通过案例快速学会Picasso图片缓存库

picasso是Square公司开源的一个Android图形缓存库，官网地址http://square.github.io/picasso/，可以实现图片下载和缓存功能。

下载地址：https://github.com/square/picasso

配置方法

MAVEN

<dependency>

  <groupId>com.squareup.picasso</groupId>

  <artifactId>picasso</artifactId>

  <version>(insert latest version)</version>

</dependency>

GRADLE

compile 'com.squareup.picasso:picasso:(insert latest version)'

基本使用

picasso仅需一行代码就能实现图片的异步加载

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

Picasso不仅实现了图片异步加载的功能，还解决了android中加载图片时需要解决的一些常见问题：

在adapter中需要取消已经不在视野范围的ImageView图片资源的加载，否则会导致图片错位，Picasso已经解决了这个问题。
使用复杂的图片压缩转换来尽可能的减少内存消耗
自带内存和硬盘二级缓存功能

Picasso特性

ADAPTER 中的下载：Adapter的重用会被自动检测到，Picasso会取消上次的加载

@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：转换图片以适应布局大小并减少内存占用

Picasso.with(context)

  .load(url)

  .resize(50, 50)

  .centerCrop()

  .into(imageView)

你还可以自定义转换：

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()"; }

}

将CropSquareTransformation 的对象传递给transform 方法即可。

Place holders-空白或者错误占位图片

picasso提供了两种占位图片，未加载完成或者加载发生错误的时需要一张图片作为提示。

Picasso.with(context)

    .load(url)

    .placeholder(R.drawable.user_placeholder)//没有加载图片时显示的默认图像

    .error(R.drawable.user_placeholder_error)// 图像加载错误时显示的图像

    .into(imageView);// 被加载的控件

如果加载发生错误会重复三次请求，三次都失败才会显示erro Place holder。

资源文件的加载

除了加载网络图片以外，picasso还支持加载Resources, assets, files, content providers中的资源文件。

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);

列表使用Picasso加载图片案例

首先在dependencies 中添加Picasso相关配置

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

然后上源码：

ListViewActivity.java

import android.os.Bundle;

import android.support.v7.app.AppCompatActivity;

import android.widget.ListView;

import java.util.ArrayList;

import java.util.List;

/**

 * Created by huangshuai on 2016/5/20.

 * Email：huangshuai@wooyun.org

 * 列表Activity

 */

public class ListViewActivity extends AppCompatActivity {

    private ListView lv_main;

    private List<SunBean> listDatas;

    private SunAdapter lAdapter;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.list_view);

        lv_main = (ListView) findViewById(R.id.list_sun);

        listDatas = new ArrayList<>();

        initData();

        lAdapter = new SunAdapter(listDatas, this);

        lv_main.setAdapter(lAdapter);

    }

    /**

     * 初始化数据

     */

    private void initData() {

        SunBean sun1 = new SunBean();

        sun1.setTitle("太阳的后裔一");

        sun1.setPicUrl("http://img3.imgtn.bdimg.com/it/u=3746050745,4184822198&fm=11&gp=0.jpg");

        listDatas.add(sun1);

        SunBean sun2 = new SunBean();

        sun2.setTitle("太阳的后裔二");

        sun2.setPicUrl("http://cdn.duitang.com/uploads/item/201603/02/20160302223503_jmdXT.thumb.224_0.jpeg");

        listDatas.add(sun2);

        SunBean news3 = new SunBean();

        news3.setTitle("太阳的后裔三");

        news3.setPicUrl("http://img5.duitang.com/uploads/item/201602/23/20160223132131_RNSQ3.thumb.224_0.jpeg");

        listDatas.add(news3);

        SunBean sun4 = new SunBean();

        sun4.setTitle("太阳的后裔四");

        sun4.setPicUrl("http://img5.duitang.com/uploads/item/201603/03/20160303163634_iveUE.thumb.224_0.jpeg");

        listDatas.add(sun4);

        SunBean sun5 = new SunBean();

        sun5.setTitle("太阳的后裔五");

        sun5.setPicUrl("http://cdn.duitang.com/uploads/item/201603/07/20160307013640_FQCTc.thumb.224_0.jpeg");

        listDatas.add(sun5);

