1. 前言
最近项目开发中使用到来Picasso,并且碰到了一个部分图片无法加载的问题,使用Glide可以正常加载,使用最新的Picasso3.0.0-SNAPSHOT版本也没有问题,最后使用Picasso自带的异常监听获取堆栈,并且在Github中找到的同样的问题,具体查阅:
所以,想详细的看看Picasso的详细用法和master分支上的源代码。
2. 基本介绍
Picasso 是Square 公司开源的Android 端的图片加载和缓存框架。图片的加载是一个令人很头疼的问题,因为涉及的内容比较多,比如网络请求,本地存储,图片缓存,图片回收等等,处理不好可能会出现图片变形内存泄漏等令开发者焦头烂额的问题,但是Picasso帮我们做了很多事情,比如:
- 处理Adapter 中ImageView的回收和取消下载。
- 使用最小的内存 来做复杂的图片变换。比如高斯模糊,圆角、圆形等处理。
- 自动帮我们缓存图片。内存和磁盘缓存
3. 基本使用
以Android Studio为开发环境,以Gradle为构建工具,其他的本文不做介绍。
3.1 添加依赖
compile 'com.squareup.picasso:picasso:2.5.2'
3.2 混淆
-dontwarn com.squareup.okhttp.**
3.3 加载图片
/* 网络图片 */
Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView);
/* 图片资源 */
Picasso.with(context).load(R.drawable.landing_screen).into(imageView1);
/* 本地图片*/
Picasso.with(context).load("file:///android_asset/DvpvklR.png").into(imageView2);
/* 本地图片文件 */
Picasso.with(context).load(new File(...)).into(imageView3);
3.3.1 Picasso.with(Context context)
/**
* The global default {@link Picasso} instance.
* <p>
* 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+ <em>or</em> 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>
* <p>
* 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;
}
构造一个默认的Picasso单例,如果注释中的一些基本配置需求无法满足你,你可以使用Picasso.Builder来构造自己的Picasso实例。
3.3.2 load()
很显然,四个load方法,不同的入参,相同类型的返回值。针对不同的图片来源,执行不同的加载方式。
/**
* Start an image request using the specified URI.
* <p>
* 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)}.
* <p>
* 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:}.
* <p>
* 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)}.
* <p>
* Passing {@code null} as a {@code file} will not trigger any request but will set a
* placeholder, if one is specified.
* <p>
* 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);
}
每一个load()方法都是创建一个RequestCreator实例,load(File file)和load(String path)最后都是通过转换成load(Uri uri)的方式实现加载。而RequestCreator的构造方法中三个参数,第一个是Picasso实例,一个是uri,一个是resourceId,通过后两个参数覆盖所有的加载场景。
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中很多方法都是我们经常会用到的。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.
* <p>
* 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>
* <p>
* <em>Note:</em> 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}.
* <p>
* <em>Note:</em> This method keeps a weak reference to the {@link ImageView} instance and will
* automatically support object recycling.
*/
public void into(ImageView target) {
into(target, null);
}
/**
* Asynchronously fulfills the request into the specified {@link ImageView} and invokes the
* target {@link Callback} if it's not {@code null}.
* <p>
* <em>Note:</em> The {@link Callback} param is a strong reference and will prevent your
* {@link android.app.Activity} or {@link android.app.Fragment} from being garbage collected. If
* you use this method, it is <b>strongly</b> recommended you invoke an adjacent
* {@link Picasso#cancelRequest(android.widget.ImageView)} call to prevent temporary leaking.
