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《Android开源库》Picasso

發(fā)布時(shí)間：2024/1/18 Android 40 豆豆

生活随笔收集整理的這篇文章主要介紹了《Android开源库》Picasso 小編覺(jué)得挺不錯(cuò)的,現(xiàn)在分享給大家,幫大家做個(gè)參考.

1. 前言

最近項(xiàng)目開(kāi)發(fā)中使用到來(lái)Picasso，并且碰到了一個(gè)部分圖片無(wú)法加載的問(wèn)題，使用Glide可以正常加載，使用最新的Picasso3.0.0-SNAPSHOT版本也沒(méi)有問(wèn)題，最后使用Picasso自帶的異常監(jiān)聽(tīng)獲取堆棧，并且在Github中找到的同樣的問(wèn)題，具體查閱：

https://github.com/square/picasso/issues/1514

所以，想詳細(xì)的看看Picasso的詳細(xì)用法和master分支上的源代碼。

2. 基本介紹

Picasso 是Square 公司開(kāi)源的Android 端的圖片加載和緩存框架。圖片的加載是一個(gè)令人很頭疼的問(wèn)題，因?yàn)樯婕暗膬?nèi)容比較多，比如網(wǎng)絡(luò)請(qǐng)求，本地存儲(chǔ)，圖片緩存，圖片回收等等，處理不好可能會(huì)出現(xiàn)圖片變形內(nèi)存泄漏等令開(kāi)發(fā)者焦頭爛額的問(wèn)題，但是Picasso幫我們做了很多事情，比如：

處理Adapter 中ImageView的回收和取消下載。
使用最小的內(nèi)存來(lái)做復(fù)雜的圖片變換。比如高斯模糊，圓角、圓形等處理。
自動(dòng)幫我們緩存圖片。內(nèi)存和磁盤緩存

3. 基本使用

以Android Studio為開(kāi)發(fā)環(huán)境，以Gradle為構(gòu)建工具，其他的本文不做介紹。

3.1 添加依賴

compile 'com.squareup.picasso:picasso:2.5.2'

3.2 混淆

-dontwarn com.squareup.okhttp.**

3.3 加載圖片

/* 網(wǎng)絡(luò)圖片 */ Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView);/* 圖片資源 */ Picasso.with(context).load(R.drawable.landing_screen).into(imageView1);/* 本地圖片*/ Picasso.with(context).load("file:///android_asset/DvpvklR.png").into(imageView2);/* 本地圖片文件 */ Picasso.with(context).load(new File(...)).into(imageView3);

3.3.1 Picasso.with(Context context)

/*** The global default {@link Picasso} instance.* * This instance is automatically initialized with defaults that are suitable to most* implementations.* <ul>* <li>LRU memory cache of 15% the available application RAM</li>* <li>Disk cache of 2% storage space up to 50MB but no less than 5MB. (Note: this is only* available on API 14+ or if you are using a standalone library that provides a disk* cache on all API levels like OkHttp)</li>* <li>Three download threads for disk and network access.</li>* </ul>* * If these settings do not meet the requirements of your application you can construct your own* with full control over the configuration by using {@link Picasso.Builder} to create a* {@link Picasso} instance. You can either use this directly or by setting it as the global* instance with {@link #setSingletonInstance}.*/public static Picasso with(Context context) {if (singleton == null) {synchronized (Picasso.class) {if (singleton == null) {singleton = new Builder(context).build();}}}return singleton;}

構(gòu)造一個(gè)默認(rèn)的Picasso單例，如果注釋中的一些基本配置需求無(wú)法滿足你，你可以使用Picasso.Builder來(lái)構(gòu)造自己的Picasso實(shí)例。

3.3.2 load()

