catch (IllegalArgumentException ignored) {
}
// Bound inside min/max size for disk cache.
return Math.max(Math.min(size, MAX_DISK_CACHE_SIZE), MIN_DISK_CACHE_SIZE);
}
public OkHttp3Downloader(final File cacheDir, final long maxSize) {
this(new OkHttpClient.Builder().cache(new Cache(cacheDir, maxSize)).build());
sharedClient = false;
}
先看OkHttp3Downloader的构造方法，这里的OkHttp3Downloader是默认基于okhttp封装的请求下载器，我们看下createDefaultCacheDir方法，根据命名其实可以猜出这个方法就是创建默认的缓存文件的目录，而这个目录的路径就是context.getApplicationContext().getCacheDir()，然后回到OkHttp3Downloader的this方法，继续跟下去调用了calculateDiskCacheSize方法并把缓存目录传进去，跟进去看，这个方法就是计算默认缓存文件的大小，通过获取本地存储空间来计算能缓存的最大文件空间长度是多少。然后继续看最下面的OkHttp3Downloader的this方法，通过新建OkHttpClient的Builder设置缓存的目录和size并完成初始化。
build()之二 LruCache内存缓存
public final class LruCache implements Cache {
final android.util.LruCache cache;
/** Create a cache using an appropriate portion of the available RAM as the maximum size. */
public LruCache(@NonNull Context context) {
this(Utils.calculateMemoryCacheSize(context));
}
0
/** Create a cache with a given maximum size in bytes. */
public LruCache(int maxByteCount) {
cache = new android.util.LruCache(maxByteCount) {
@Override protected int sizeOf(String key, BitmapAndSize value) {
return value.byteCount;
}
};
}
static int calculateMemoryCacheSize(Context context) {
ActivityManager am = getService(context, ACTIVITY_SERVICE);
boolean largeHeap = (context.getApplicationInfo().flags & FLAG_LARGE_HEAP) != 0;
int memoryClass = largeHeap ? am.getLargeMemoryClass() : am.getMemoryClass();
// Target ~15% of the available heap.
return (int) (1024L * 1024L * memoryClass / 7);
}
这里我们还是只看简单的构造方法吧，其实内部持有了一个LruCache引用，相当于自己封装了一层LruCache，通过calculateMemoryCacheSize方法，计算能此初始化的内存缓存的大小并完成LruCache初始化。这里是获取app能用缓存的七分之一大小设置内存缓存，关于LRUCache后面我们会单独拿出来分析源码，只要理解作为最近最少的使用的缓存优先会被回收就行了，我们接着看下面：
build()之三 PicassoExecutorService线程池
if (service == null) {
service = new PicassoExecutorService();
}
class PicassoExecutorService extends ThreadPoolExecutor {
private static final int DEFAULT_THREAD_COUNT = 3;
PicassoExecutorService() {
super(DEFAULT_THREAD_COUNT, DEFAULT_THREAD_COUNT, 0, TimeUnit.MILLISECONDS,
new PriorityBlockingQueue(), new Utils.PicassoThreadFactory());
}
}
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue workQueue, ThreadFactory threadFactory) {
throw new RuntimeException("Stub!");
}
下面初始化了一个自定义的线程池PicassoExecutorService，继承的还是自带的线程池ThreadPoolExecutor，那么构造方法里传入的五个参数分别表示什么意思呢
- corePoolSize
表示默认初始化的核心线程数 - maximumPoolSize
表示线程池能创建的最大线程个数，超过则不会继续创建 - keepAliveTime
线程最大空闲时间 - TimeUnit
时间单位 - BlockingQueue
线程等待队列 - ThreadFactory
线程创建工厂
build()之四 Dispatcher
class Dispatcher {
Dispatcher(Context context, ExecutorService service, Handler mainThreadHandler,