*/
public void into(ImageView target, Callback callback) {
long started = System.nanoTime();
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个into()方法,可以看到Picasso常用的是直接往ImageView,但是它的功能远远不止如此,我们可以通过实现Target接口来实现自定义的图片处理,可以用来设背景,可以用来填充布局,可扩展性很强,也支持RemoteView。而我们常用的
public void into(ImageView target) {
into(target, null);
}
转到有Callback的执行方法。我们可以通过使用带有回调的into方法来执行图片加载成功或者失败后的事件处理。
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)这个方法:
1. checkMain() 判断是否为主进程
2. 非空检查。判断target是否为空,uri 或者 resourceId是否为空;
3. deferred是否为true。这个和是否调用fit方法有关。由于fit方法是让图片适应ImageView的大小,所以需要ImageView大小明确之后才执行请求,所以需要延迟执行。
4. 创建Request和requestKey
5. 是否在Cache查找该图片,通过requestKey,这个Key的构建过程和很多内容有关系。
6. 设置占位图片
7. 构建Action并提交
以上就是图片加载的主要过程。
4. 进阶用法(链式使用)
主要介绍一下Picasso的其他用法
4.1 noPlaceholder()
不设置占位图片,不能于placeHolder同时使用
/**
* Explicitly opt-out to having a placeholder set when calling {@code into}.
* <p>
* 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()
设置占位图片
/**
* 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()
设置加载出错时的图片
/** 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()
设置tag,方便后续pause,cancel,或者resume。这个应该用的比较少,有可能通过tag来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.
* <p>
* 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}.
*
* <strong>WARNING:</strong>: 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()
调整图片大小来适配target的边界,这个会导致图片的加载过程在ImageView布局确定之后进行。
/**
* 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.
* <p>
* <em>Note:</em> 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()
调整图片大小。
/** 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:充满ImageView居中裁剪 centerInside: 完整显示图片但是可能无法充满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()
当调用了resize 方法重新设置图片尺寸的时候,调用onlyScaleDown 方法,只有当原始图片的尺寸大于我们指定的尺寸时,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()
旋转图片角度,可指定旋转中心点
/** 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()
设置图片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.
* <p>
* 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()
设置固定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()
设置优先级,这个对图片请求的执行顺序有影响,默认的优先级均为NORMAL
/**
* Set the priority of this request.
* <p>
* 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()
添加自定义Transformation,方便执行图形转换。
更多transformation请移步picasso-transformations
/**
* Add a custom transformation to be applied to the image.
* <p>
* Custom transformations will always be run after the built-in transformations.
*/
// TODO show example of calling resize after a transform in the javadoc
public RequestCreator transform(Transformation transformation) {
data.transform(transformation);
return this;
}
/**
* Add a list of custom transformations to be applied to the image.
* <p>
* 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:表示处理请求的时候跳过检查内存缓存
NO_STORE: 表示请求成功之后,结果不存到内存。
/**
* 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: 表示处理请求的时候跳过处理磁盘缓存
NO_STORE: 表示请求成功后,结果不缓存到Disk。
OFFLINE: 强制这次请求从缓存中获取结果,不会发起网络请求,不管缓存中能否获取到结果。
/**
* 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()
去掉默认的图片加载过程中的渐入效果
/** 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,获取Bitmap。所以不要在主线程执行此操作 用法:
try {
Bitmap bitmap = Picasso.with(this).load(URL).get();
} catch (IOException e) {
e.printStackTrace();
}
源码实现:
/**
* Synchronously fulfill this request. Must not be called from the main thread.
* <p>
* <em>Note</em>: 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()
毫无结果的获取图片,也没有目标,也没有返回bitmap,难道只是为了暖场?考虑到Picasso有缓存机制,这个方法还是有一点价值的,提前加载放到缓存,后面加载速度会更快。
/**
* 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.
* <p>
* <em>Note:</em> The {@link Callback} param is a strong reference and will prevent your
* {@link android.app.Activity} or {@link android.app.Fragment} from being garbage collected
* 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. 自定义Picasso
默认使用Picasso.with(context)可以快速的构建Picasso实例,但是Picasso支持扩展自定义Picasso,也就是一些默认的配置,我们可以自己来调控。
可以看到Picasso.Builder方法存在很多可配置的内容,downloader, executor, memoryCache,listener等等都是可以自己设置的,配置完成后调用
builder()方法即可返回Picasso实例。
还可以通过如下方法设置全局Picasso实例。
/**
* Set the global instance returned from {@link #with}.
* <p>
* 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;
}
}