Picasso 图片下载缓存库

阳光穿透心脏的1/2处 2022-06-09 07:51 203阅读 0赞

转：[http://blog.csdn.net/theone10211024/article/details/45604373][http_blog.csdn.net_theone10211024_article_details_45604373]

对于Bitmap的对象池，重用对象，通常使用WeakReference来保存要引用的Bitmap对象，这里需要加一个引用队列，在发生GC时，可以对Bitmap做recycle处理。

[https://www.jianshu.com/p/f86d3a43eec5][https_www.jianshu.com_p_f86d3a43eec5]

[https://blog.csdn.net/gdutxiaoxu/article/details/80738581][https_blog.csdn.net_gdutxiaoxu_article_details_80738581]

在[Android][]应用中，图片消费了大量的资源，却为应用提供了很好的视觉体验。幸运的是，Picasso为你的应用提供了非常容易的图片加载方式——通常一行代码就可以搞定！

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1.  Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView);

Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView);

Picasso处理了[android][Android]上图片加载的许多坑：

1）在Adapter中，处理了ImageView的循环利用和取消下载。

2）耗费最小的内存处理了复杂的图形变换。

3）在内存和磁盘中自动缓存图片。

![Center][]

## 特点 ##

### Adapter中的下载 ###

自动检测adapter中的重用功能，且一旦发现重用，将自动取消之前的下载。

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1.  @Override public void getView(int position, View convertView, ViewGroup parent) \{
2.  SquaredImageView view = (SquaredImageView) convertView;
3.  if (view == null) \{
4.  view = new SquaredImageView(context);
5.  \}
6.  String url = getItem(position);
7.  
8.  Picasso.with(context).load(url).into(view);
9.  \}

@Override public void getView(int position, View convertView, ViewGroup parent) {
      SquaredImageView view = (SquaredImageView) convertView;
      if (view == null) {
        view = new SquaredImageView(context);
      }
      String url = getItem(position);
    
      Picasso.with(context).load(url).into(view);
    }

### 图片转换 ###

变换图片以便更好的适应布局，同时也减少了内存的使用。

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1.  Picasso.with(context)
2.  .load(url)
3.  .resize(50, 50)
4.  .centerCrop()
5.  .into(imageView)

Picasso.with(context)
      .load(url)
      .resize(50, 50)
      .centerCrop()
      .into(imageView)

你也可以自定义图片的转换方式以达到更复杂的变换要求。

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1.  public class CropSquareTransformation implements Transformation \{
2.  @Override public Bitmap transform(Bitmapsource) \{
3.  int size = Math.min(source.getWidth(),source.getHeight());
4.  int x = (source.getWidth() - size) / 2;
5.  int y = (source.getHeight() - size) / 2;
6.  Bitmap result = Bitmap.createBitmap(source,x, y, size, size);
7.  if (result != source) \{
8.  source.recycle();
9.  \}
10. return result;
11. \}
12. 
13. @Override public String key() \{ return"square()"; \}
14. \}

public class CropSquareTransformation implements Transformation {
      @Override public Bitmap transform(Bitmapsource) {
        int size = Math.min(source.getWidth(),source.getHeight());
        int x = (source.getWidth() - size) / 2;
        int y = (source.getHeight() - size) / 2;
        Bitmap result = Bitmap.createBitmap(source,x, y, size, size);
        if (result != source) {
          source.recycle();
        }
        return result;
      }
     
      @Override public String key() { return"square()"; }
    }

将**CropSquareTransformation**这个类的一个实例传递给transform()函数即可。

### 占位图片 ###

Picasso同时支持“正在下载”时和“图片下载出错”后这两种状态展示的默认图片。

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1.  Picasso.with(context)
2.  .load(url)
3.  .placeholder(R.drawable.user\_placeholder)
4.  .error(R.drawable.user\_placeholder\_error)
5.  .into(imageView);

Picasso.with(context)
        .load(url)
        .placeholder(R.drawable.user_placeholder)
        .error(R.drawable.user_placeholder_error)
    .into(imageView);

Picasso至多会尝试三次下载，如果三次下载都失败了，就会在原图位置上展示“图片下载出错”时的图片。

### 资源加载 ###

Picasso支持将resources、assets、files和contentprovider作为图片的加载源。

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1.  Picasso.with(context).load(R.drawable.landing\_screen).into(imageView1);
2.  Picasso.with(context).load("file:///android\_asset/DvpvklR.png").into(imageView2);
3.  Picasso.with(context).load(newFile(...)).into(imageView3);