很顯然，四個(gè)load方法，不同的入?yún)?#xff0c;相同類型的返回值。針對(duì)不同的圖片來(lái)源，執(zhí)行不同的加載方式。

/*** Start an image request using the specified URI.* * Passing {@code null} as a {@code uri} will not trigger any request but will set a placeholder,* if one is specified.** @see #load(File)* @see #load(String)* @see #load(int)*/public RequestCreator load(Uri uri) {return new RequestCreator(this, uri, 0);}/*** Start an image request using the specified path. This is a convenience method for calling* {@link #load(Uri)}.* * This path may be a remote URL, file resource (prefixed with {@code file:}), content resource* (prefixed with {@code content:}), or android resource (prefixed with {@code* android.resource:}.* * Passing {@code null} as a {@code path} will not trigger any request but will set a* placeholder, if one is specified.** @see #load(Uri)* @see #load(File)* @see #load(int)* @throws IllegalArgumentException if {@code path} is empty or blank string.*/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));}/*** Start an image request using the specified image file. This is a convenience method for* calling {@link #load(Uri)}.* * Passing {@code null} as a {@code file} will not trigger any request but will set a* placeholder, if one is specified.* * Equivalent to calling {@link #load(Uri) load(Uri.fromFile(file))}.** @see #load(Uri)* @see #load(String)* @see #load(int)*/public RequestCreator load(File file) {if (file == null) {return new RequestCreator(this, null, 0);}return load(Uri.fromFile(file));}/*** Start an image request using the specified drawable resource ID.** @see #load(Uri)* @see #load(String)* @see #load(File)*/public RequestCreator load(int resourceId) {if (resourceId == 0) {throw new IllegalArgumentException("Resource ID must not be zero.");}return new RequestCreator(this, null, resourceId);}

每一個(gè)load()方法都是創(chuàng)建一個(gè)RequestCreator實(shí)例，load(File file)和load(String path)最后都是通過(guò)轉(zhuǎn)換成load(Uri uri)的方式實(shí)現(xiàn)加載。而RequestCreator的構(gòu)造方法中三個(gè)參數(shù)，第一個(gè)是Picasso實(shí)例，一個(gè)是uri，一個(gè)是resourceId，通過(guò)后兩個(gè)參數(shù)覆蓋所有的加載場(chǎng)景。

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);}

3.3.3 into()

RequestCreator中很多方法都是我們經(jīng)常會(huì)用到的。先看into()方法

/*** Asynchronously fulfills the request into the specified {@link Target}. In most cases, you* should use this when you are dealing with a custom {@link android.view.View View} or view* holder which should implement the {@link Target} interface.* * Implementing on a {@link android.view.View View}:* <blockquote><pre>* public class ProfileView extends FrameLayout implements Target {* {@literal @}Override public void onBitmapLoaded(Bitmap bitmap, LoadedFrom from) {* setBackgroundDrawable(new BitmapDrawable(bitmap));* }** {@literal @}Override public void onBitmapFailed() {* setBackgroundResource(R.drawable.profile_error);* }** {@literal @}Override public void onPrepareLoad(Drawable placeHolderDrawable) {* frame.setBackgroundDrawable(placeHolderDrawable);* }* }* </pre></blockquote>* Implementing on a view holder object for use inside of an adapter:* <blockquote><pre>* public class ViewHolder implements Target {* public FrameLayout frame;* public TextView name;** {@literal @}Override public void onBitmapLoaded(Bitmap bitmap, LoadedFrom from) {* frame.setBackgroundDrawable(new BitmapDrawable(bitmap));* }** {@literal @}Override public void onBitmapFailed() {* frame.setBackgroundResource(R.drawable.profile_error);* }** {@literal @}Override public void onPrepareLoad(Drawable placeHolderDrawable) {* frame.setBackgroundDrawable(placeHolderDrawable);* }* }* </pre></blockquote>* * Note: This method keeps a weak reference to the {@link Target} instance and will be* garbage collected if you do not keep a strong reference to it. To receive callbacks when an* image is loaded use {@link #into(android.widget.ImageView, Callback)}.*/public void into(Target target) {long started = System.nanoTime();checkMain();if (target == null) {throw new IllegalArgumentException("Target must not be null.");}if (deferred) {throw new IllegalStateException("Fit cannot be used with a Target.");}if (!data.hasImage()) {picasso.cancelRequest(target);target.onPrepareLoad(setPlaceholder ? getPlaceholderDrawable() : null);return;}Request request = createRequest(started);String requestKey = createKey(request);if (shouldReadFromMemoryCache(memoryPolicy)) {Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);if (bitmap != null) {picasso.cancelRequest(target);target.onBitmapLoaded(bitmap, MEMORY);return;}}target.onPrepareLoad(setPlaceholder ? getPlaceholderDrawable() : null);Action action =new TargetAction(picasso, target, request, memoryPolicy, networkPolicy, errorDrawable,requestKey, tag, errorResId);picasso.enqueueAndSubmit(action);}/*** Asynchronously fulfills the request into the specified {@link RemoteViews} object with the* given {@code viewId}. This is used for loading bitmaps into a {@link Notification}.*/public void into(RemoteViews remoteViews, int viewId, int notificationId,Notification notification) {long started = System.nanoTime();if (remoteViews == null) {throw new IllegalArgumentException("RemoteViews must not be null.");}if (notification == null) {throw new IllegalArgumentException("Notification must not be null.");}if (deferred) {throw new IllegalStateException("Fit cannot be used with RemoteViews.");}if (placeholderDrawable != null || placeholderResId != 0 || errorDrawable != null) {throw new IllegalArgumentException("Cannot use placeholder or error drawables with remote views.");}Request request = createRequest(started);String key = createKey(request, new StringBuilder()); // Non-main thread needs own builder.RemoteViewsAction action =new NotificationAction(picasso, request, remoteViews, viewId, notificationId, notification,memoryPolicy, networkPolicy, key, tag, errorResId);performRemoteViewInto(action);}/*** Asynchronously fulfills the request into the specified {@link RemoteViews} object with the* given {@code viewId}. This is used for loading bitmaps into all instances of a widget.*/public void into(RemoteViews remoteViews, int viewId, int[] appWidgetIds) {long started = System.nanoTime();if (remoteViews == null) {throw new IllegalArgumentException("remoteViews must not be null.");}if (appWidgetIds == null) {throw new IllegalArgumentException("appWidgetIds must not be null.");}if (deferred) {throw new IllegalStateException("Fit cannot be used with remote views.");}if (placeholderDrawable != null || placeholderResId != 0 || errorDrawable != null) {throw new IllegalArgumentException("Cannot use placeholder or error drawables with remote views.");}Request request = createRequest(started);String key = createKey(request, new StringBuilder()); // Non-main thread needs own builder.RemoteViewsAction action =new AppWidgetAction(picasso, request, remoteViews, viewId, appWidgetIds, memoryPolicy,networkPolicy, key, tag, errorResId);performRemoteViewInto(action);}/*** Asynchronously fulfills the request into the specified {@link ImageView}.* * Note: 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}.* * Note: 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 strongly 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();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);}