Downloader downloader, Cache cache, Stats stats) {
this.dispatcherThread = new DispatcherThread();
this.context = context;
this.service = service;
this.handler = new DispatcherHandler(dispatcherThread.getLooper(), this);
this.downloader = downloader;
this.mainThreadHandler = mainThreadHandler;
this.cache = cache;
this.stats = stats;
this.receiver = new NetworkBroadcastReceiver(this);
receiver.register();
}
Dispatcher是一个比较重要的类，这里只截出了几个比较关键的参数信息，dispatcher里包含了缓存信息，请求结果的调度，状态的切换，其中最重要的两个handler，DispatcherHandler和mainThreadHandler ，我们来看看内部的消息的处理是怎么样的:
private static class DispatcherHandler extends Handler {
private final Dispatcher dispatcher;
DispatcherHandler(Looper looper, Dispatcher dispatcher) {
super(looper);
this.dispatcher = dispatcher;
}
@Override public void handleMessage(final Message msg) {
switch (msg.what) {
case REQUEST_SUBMIT: {
Action action = (Action) msg.obj;
dispatcher.performSubmit(action);
break;
}
case REQUEST_CANCEL: {
Action action = (Action) msg.obj;
dispatcher.performCancel(action);
break;
}
case TAG_PAUSE: {
Object tag = msg.obj;
dispatcher.performPauseTag(tag);
break;
}
case TAG_RESUME: {
Object tag = msg.obj;
dispatcher.performResumeTag(tag);
break;
}
case HUNTER_COMPLETE: {
BitmapHunter hunter = (BitmapHunter) msg.obj;
dispatcher.performComplete(hunter);
break;
}
case HUNTER_RETRY: {
BitmapHunter hunter = (BitmapHunter) msg.obj;
dispatcher.performRetry(hunter);
break;
}
case HUNTER_DECODE_FAILED: {
BitmapHunter hunter = (BitmapHunter) msg.obj;
dispatcher.performError(hunter, false);
break;
}
case HUNTER_DELAY_NEXT_BATCH: {
dispatcher.performBatchComplete();
break;
}
case NETWORK_STATE_CHANGE: {
NetworkInfo info = (NetworkInfo) msg.obj;
dispatcher.performNetworkStateChange(info);
break;
}
case AIRPLANE_MODE_CHANGE: {
dispatcher.performAirplaneModeChange(msg.arg1 == AIRPLANE_MODE_ON);
break;
}
default:
Picasso.HANDLER.post(new Runnable() {
@Override public void run() {
throw new AssertionError("Unknown handler message received: " + msg.what);
}
});
}
}
}
可以看到DispatcherHandler里面全是对各种状态和结果的回调和处理，DispatcherHandler的构造函数传入的是dispatcherThread.getLooper()，那继续看看这个dispatcherThread是什么意思：
static class DispatcherThread extends HandlerThread {
DispatcherThread() {
super(Utils.THREAD_PREFIX + DISPATCHER_THREAD_NAME, THREAD_PRIORITY_BACKGROUND);
}
}
其实就是继承的HandlerThread，这里不去细看，dispatcher的构造方法中还有一个this.receiver = new NetworkBroadcastReceiver(this).那我们重点看下这个广播接收者的onReceive方法：
@Override public void onReceive(Context context, Intent intent) {
if (intent == null) {
return;
}
final String action = intent.getAction();
if (ACTION_AIRPLANE_MODE_CHANGED.equals(action)) {
if (!intent.hasExtra(EXTRA_AIRPLANE_STATE)) {
return; // No airplane state, ignore it. Should we query Utils.isAirplaneModeOn?
