Android Picasso的基本使用

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Picasso是Android中强大的图像下载和缓存库。

Picasso Introduction（介绍）：

在Android应用程序中图片的添加有必须的环境和良好的视觉风格，Picasso可以很轻松的加载图片在你的应用程序中-通常在一行代码中。

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

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

在Android中图片加载大多数常见的陷阱是通过Picasso自动处理的。
1.处理图片回收和下载取消在Adapter中。
2.复杂的图像转换以最小的内存使用。
3.自动存储和磁盘缓存。

Picass Features（特点）：

1.ADAPTER DOWNLOADS（适配器中加载）：

适配器自动发现和重用以前的取消下载。

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@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);
}

@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);}

2.IMAGE TRANSFORMATIONS（图片转换）：

转换图片以更好地适应到布局和减少内存的大小。

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

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

您还可以指定为更高级的效果自定义转换。

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public class CropSquareTransformation implements Transformation {
@Override public Bitmap transform(Bitmap source) {
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()”; }
}

public class CropSquareTransformation implements Transformation {  @Override public Bitmap transform(Bitmap source) {    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()"; }}

该类的实例传递给transform方法。

3.PLACE HOLDERS（占位符）

Picasso同时支持下载和错误占位符的可选功能。

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

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

一个要求请求将重试三次之后错误的占位符被显示。

4.RESOURCE LOADING（资源加载）

Resources, assets, files, content providers都支持作为图像源。

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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(new File(…)).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(new File(...)).into(imageView3);

5.DEBUG INDICATORS（）

对于开发可以启用彩色指示的图像源的显示，在Picasso实例上调用setIndicatorsEnabled(true)。

红色：代表从网络中获取。
蓝色：代表从磁盘中获取。
绿色：代表从缓存中获取。

6.Download（下载）

Picasso的源码在GitHub中：https://github.com/square/picasso

引入

MAVEN：

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<dependency>
<groupId>com.squareup.picasso</groupId>
<artifactId>picasso</artifactId>
<version>2.5.2</version>
</dependency>

<dependency>  <groupId>com.squareup.picasso</groupId>  <artifactId>picasso</artifactId>  <version>2.5.2</version></dependency>

GRADLE

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compile ‘com.squareup.picasso:picasso:2.5.2’

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

使用例子

根据前面的介绍，简单的加载一张图片很简单，那就写一个用ListView加载图片的情况吧。

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public class MainActivity extends AppCompatActivity {
String urlPicasso = ”http://img1.imgtn.bdimg.com/it/u=998216620,3976144567&fm=21&gp=0.jpg”;
String urlGlide = ”http://h.hiphotos.baidu.com/image/h%3D200/sign=cd65e7fa13d5ad6eb5f963eab1cb39a3/377adab44aed2e7394aa5a128f01a18b87d6fa49.jpg”;
String urlVideo = ”http://www.iqiyi.com/v_19rrm83ayw.html”;
ListView mListView;
MyAdapter mAdpater;
List<String> mData = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mListView = (ListView) findViewById(R.id.lv);
for (int i = 0; i < 20; i++) {
mData.add(urlGlide);
}
mAdpater = new MyAdapter();
mListView.setAdapter(mAdpater);
}
class MyAdapter extends BaseAdapter {
@Override
public int getCount() {
return mData.size();
}
@Override
public Object getItem(int position) {
return mData.get(position);
}
@Override
public long getItemId(int position) {
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder = null;
if (convertView == null) {
holder = new ViewHolder();
convertView = LayoutInflater.from(getApplicationContext()).inflate(R.layout.image_item, null);
holder.mImageView = (ImageView) convertView.findViewById(R.id.id_image);
convertView.setTag(holder);
} else {
holder = (ViewHolder) convertView.getTag();
}
Picasso.with(MainActivity.this).load(mData.get(position)).into(holder.mImageView);
return convertView;
}
class ViewHolder {
ImageView mImageView;
}
}
}

public class MainActivity extends AppCompatActivity {    String urlPicasso = "http://img1.imgtn.bdimg.com/it/u=998216620,3976144567&fm=21&gp=0.jpg";    String urlGlide = "http://h.hiphotos.baidu.com/image/h%3D200/sign=cd65e7fa13d5ad6eb5f963eab1cb39a3/377adab44aed2e7394aa5a128f01a18b87d6fa49.jpg";    String urlVideo = "http://www.iqiyi.com/v_19rrm83ayw.html";    ListView mListView;    MyAdapter mAdpater;    List<String> mData = new ArrayList<>();    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mListView = (ListView) findViewById(R.id.lv);        for (int i = 0; i < 20; i++) {            mData.add(urlGlide);        }        mAdpater = new MyAdapter();        mListView.setAdapter(mAdpater);    }    class MyAdapter extends BaseAdapter {        @Override        public int getCount() {            return mData.size();        }        @Override        public Object getItem(int position) {            return mData.get(position);        }        @Override        public long getItemId(int position) {            return position;        }        @Override        public View getView(int position, View convertView, ViewGroup parent) {            ViewHolder holder = null;            if (convertView == null) {                holder = new ViewHolder();                convertView = LayoutInflater.from(getApplicationContext()).inflate(R.layout.image_item, null);                holder.mImageView = (ImageView) convertView.findViewById(R.id.id_image);                convertView.setTag(holder);            } else {                holder = (ViewHolder) convertView.getTag();            }            Picasso.with(MainActivity.this).load(mData.get(position)).into(holder.mImageView);            return convertView;        }        class ViewHolder {            ImageView mImageView;        }    }}

这是使用Picasso在ListView中简单的加载图片，布局中ImageView的宽高设置为自适应，下面是显示的效果。

下面通过resize()方法改变图片的尺寸，图片会根据设置的值改变大小，效果如下。

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Picasso.with(MainActivity.this).load(mData.get(position)).resize(200, 200).into(holder.mImageView);

            Picasso.with(MainActivity.this).load(mData.get(position)).resize(200, 200).into(holder.mImageView);

下面自定义Transformation，实现图片的转换。

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class MyTransfrom implements Transformation {
@Override
public Bitmap transform(Bitmap source) {
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 “key”;
}
}

       class MyTransfrom implements Transformation {            @Override            public Bitmap transform(Bitmap source) {                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 "key";            }        }

这里需要注意，返回的key一定要设置，不然会报错。

通过transform()方法设置，下面是运行的效果。

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MyTransfrom transfrom = new MyTransfrom();
Picasso.with(MainActivity.this).load(mData.get(position)).transform(transfrom).into(holder.mImageView);

            MyTransfrom transfrom = new MyTransfrom();            Picasso.with(MainActivity.this).load(mData.get(position)).transform(transfrom).into(holder.mImageView);

通过比较可以发现，图片的大小发生了改变，picasso-transformations是一个自定义的transform，库提供了多种图像变换。

可以设置memoryPolicy()方法指定内存缓存的策略。

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Picasso.with(MainActivity.this).load(mData.get(position)).memoryPolicy(MemoryPolicy.NO_CACHE).transform(transfrom).into(holder.mImageView);

Picasso.with(MainActivity.this).load(mData.get(position)).memoryPolicy(MemoryPolicy.NO_CACHE).transform(transfrom).into(holder.mImageView);

还有很多没有使用到，先就这么多。

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