通过案例快速学会Android Picasso图片缓存库

   <p>       picasso是Square公司开源的一个Android图形缓存库，官网地址<a href="/misc/goto?guid=4958860113958143127">http://square.github.io/picasso/</a>，可以实现图片下载和缓存功能。<br>        下载地址：<a href="/misc/goto?guid=4958972548258164558">https://github.com/square/picasso</a></p>    <p><img alt="这里写图片描述" src="https://simg.open-open.com/show/dc024edaaabe3e9838d6296d3cc40b6e.png"></p>    <h2>配置方法</h2>    <pre>  <code class="language-java">MAVEN  <dependency>    <groupId>com.squareup.picasso</groupId>    <artifactId>picasso</artifactId>    <version>(insert latest version)</version>  </dependency></code></pre>    <pre>  <code class="language-java">GRADLE    compile 'com.squareup.picasso:picasso:(insert latest version)'</code></pre>    <h2>基本使用</h2>    <p>       picasso仅需一行代码就能实现图片的异步加载</p>    <pre>  <code class="language-java">Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView);</code></pre>    <p>       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：</p>    <ol>     <li>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。</li>     <li>使用复杂的图片压缩转换来尽可能的减少内存消耗</li>     <li>自带内存和硬盘二级缓存功能</li>    </ol>    <h2>Picasso特性</h2>    <p><strong>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</strong></p>    <pre>  <code class="language-java">@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);  }</code></pre>    <p><strong>图片转换：转换图片以适应布局大小并减少内存占用</strong></p>    <pre>  <code class="language-java">Picasso.with(context)    .load(url)    .resize(50, 50)    .centerCrop()    .into(imageView)</code></pre>    <p>       你还可以自定义转换：</p>    <pre>  <code class="language-java">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()"; }  }</code></pre>    <p>       将CropSquareTransformation 的对象传递给transform 方法即可。</p>    <p><strong>Place holders-空白或者错误占位图片</strong><br>        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。</p>    <pre>  <code class="language-java">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</code></pre>    <p>       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。<br> <strong>资源文件的加载</strong><br>        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。</p>    <pre>  <code class="language-java">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);</code></pre>    <p><strong>列表使用Picasso加载图片案例</strong></p>    <p><img alt="这里写图片描述" src="https://simg.open-open.com/show/3e6f95695d598c446a6a3eea8e56e3e2.png"></p>    <p>首先在dependencies 中添加Picasso相关配置</p>    <p><strong>compile ‘com.squareup.picasso:picasso:2.5.2’</strong></p>    <p>然后上源码：</p>    <p>ListViewActivity.java</p>    <pre>  <code class="language-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);      }      }</code></pre>    <p>SunAdapter.java</p>    <pre>  <code class="language-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;      }  }</code></pre>    <p>SunBean.java</p>    <pre>  <code class="language-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;      }  }</code></pre>    <p>list_itmxml</p>    <pre>  <code class="language-java"><?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></code></pre>    <p>list_view.xml</p>    <pre>  <code class="language-java"><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></code></pre>    <p>最后不要忘了增加网络权限</p>    <pre>  <code class="language-java"><uses-permission android:name="android.permission.INTERNET" />  </code></pre>    <p>来自： <a href="/misc/goto?guid=4959673479396853621" rel="nofollow">http://blog.csdn.net/mynameishuangshuai/article/details/51462601</a></p>    <p> </p>