}
dispatcher.dispatchAirplaneModeChange(intent.getBooleanExtra(EXTRA_AIRPLANE_STATE, false));
} else if (CONNECTIVITY_ACTION.equals(action)) {
ConnectivityManager connectivityManager = getService(context, CONNECTIVITY_SERVICE);
dispatcher.dispatchNetworkStateChange(connectivityManager.getActiveNetworkInfo());
}
}
其实在接收广播中可以看到对飞行模式进行了监听，ACTION_AIRPLANE_MODE_CHANGED，如果是飞行模式，就调用dispatcher.dispatchAirplaneModeChange方法，跟进去看一下:
void dispatchAirplaneModeChange(boolean airplaneMode) {
handler.sendMessage(handler.obtainMessage(AIRPLANE_MODE_CHANGE,
airplaneMode ? AIRPLANE_MODE_ON : AIRPLANE_MODE_OFF, 0));
}
void performAirplaneModeChange(boolean airplaneMode) {
this.airplaneMode = airplaneMode;
}
这里最终就是记录一个是否为飞行模式的布尔值，我们继续看下面的判读网络状态链接的部分，dispatcher.dispatchNetworkStateChange：
void performNetworkStateChange(NetworkInfo info) {
if (service instanceof PicassoExecutorService) {
((PicassoExecutorService) service).adjustThreadCount(info);
}
if (info != null && info.isConnected()) {
flushFailedActions();
}
}
void adjustThreadCount(NetworkInfo info) {
if (info == null || !info.isConnectedOrConnecting()) {
setThreadCount(DEFAULT_THREAD_COUNT);
return;
}
switch (info.getType()) {
case ConnectivityManager.TYPE_WIFI:
case ConnectivityManager.TYPE_WIMAX:
case ConnectivityManager.TYPE_ETHERNET:
setThreadCount(4);
break;
case ConnectivityManager.TYPE_MOBILE:
switch (info.getSubtype()) {
case TelephonyManager.NETWORK_TYPE_LTE: // 4G
case TelephonyManager.NETWORK_TYPE_HSPAP:
case TelephonyManager.NETWORK_TYPE_EHRPD:
setThreadCount(3);
break;
case TelephonyManager.NETWORK_TYPE_UMTS: // 3G
case TelephonyManager.NETWORK_TYPE_CDMA:
case TelephonyManager.NETWORK_TYPE_EVDO_0:
case TelephonyManager.NETWORK_TYPE_EVDO_A:
case TelephonyManager.NETWORK_TYPE_EVDO_B:
setThreadCount(2);
break;
case TelephonyManager.NETWORK_TYPE_GPRS: // 2G
case TelephonyManager.NETWORK_TYPE_EDGE:
setThreadCount(1);
break;
default:
setThreadCount(DEFAULT_THREAD_COUNT);
}
break;
default:
setThreadCount(DEFAULT_THREAD_COUNT);
}
}
这里重点分析一下adjustThreadCount方法，当网络状态改变的时候，根据当前网络的类型，创建不同的核心线程数，因为网络的影响，可能线程池的核心线程数也受到影响，这个设计是相当有意思的
，也是Glide中没有去做的东西，对网络差的图片加载来说，提高了加载的效率。继续看flushFailedActions();方法:
private void flushFailedActions() {
if (!failedActions.isEmpty()) {
Iterator iterator = failedActions.values().iterator();
while (iterator.hasNext()) {
Action action = iterator.next();
iterator.remove();
if (action.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_REPLAYING, action.getRequest().logId());
}
performSubmit(action, false);
}
}
}
failedActions这里存储的是第一次请求失败需要重新请求的集合，那么这里是空的，我们不继续往下分析了，到这里整个Picasso的build方法就结束了，我们继续往下分析load方法:
Picasso.load()
public RequestCreator load(@Nullable Uri uri) {
return new RequestCreator(this, uri, 0);
}
public RequestCreator load(@Nullable 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(@NonNull File file) {
if (file == null) {
return new RequestCreator(this, null, 0);
}
return load(Uri.fromFile(file));
}
public RequestCreator load(@DrawableRes int resourceId) {
if (resourceId == 0) {
throw new IllegalArgumentException("Resource ID must not be zero.");