可以看到5個(gè)into()方法，可以看到Picasso常用的是直接往ImageView，但是它的功能遠(yuǎn)遠(yuǎn)不止如此，我們可以通過(guò)實(shí)現(xiàn)Target接口來(lái)實(shí)現(xiàn)自定義的圖片處理，可以用來(lái)設(shè)背景，可以用來(lái)填充布局，可擴(kuò)展性很強(qiáng)，也支持RemoteView。而我們常用的

public void into(ImageView target) {into(target, null);}

轉(zhuǎn)到有Callback的執(zhí)行方法。我們可以通過(guò)使用帶有回調(diào)的into方法來(lái)執(zhí)行圖片加載成功或者失敗后的事件處理。

public interface Callback {void onSuccess();void onError();public static class EmptyCallback implements Callback {@Override public void onSuccess() {}@Override public void onError() {}} }

看下public void into(ImageView target, Callback callback)這個(gè)方法：

checkMain() 判斷是否為主進(jìn)程

非空檢查。判斷target是否為空，uri 或者 resourceId是否為空；

deferred是否為true。這個(gè)和是否調(diào)用fit方法有關(guān)。由于fit方法是讓圖片適應(yīng)ImageView的大小，所以需要ImageView大小明確之后才執(zhí)行請(qǐng)求，所以需要延遲執(zhí)行。

創(chuàng)建Request和requestKey

是否在Cache查找該圖片，通過(guò)requestKey，這個(gè)Key的構(gòu)建過(guò)程和很多內(nèi)容有關(guān)系。

設(shè)置占位圖片

構(gòu)建Action并提交

以上就是圖片加載的主要過(guò)程。

4. 進(jìn)階用法(鏈?zhǔn)绞褂?

