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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