}
return new RequestCreator(this, null, resourceId);
}
Picasso的load方法，相对于Glide来说比较少，只有四个，分别是Uri、path、file、Drawable。Uri、path、file最中都是调用的load(@Nullable Uri uri) 方法，Drawable也简单，这里我们就看传入字符串的情况。最终调用的new RequestCreator(this, uri, 0)方法，我们跟进去看RequestCreator主要是有什么作用：
RequestCreator
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);
}
Builder(Uri uri, int resourceId, Bitmap.Config bitmapConfig) {
this.uri = uri;
this.resourceId = resourceId;
this.config = bitmapConfig;
}
看代码才发现RequestCreator其实是request的build的包装类，由它生成request的build类，然后build才能创建request，还有一个作用就是RequestCreator这里包装了一系列的链式调用，包括placeholder、error、fit、centerCrop等等。到这里load方法就结束了，最终我们直接看into方法:
RequestCreator.into(ImageView)
public void into(ImageView target) {
into(target, null);
}
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);
}
boolean hasImage() {
return uri != null || resourceId != 0;
}
public RequestCreator fit() {
deferred = true;
return this;
}
boolean hasSize() {
return targetWidth != 0 || targetHeight != 0;
}
checkMain是检查时否在主线程，不是则抛出异常，我们直接看data.hasImage()方法，这里的data就是我们load时创建的request的build，而hasImage就是判读当前加载的资源是不是空，如果是空则调用picasso.cancelRequest取消请求并且设置占位图直接return。
接下来看deferred变量，这里只有我们设置了fit才会去执行，那么假设我们设了fit，data.hasSize实际上是判断我们是否设置过resize方法，如果设置了，那么不能再调用fit方法，因为这两者是冲突的。我们继续看通过target也就是传入的image view获取宽和高，如果获取到的宽或者高有一个为0，表示imageview还没能绘制成功，这里设置占位图，并且调用picasso.defer方法，我们跟进去看一下:
class DeferredRequestCreator implements OnPreDrawListener, OnAttachStateChangeListener {
private final RequestCreator creator;
@VisibleForTesting final WeakReference target;
@VisibleForTesting Callback callback;
DeferredRequestCreator(RequestCreator creator, ImageView target, Callback callback) {
this.creator = creator;
this.target = new WeakReference<>(target);
this.callback = callback;
target.addOnAttachStateChangeListener(this);
if (target.getWindowToken() != null) {
onViewAttachedToWindow(target);
}
}
@Override public void onViewAttachedToWindow(View view) {
view.getViewTreeObserver().addOnPreDrawListener(this);
}
@Override public void onViewDetachedFromWindow(View view) {
view.getViewTreeObserver().removeOnPreDrawListener(this);
}
@Override public boolean onPreDraw() {
ImageView target = this.target.get();
if (target == null) {
return true;
}
ViewTreeObserver vto = target.getViewTreeObserver();
if (!vto.isAlive()) {
return true;
}
int width = target.getWidth();
int height = target.getHeight();
if (width <= 0 || height <= 0) {
return true;
}
target.removeOnAttachStateChangeListener(this);
vto.removeOnPreDrawListener(this);
this.target.clear();
this.creator.unfit().resize(width, height).into(target, callback);
return true;
}
这里其实大致看出来DeferredRequestCreator 实现了OnPreDrawListener和OnAttachStateChangeListener ，那么就是在image view绘制成功后，重新再去获取宽和高，并且调用creator.unfit().resize反法重新给build也就是data传递了新的图片宽高。并将这个DeferredRequestCreator存储到targetToDeferredRequestCreator集合中。那么回到上面的into方法，如果没有设置或fit，则开始调用Request request = createRequest(started)创建一个请求，我们跟进去看是如何创建的。
createRequest()
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;
}
public Request build() {
......