主要介紹一下Picasso的其他用法

4.1 noPlaceholder()

不設(shè)置占位圖片，不能于placeHolder同時(shí)使用

/*** Explicitly opt-out to having a placeholder set when calling {@code into}.* * By default, Picasso will either set a supplied placeholder or clear the target* {@link ImageView} in order to ensure behavior in situations where views are recycled. This* method will prevent that behavior and retain any already set image.*/public RequestCreator noPlaceholder() {if (placeholderResId != 0) {throw new IllegalStateException("Placeholder resource already set.");}if (placeholderDrawable != null) {throw new IllegalStateException("Placeholder image already set.");}setPlaceholder = false;return this;}

4.2 placeholder()

設(shè)置占位圖片

/*** A placeholder drawable to be used while the image is being loaded. If the requested image is* not immediately available in the memory cache then this resource will be set on the target* {@link ImageView}.*/public RequestCreator placeholder(int placeholderResId) {if (!setPlaceholder) {throw new IllegalStateException("Already explicitly declared as no placeholder.");}if (placeholderResId == 0) {throw new IllegalArgumentException("Placeholder image resource invalid.");}if (placeholderDrawable != null) {throw new IllegalStateException("Placeholder image already set.");}this.placeholderResId = placeholderResId;return this;}

4.3 error()

設(shè)置加載出錯(cuò)時(shí)的圖片

/** An error drawable to be used if the request image could not be loaded. */public RequestCreator error(int errorResId) {if (errorResId == 0) {throw new IllegalArgumentException("Error image resource invalid.");}if (errorDrawable != null) {throw new IllegalStateException("Error image already set.");}this.errorResId = errorResId;return this;}/** An error drawable to be used if the request image could not be loaded. */public RequestCreator error(Drawable errorDrawable) {if (errorDrawable == null) {throw new IllegalArgumentException("Error image may not be null.");}if (errorResId != 0) {throw new IllegalStateException("Error image already set.");}this.errorDrawable = errorDrawable;return this;}

4.4 tag()

設(shè)置tag，方便后續(xù)pause，cancel，或者resume。這個(gè)應(yīng)該用的比較少，有可能通過(guò)tag來(lái)cancel一下。

/*** Assign a tag to this request. Tags are an easy way to logically associate* related requests that can be managed together e.g. paused, resumed,* or canceled.* * You can either use simple {@link String} tags or objects that naturally* define the scope of your requests within your app such as a* {@link android.content.Context}, an {@link android.app.Activity}, or a* {@link android.app.Fragment}.** WARNING:: Picasso will keep a reference to the tag for* as long as this tag is paused and/or has active requests. Look out for* potential leaks.** @see Picasso#cancelTag(Object)* @see Picasso#pauseTag(Object)* @see Picasso#resumeTag(Object)*/public RequestCreator tag(Object tag) {if (tag == null) {throw new IllegalArgumentException("Tag invalid.");}if (this.tag != null) {throw new IllegalStateException("Tag already set.");}this.tag = tag;return this;}

4.5 fit() & unfit()

調(diào)整圖片大小來(lái)適配target的邊界，這個(gè)會(huì)導(dǎo)致圖片的加載過(guò)程在ImageView布局確定之后進(jìn)行。

/*** Attempt to resize the image to fit exactly into the target {@link ImageView}'s bounds. This* will result in delayed execution of the request until the {@link ImageView} has been laid out.* * Note: This method works only when your target is an {@link ImageView}.*/public RequestCreator fit() {deferred = true;return this;}/** Internal use only. Used by {@link DeferredRequestCreator}. */RequestCreator unfit() {deferred = false;return this;}

4.6 resizeDimen() & resize()

調(diào)整圖片大小。

/** Resize the image to the specified dimension size. */public RequestCreator resizeDimen(int targetWidthResId, int targetHeightResId) {Resources resources = picasso.context.getResources();int targetWidth = resources.getDimensionPixelSize(targetWidthResId);int targetHeight = resources.getDimensionPixelSize(targetHeightResId);return resize(targetWidth, targetHeight);}/** Resize the image to the specified size in pixels. */public RequestCreator resize(int targetWidth, int targetHeight) {data.resize(targetWidth, targetHeight);return this;}

4.7 centerCrop() & centerInside()

centerCrop：充滿ImageView居中裁剪
centerInside：完整顯示圖片但是可能無(wú)法充滿ImageView

/*** Crops an image inside of the bounds specified by {@link #resize(int, int)} rather than* distorting the aspect ratio. This cropping technique scales the image so that it fills the* requested bounds and then crops the extra.*/public RequestCreator centerCrop() {data.centerCrop();return this;}/*** Centers an image inside of the bounds specified by {@link #resize(int, int)}. This scales* the image so that both dimensions are equal to or less than the requested bounds.*/public RequestCreator centerInside() {data.centerInside();return this;}