Picasso.with(context).load(R.drawable.landing_screen).into(imageView1);
    Picasso.with(context).load("file:///android_asset/DvpvklR.png").into(imageView2);
    Picasso.with(context).load(newFile(...)).into(imageView3);

### 调试指示器 ###

开发者可以在图片的左上角展示一个彩色的小三角，不同的颜色指明了图片资源的不同来源。调用Picasso对象的setIndicatorsEnabled(true)方法就可以了。

![Center 1][]

## 下载 ##

[http://repo1.maven.org/maven2/com/squareup/picasso/picasso/2.5.2/picasso-2.5.2.jar][http_repo1.maven.org_maven2_com_squareup_picasso_picasso_2.5.2_picasso-2.5.2.jar]

Picasso的源代码，例子和这个网址（http://square.github.io/picasso/）都放在了[GitHub][]上

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1.  MAVEN
2.  
3.  <dependency>
4.  <groupId>com.squareup.picasso</groupId>
5.  <artifactId>picasso</artifactId>
6.  <version>2.5.2</version>
7.  </dependency>
8.  
9.  
10. GRADLE
11. 
12. compile 'com.squareup.picasso:picasso:2.5.2'

MAVEN
    
    <dependency>
      <groupId>com.squareup.picasso</groupId>
      <artifactId>picasso</artifactId>
      <version>2.5.2</version>
    </dependency>
    
    
    GRADLE
    
    compile 'com.squareup.picasso:picasso:2.5.2'

## 为Picasso做贡献 ##

如果你想为Picasso开发贡献你的代码，你可以上github，然后fork该代码库，再把你的修改发给我们（pull request）。

在提交代码的时候，请保持代码的习惯和风格与原来的一致以便于尽可能保持代码的可阅读性。同时，为了保证你的代码正确编译，请运行mvn clean verify。

你需要同意[Individual ContributorLicense Agreement (CLA)][Individual ContributorLicense Agreement _CLA]才能将你的代码合并到Picasso工程中。

## 重要资料 ##

1\]Javadoc : [ http://square.github.io/picasso/javadoc/index.html][http_square.github.io_picasso_javadoc_index.html]

\[2\]StackOverflow:[http://stackoverflow.com/questions/tagged/picasso?sort=active][http_stackoverflow.com_questions_tagged_picasso_sort_active]

## 许可证 ##

Licensed under the Apache License, Version2.0 (the "License");

you may not use this file except incompliance with the License.

You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreedto in writing, software

distributed under the License isdistributed on an "AS IS" BASIS,

WITHOUT WARRANTIES OR CONDITIONS OF ANYKIND, either express or implied.

See the License for the specific languagegoverning permissions and

limitations under the License.

原文链接：[http://square.github.io/picasso/][http_square.github.io_picasso]

[http_blog.csdn.net_theone10211024_article_details_45604373]: http://blog.csdn.net/theone10211024/article/details/45604373
[https_www.jianshu.com_p_f86d3a43eec5]: https://www.jianshu.com/p/f86d3a43eec5
[https_blog.csdn.net_gdutxiaoxu_article_details_80738581]: https://blog.csdn.net/gdutxiaoxu/article/details/80738581
[Android]: http://lib.csdn.net/base/android
[view plain]: http://blog.csdn.net/theone10211024/article/details/45604373#
[Center]: /images/20220609/af985d572bc549cdb9b2296360427f09.png
[Center 1]: /images/20220609/de204373ed0b4ceaa9434ac0b821fd6c.png
[http_repo1.maven.org_maven2_com_squareup_picasso_picasso_2.5.2_picasso-2.5.2.jar]: http://repo1.maven.org/maven2/com/squareup/picasso/picasso/2.5.2/picasso-2.5.2.jar
[GitHub]: http://github.com/square/picasso
[Individual ContributorLicense Agreement _CLA]: http://squ.re/sign-the-cla
[http_square.github.io_picasso_javadoc_index.html]: http://square.github.io/picasso/javadoc/index.html
[http_stackoverflow.com_questions_tagged_picasso_sort_active]: http://stackoverflow.com/questions/tagged/picasso?sort=active
[http_square.github.io_picasso]: http://square.github.io/picasso/