if (priority == null) {
priority = Priority.NORMAL;
}
return new Request(uri, resourceId, stableKey, transformations, targetWidth, targetHeight,
centerCrop, centerInside, centerCropGravity, onlyScaleDown, rotationDegrees,
rotationPivotX, rotationPivotY, hasRotationPivot, purgeable, config, priority);
}
这里调用了data.build()创建Request，我们进去看一下，直看重点，直接new了一个Request对象，传入了很多参数，比如uri、资源id、key、转换、宽高、旋转、scaletype、优先级等。然后回到build方法，调用了transformRequest方法，其实如果没有设置transform，这里等于没有变化，所以回到createRequest方法，通过createKey(request)创建了一个请求key，然后调用了shouldReadFromMemoryCache方法，这里如果我们设置了可以存储到内存缓存，那么是会先去从内存缓存读取的，所以看看下面的Picasso的quickMemoryCacheCheck方法:
获取内存缓存
Bitmap quickMemoryCacheCheck(String key) {
Bitmap cached = cache.get(key);
if (cached != null) {
stats.dispatchCacheHit();
} else {
stats.dispatchCacheMiss();
}
return cached;
}
void dispatchCacheHit() {
handler.sendEmptyMessage(CACHE_HIT);
}
@Override public void handleMessage(final Message msg) {
switch (msg.what) {
case CACHE_HIT:
stats.performCacheHit();
break;
case CACHE_MISS:
stats.performCacheMiss();
break;
case BITMAP_DECODE_FINISHED:
stats.performBitmapDecoded(msg.arg1);
break;
case BITMAP_TRANSFORMED_FINISHED:
stats.performBitmapTransformed(msg.arg1);
break;
case DOWNLOAD_FINISHED:
stats.performDownloadFinished((Long) msg.obj);
break;
default:
Picasso.HANDLER.post(new Runnable() {
@Override public void run() {
throw new AssertionError("Unhandled stats message." + msg.what);
}
});
}
}
void performCacheHit() {
cacheHits++;
}
void performCacheMiss() {
cacheMisses++;
}
这里很简单，直接从Picasso的cache中获取缓存的bitmap，如果没有则调用stats.dispatchCacheHit()方法，stats其实就是对各种结果和回调做一个通知的作用。也可以看一下，最终调用到performCacheHit，对成功获取的缓存数++；那么dispatchCacheMiss应该就是没拿到缓存的计数++了。
回到上面的quickMemoryCacheCheck方法返回的bitmap,，如果bitmap 不为空表示获取缓存成功，没必要继续请求了，因此调用cancelRequest，然后调用setBitmap方法：
static void setBitmap(ImageView target, Context context, Bitmap bitmap,
Picasso.LoadedFrom loadedFrom, boolean noFade, boolean debugging) {
Drawable placeholder = target.getDrawable();
if (placeholder instanceof Animatable) {
((Animatable) placeholder).stop();
}
PicassoDrawable drawable =
new PicassoDrawable(context, bitmap, placeholder, loadedFrom, noFade, debugging);
target.setImageDrawable(drawable);
}
直接看重点的一行，target.setImageDrawable(drawable)，到这里就是将bitmap设置到imageview了，PicassoDrawable 其实是对bitmap进行的封装。再回到获取内存缓存并setbitmap的位置，这里我们第一次加载应该是没有缓存的，所以我们继续看下去，没有缓存的情况。
setPlaceholder就是设置占位符，这个不是重点，看下面的new ImageViewAction方法，传入了picasso, target, request, memoryPolicy, networkPolicy, errorResId,errorDrawable, requestKey, tag, callback, noFade一系列参数，生成了一个ImageViewAction，那么我们重点看这个action是干嘛的：
class ImageViewAction extends Action {
Callback callback;
ImageViewAction(Picasso picasso, ImageView imageView, Request data, int memoryPolicy,
int networkPolicy, int errorResId, Drawable errorDrawable, String key, Object tag,
Callback callback, boolean noFade) {
super(picasso, imageView, data, memoryPolicy, networkPolicy, errorResId, errorDrawable, key,
tag, noFade);
this.callback = callback;
}
@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();
}
}
@Override public void error(Exception e) {
ImageView target = this.target.get();
if (target == null) {
return;
}
Drawable placeholder = target.getDrawable();
if (placeholder instanceof Animatable) {
((Animatable) placeholder).stop();