4.8 onlyScaleDown()

當(dāng)調(diào)用了resize 方法重新設(shè)置圖片尺寸的時(shí)候，調(diào)用onlyScaleDown 方法，只有當(dāng)原始圖片的尺寸大于我們指定的尺寸時(shí)，resize才起作用。

/*** Only resize an image if the original image size is bigger than the target size* specified by {@link #resize(int, int)}.*/public RequestCreator onlyScaleDown() {data.onlyScaleDown();return this;}

4.9 rotate()

旋轉(zhuǎn)圖片角度，可指定旋轉(zhuǎn)中心點(diǎn)

/** Rotate the image by the specified degrees. */public RequestCreator rotate(float degrees) {data.rotate(degrees);return this;}/** Rotate the image by the specified degrees around a pivot point. */public RequestCreator rotate(float degrees, float pivotX, float pivotY) {data.rotate(degrees, pivotX, pivotY);return this;}

4.10 config()

設(shè)置圖片Bitmap格式，如
ALPHA_8，ARGB_4444，ARGB_8888，HARDWARE，RGBA_F16，RGB_565等等。

/*** Attempt to decode the image using the specified config.* * Note: This value may be ignored by {@link BitmapFactory}. See* {@link BitmapFactory.Options#inPreferredConfig its documentation} for more details.*/public RequestCreator config(Bitmap.Config config) {data.config(config);return this;}

4.11 stableKey()

設(shè)置固定Key

/*** Sets the stable key for this request to be used instead of the URI or resource ID when* caching. Two requests with the same value are considered to be for the same resource.*/public RequestCreator stableKey(String stableKey) {data.stableKey(stableKey);return this;}

4.12 priority()

設(shè)置優(yōu)先級(jí)，這個(gè)對(duì)圖片請(qǐng)求的執(zhí)行順序有影響，默認(rèn)的優(yōu)先級(jí)均為NORMAL

/*** Set the priority of this request.* * This will affect the order in which the requests execute but does not guarantee it.* By default, all requests have {@link Priority#NORMAL} priority, except for* {@link #fetch()} requests, which have {@link Priority#LOW} priority by default.*/public RequestCreator priority(Priority priority) {data.priority(priority);return this;}

4.13 transform()

添加自定義Transformation，方便執(zhí)行圖形轉(zhuǎn)換。
更多transformation請(qǐng)移步picasso-transformations

/*** Add a custom transformation to be applied to the image.* * Custom transformations will always be run after the built-in transformations.*/// TODO show example of calling resize after a transform in the javadocpublic RequestCreator transform(Transformation transformation) {data.transform(transformation);return this;}/*** Add a list of custom transformations to be applied to the image.* * Custom transformations will always be run after the built-in transformations.*/public RequestCreator transform(List<? extends Transformation> transformations) {data.transform(transformations);return this;}

4.14 memoryPolicy()

指定memoryPolicy。NO_CACHE，NO_STORE
NO_CACHE：表示處理請(qǐng)求的時(shí)候跳過(guò)檢查內(nèi)存緩存
NO_STORE: 表示請(qǐng)求成功之后，結(jié)果不存到內(nèi)存。

/*** Specifies the {@link MemoryPolicy} to use for this request. You may specify additional policy* options using the varargs parameter.*/public RequestCreator memoryPolicy(MemoryPolicy policy, MemoryPolicy... additional) {if (policy == null) {throw new IllegalArgumentException("Memory policy cannot be null.");}this.memoryPolicy |= policy.index;if (additional == null) {throw new IllegalArgumentException("Memory policy cannot be null.");}if (additional.length > 0) {for (MemoryPolicy memoryPolicy : additional) {if (memoryPolicy == null) {throw new IllegalArgumentException("Memory policy cannot be null.");}this.memoryPolicy |= memoryPolicy.index;}}return this;}

4.15 networkPolicy()

指定NetworkPolicy。NO_CACHE，NO_STORE，OFFLINE
NO_CACHE: 表示處理請(qǐng)求的時(shí)候跳過(guò)處理磁盤緩存
NO_STORE: 表示請(qǐng)求成功后，結(jié)果不緩存到Disk。
OFFLINE: 強(qiáng)制這次請(qǐng)求從緩存中獲取結(jié)果，不會(huì)發(fā)起網(wǎng)絡(luò)請(qǐng)求，不管緩存中能否獲取到結(jié)果。