        SunBean sun6 = new SunBean();

        sun6.setTitle("太阳的后裔六");

        sun6.setPicUrl("http://img4.duitang.com/uploads/item/201603/03/20160303102359_vzwPC.thumb.224_0.jpeg");

        listDatas.add(sun6);

        SunBean news7 = new SunBean();

        news7.setTitle("太阳的后裔七");

        news7.setPicUrl("http://img4.duitang.com/uploads/item/201603/03/20160303101132_AMPRv.thumb.224_0.jpeg");

        listDatas.add(news7);

        SunBean sun8 = new SunBean();

        sun8.setTitle("太阳的后裔八");

        sun8.setPicUrl("http://img0.imgtn.bdimg.com/it/u=1043934750,464206674&fm=11&gp=0.jpg");

        listDatas.add(sun8);

    }

}

SunAdapter.java

import android.content.Context;

import android.view.LayoutInflater;

import android.view.View;

import android.view.ViewGroup;

import android.widget.BaseAdapter;

import android.widget.ImageView;

import android.widget.TextView;

import com.squareup.picasso.Picasso;

import java.util.List;

/**

 * Created by huangshuai on 2016/5/20.

 * Email：huangshuai@wooyun.org

 * 列表Adapter

 */

public class SunAdapter extends BaseAdapter {

    private List<SunBean> list_sun;

    private LayoutInflater inflater;

    private Context context;

    public SunAdapter(List<SunBean> list_new, Context context) {

        this.list_sun = list_new;

        this.context = context;

        this.inflater = LayoutInflater.from(context);

    }

    @Override

    public int getCount() {

        return list_sun.size();

    }

    @Override

    public Object getItem(int position) {

        return list_sun.get(position);

    }

    @Override

    public long getItemId(int position) {

        return position;

    }

    @Override

    public View getView(int position, View convertView, ViewGroup parent) {

        ItemViewHodler viewHolder;

        if (convertView == null) {

            viewHolder = new ItemViewHodler();

            convertView = inflater.inflate(R.layout.list_item, null);

            viewHolder.title = (TextView) convertView.findViewById(R.id.item_title);

            viewHolder.pic = (ImageView) convertView.findViewById(R.id.item_pic);

            convertView.setTag(viewHolder);

        } else {

            viewHolder = (ItemViewHodler) convertView.getTag();

        }

        viewHolder.title.setText(list_sun.get(position).getTitle());

        Picasso.with(context).load(list_sun.get(position).getPicUrl()).fit()

                .placeholder(R.mipmap.alibg)

                .error(R.mipmap.ic_launcher)

                .into(viewHolder.pic);

        return convertView;

    }

    class ItemViewHodler {

        TextView title;

        ImageView pic;

    }

}

SunBean.java

/**

 * Created by huangshuai on 2016/5/20.

 * Email：huangshuai@wooyun.org

 * 列表图片Bean

 */

public class SunBean {

    private String title;

    private String picUrl;

    public SunBean(String title, String picUrl) {

        this.title = title;

        this.picUrl = picUrl;

    }

    public SunBean() {

    }

    public String getTitle() {

        return title;

    }

    public void setTitle(String title) {

        this.title = title;

    }

    public String getPicUrl() {

        return picUrl;

    }

    public void setPicUrl(String picUrl) {

        this.picUrl = picUrl;

    }

}

list_itmxml

<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:layout_width="match_parent"

    android:layout_height="match_parent">

    <TextView

        android:id="@+id/item_title"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:layout_centerHorizontal="true"

        android:text="太阳的后裔" />

    <ImageView

        android:id="@+id/item_pic"

        android:layout_width="160dp"

        android:layout_height="240dp"

        android:layout_centerHorizontal="true"

        android:layout_below="@id/item_title"

        android:src="@mipmap/alibg" />

</RelativeLayout>

list_view.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:layout_width="match_parent"

    android:layout_height="match_parent">

    <ListView

        android:id="@+id/list_sun"

        android:layout_width="match_parent"

        android:layout_height="match_parent"></ListView>

</RelativeLayout>

最后不要忘了增加网络权限

<uses-permission android:name="android.permission.INTERNET" />

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