}
if (errorResId != 0) {
target.setImageResource(errorResId);
} else if (errorDrawable != null) {
target.setImageDrawable(errorDrawable);
}
if (callback != null) {
callback.onError(e);
}
}
ImageViewAction 主要实现的方法就是complete和error方法，这两个的调用还是父类action中的，主要是对请求完成后的回调做处理，设置成功的图片或者设置加载出错的图片。所以主要的功能还是再父类Action中。回到ImageViewAction创建完成后，into方法的最后一行，调用的是picasso.enqueueAndSubmit(action)执行这个action，我们跟进去看一下：
执行请求
void enqueueAndSubmit(Action action) {
Object target = action.getTarget();
if (target != null && targetToAction.get(target) != action) {
cancelExistingRequest(target);
targetToAction.put(target, action);
}
submit(action);
}
void submit(Action action) {
dispatcher.dispatchSubmit(action);
}
case REQUEST_SUBMIT: {
Action action = (Action) msg.obj;
dispatcher.performSubmit(action);
break;
}
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());
}
}
这里的if判断，就是判断如果之前存储过跟这次不一样的加载请求，那么取消掉上一次的加载，存储当前这一次的加载请求，然后调用submit方法，我们看submit方法通过dispatcher下发了提交请求任务的消息，最终调用到了performSubmit方法。这里我们看第一个if判断
在了解判断之前，来看看这个pausedTags是什么，怎么赋值的：
public class SampleScrollListener implements AbsListView.OnScrollListener {
private final Context context;
public SampleScrollListener(Context context) {
this.context = context;
}
@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
final Picasso picasso = Picasso.get();
if (scrollState == SCROLL_STATE_IDLE || scrollState == SCROLL_STATE_TOUCH_SCROLL) {
picasso.resumeTag(context);
} else {
picasso.pauseTag(context);
}
}
}
最终发现，pausedTags添加的地方，就是滚动监听，当屏幕暂停时，就将当前的context存储到暂停的tag中，否则就从暂停的pausedTags中移除。那么回到上面的performSubmit方法我们就明白了，意思就是只有当屏幕没有滑动的时候，才允许去加载当前的请求，那么继续往下看，hunterMap.get方法获取到了BitmapHunter，这里的action.getKey()就是前面我们获取内存缓存的key，那么bitmapHunter是什么呢，他说一个实现了Runnable接口的类，所有的耗时操作包括请求图片，解码图片，图片转换等都在bitmapHunter中执行。这里获取BitmapHunter 后去判断是否为空，如果不为空表示当前正在执行请求，那么就不会再去执行直接return，否则我们看下面的forRequest方法：
static BitmapHunter forRequest(Picasso picasso, Dispatcher dispatcher, Cache cache, Stats stats,
Action action) {
Request request = action.getRequest();
List requestHandlers = picasso.getRequestHandlers();
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);
}
这里没有BitmapHunter ，那么就去创建一个BitmapHunter 并且返回，通过找到对应的requestHandlers 去生成对应的BitmapHunter，这里我们地方requestHandler应该就是请求相关的NetworkRequestHandler，我们回到返回值的地方，下面调用service.submit(hunter)表示去执行请求，请求的结果存储在hunter.future中，完成后缓存到hunterMap中，我们跟进去看一下：
@Override
public Future submit(Runnable task) {
PicassoFutureTask ftask = new PicassoFutureTask((BitmapHunter) task);
execute(ftask);
return ftask;
}
@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 (NetworkRequestHandler.ResponseException e) {
if (!NetworkPolicy.isOfflineOnly(e.networkPolicy) || e.code != 504) {
exception = e;
}
dispatcher.dispatchFailed(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);
}
}
这里直接执行execute方法，那么我们就看BitmapHunter的run方法了，直接看result = hunt();这一行，可以看到通过hunt方法拿到请求的返回值了，result是一个Bitmap类型，那么我们就重点看hunt方法了。
hunt()
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;
}
}
networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;
RequestHandler.Result result = requestHandler.load(data, networkPolicy);
if (result != null) {
loadedFrom = result.getLoadedFrom();
exifOrientation = result.getExifOrientation();
bitmap = result.getBitmap();
// If there was no Bitmap then we need to decode it from the stream.