/*** Specifies the {@link NetworkPolicy} to use for this request. You may specify additional policy* options using the varargs parameter.*/public RequestCreator networkPolicy(NetworkPolicy policy, NetworkPolicy... additional) {if (policy == null) {throw new IllegalArgumentException("Network policy cannot be null.");}this.networkPolicy |= policy.index;if (additional == null) {throw new IllegalArgumentException("Network policy cannot be null.");}if (additional.length > 0) {for (NetworkPolicy networkPolicy : additional) {if (networkPolicy == null) {throw new IllegalArgumentException("Network policy cannot be null.");}this.networkPolicy |= networkPolicy.index;}}return this;}

4.16 noFade()

去掉默認(rèn)的圖片加載過(guò)程中的漸入效果

/** Disable brief fade in of images loaded from the disk cache or network. */public RequestCreator noFade() {noFade = true;return this;}

5. 其他用法

5.1 get()

同步方法，獲取Bitmap。所以不要在主線程執(zhí)行此操作
用法：

try {Bitmap bitmap = Picasso.with(this).load(URL).get();} catch (IOException e) {e.printStackTrace();}

源碼實(shí)現(xiàn)：

/*** Synchronously fulfill this request. Must not be called from the main thread.* * Note: The result of this operation is not cached in memory because the underlying* {@link Cache} implementation is not guaranteed to be thread-safe.*/public Bitmap get() throws IOException {long started = System.nanoTime();checkNotMain();if (deferred) {throw new IllegalStateException("Fit cannot be used with get.");}if (!data.hasImage()) {return null;}Request finalData = createRequest(started);String key = createKey(finalData, new StringBuilder());Action action = new GetAction(picasso, finalData, memoryPolicy, networkPolicy, tag, key);return forRequest(picasso, picasso.dispatcher, picasso.cache, picasso.stats, action).hunt();}

5.2 fetch()

毫無(wú)結(jié)果的獲取圖片，也沒(méi)有目標(biāo)，也沒(méi)有返回bitmap，難道只是為了暖場(chǎng)？考慮到Picasso有緩存機(jī)制，這個(gè)方法還是有一點(diǎn)價(jià)值的，提前加載放到緩存，后面加載速度會(huì)更快。

/*** Asynchronously fulfills the request without a {@link ImageView} or {@link Target},* and invokes the target {@link Callback} with the result. This is useful when you want to warm* up the cache with an image.* * Note: 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* until the request is completed.*/public void fetch(Callback callback) {long started = System.nanoTime();if (deferred) {throw new IllegalStateException("Fit cannot be used with fetch.");}if (data.hasImage()) {// Fetch requests have lower priority by default.if (!data.hasPriority()) {data.priority(Priority.LOW);}Request request = createRequest(started);String key = createKey(request, new StringBuilder());Bitmap bitmap = picasso.quickMemoryCacheCheck(key);if (bitmap != null) {if (picasso.loggingEnabled) {log(OWNER_MAIN, VERB_COMPLETED, request.plainId(), "from " + MEMORY);}if (callback != null) {callback.onSuccess();}} else {Action action =new FetchAction(picasso, request, memoryPolicy, networkPolicy, tag, key, callback);picasso.submit(action);}}}

6. 自定義Picasso

默認(rèn)使用Picasso.with(context)可以快速的構(gòu)建Picasso實(shí)例，但是Picasso支持?jǐn)U展自定義Picasso，也就是一些默認(rèn)的配置，我們可以自己來(lái)調(diào)控。

可以看到Picasso.Builder方法存在很多可配置的內(nèi)容，downloader, executor, memoryCache,listener等等都是可以自己設(shè)置的，配置完成后調(diào)用
builder()方法即可返回Picasso實(shí)例。
還可以通過(guò)如下方法設(shè)置全局Picasso實(shí)例。

/*** Set the global instance returned from {@link #with}.* * This method must be called before any calls to {@link #with} and may only be called once.*/public static void setSingletonInstance(Picasso picasso) {synchronized (Picasso.class) {if (singleton != null) {throw new IllegalStateException("Singleton instance already exists.");}singleton = picasso;}}

7. 類圖

請(qǐng)適當(dāng)放大后查看。

8. 主要流程圖