Picasso学习记录

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2017-10-08
原文链接:blog.csdn.net

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:

github.com/square/pica…

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()

这里写图片描述
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
这里写图片描述

  /**
   * 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

这里写图片描述

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。
这里写图片描述

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;
    }
  }
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