if (bitmap == null) {
Source source = result.getSource();
try {
bitmap = decodeStream(source, data);
} finally {
try {
//noinspection ConstantConditions If bitmap is null then source is guranteed non-null.
source.close();
} catch (IOException ignored) {
}
}
}
}
if (bitmap != null) {
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_DECODED, data.logId());
}
stats.dispatchBitmapDecoded(bitmap);
if (data.needsTransformation() || exifOrientation != 0) {
synchronized (DECODE_LOCK) {
if (data.needsMatrixTransform() || exifOrientation != 0) {
bitmap = transformResult(data, bitmap, exifOrientation);
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;
}
这里我们又调了一次shouldReadFromMemoryCache，这里再次获取内存缓存也没有冲突，防止前面请求的时候获取缓存不及时，这里我们直接看requestHandler.load返回RequestHandler.Result的方法，跟进去看一下：
@Override public Result load(Request request, int networkPolicy) throws IOException {
okhttp3.Request downloaderRequest = createRequest(request, networkPolicy);
Response response = downloader.load(downloaderRequest);
ResponseBody body = response.body();
if (!response.isSuccessful()) {
body.close();
throw new ResponseException(response.code(), request.networkPolicy);
}
Picasso.LoadedFrom loadedFrom = response.cacheResponse() == null ? NETWORK : DISK;
if (loadedFrom == DISK && body.contentLength() == 0) {
body.close();
throw new ContentLengthException("Received response with 0 content-length header.");
}
if (loadedFrom == NETWORK && body.contentLength() > 0) {
stats.dispatchDownloadFinished(body.contentLength());
}
return new Result(body.source(), loadedFrom);
}
@NonNull @Override public Response load(@NonNull Request request) throws IOException {
return client.newCall(request).execute();
}
到这里就是直接通过okhttp3请求去获取结果了，可能你奇怪为什么看到这里一直都是内存缓存，没有看到过磁盘缓存，因为磁盘缓存是在okhttp3请求框架里面自己去做了，所以这个代码里面是没有的。看downloader.load方法内部其实还是使用的OK HTTP的newCall创建请求并执行的，那么回调结果，返回上一层通过result.getBitmap()获取到bitmap对象，如果为空，那么就再去解码拿到Source并转换成bitmap。后面就是回调了，返回到BitmapHunter的Run方法，拿到bitmap后调用dispatcher.dispatchComplete(this)方法：
拿到bitmap，回调结果
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");
}
}
在拿到hunter后，立马通过判断是否需要内存缓存来决定是否将结果缓存到内存缓存，而下面的hunterMap也在请求成功后移除对应的请求缓存，然后调用batch方法：
private void batch(BitmapHunter hunter) {
if (hunter.isCancelled()) {
return;
}
if (hunter.result != null) {
hunter.result.prepareToDraw();
}
batch.add(hunter);
if (!handler.hasMessages(HUNTER_DELAY_NEXT_BATCH)) {
handler.sendEmptyMessageDelayed(HUNTER_DELAY_NEXT_BATCH, BATCH_DELAY);
}
}
拿到hunter.result后调用bitmap的prepareToDraw方法准备填充到image view，然后存储到batch中。方便后面对bitmapHunter同意管理：
case HUNTER_BATCH_COMPLETE: {
@SuppressWarnings("unchecked") List batch = (List) 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 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, exception);
}
if (hasMultiple) {
//noinspection ForLoopReplaceableByForEach
for (int i = 0, n = joined.size(); i < n; i++) {
Action join = joined.get(i);
deliverAction(result, from, join, exception);
}
}
if (listener != null && exception != null) {
listener.onImageLoadFailed(this, uri, exception);
}
}
第一行的single也就是表示是否只有这一个，如果是则获取到bitmap后直接回调，否则就遍历每一个Action都调用deliverAction回调，我们继续看deliverAction方法：
private void deliverAction(Bitmap result, LoadedFrom from, Action action, Exception e) {
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(e);
if (loggingEnabled) {
log(OWNER_MAIN, VERB_ERRORED, action.request.logId(), e.getMessage());
}
}
}
重点还是关注在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();
}
}
static void setBitmap(ImageView target, Context context, Bitmap bitmap,
Picasso.LoadedFrom loadedFrom, boolean noFade, boolean debugging) {
Drawable placeholder = target.getDrawable();
if (placeholder instanceof Animatable) {
((Animatable) placeholder).stop();
}
PicassoDrawable drawable =
new PicassoDrawable(context, bitmap, placeholder, loadedFrom, noFade, debugging);
target.setImageDrawable(drawable);
}
最终调用action.complete实际上还是调用的image viewAction，因为一开始的into方法，我们创建的action就是image viewAction，因此在complete中可以看到target.get()获取我们传进来的imageview，然后调用PicassoDrawable的setBitmap方法设置图片到view，最终调用callback.onSuccess()回调到我们使用Picasso加载图片到地方，前提是我们设置了这个callback。
总结
Picasso通过get方法创建单例实例，然后通过build方法创建请求下载器、内存缓存、线程池和Dispatcher ，并最终生成Picasso对象。磁盘缓存和内存缓存都是根据手机容量和内存来判断的，内存为app可用内存的七分之一。Dispatcher则在全程中起到的一个调度的作用，回调和传递各种信息，内部有一个网络状态的广播监听，根据手机网络的情况2G-4G来设置线程池不同的核心线程数，然后还可以监听飞行模式。load方法则是通过RequestCreator 创建Request的build对象，最终调用into方法通过build对象创建Request对象并生成一个缓存key，通过缓存key在内存缓存中查到是否存在缓存，不存在则创建一个Action对象，通过action对象找到对应NetworkRequestHandler 并生成BitmapHunter，BitmapHunter本身是一个线程，最终调用PicassoExecutorService线程池的submit方法执行BitmapHunter，最终调用requestHandler的load方法通过OK HTTP请求做请求和磁盘缓存的操作获得Response ，然后解码拿到bitmap回调给dispatcher，然后缓存到内存，并且回调到image viewAction的complete方法，最终通过PicassoDrawable的setBitmap方法设置图片资源到image view中。
到这里Picasso的源码分析就结束了，在看过了Glide源码之后，再看Picasso可能突然觉得轻松了许多，毕竟Glide的功能还是很丰富的，类有比较复杂，所以阅读起来相对比较麻烦。那么对比我们之前分析的Glide源码，我们看看他们的区别在哪：
- 加载数据类型的区别
glide支持gif图加载，Picasso不支持。 - 缓存的区别
glide内存缓存和磁盘缓存都做了二级缓存，Picasso只有一步内存缓存，磁盘缓存还是OkHttp做的。 - Picasso对飞行模式做了单独的监听，并对网络状态的234G做了监听，根据不同的网络设置不同的线程池核心数
- Picasso对在滑动状态的view不会去请求，只有当停止滑动的时候才会请求
- Glide绑定了activity的生命周期，当activity销毁则取消对应的请求和回收资源避免内存泄漏。
- Glide的默认图片格式是RGB_565，Picasso则是ARGB_8888，所以Picasso加载的图片缓存是Glide的2倍，但是清晰度比Glide高
- Picasso会默认加载url对应的全尺寸图片，Glide则会根据imageview的大小缓存对应尺寸的图片，所以在内存方面，Glide优于Picasso