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我们不能每次加载图片的时候都让用户从网络上下载,这样不仅浪费流量又会影响用户体验,所以Android中引入了图片的缓存这一操作机制。
原理:
1 <?xml version="1.0" encoding="utf-8"?> 2 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 3 android:layout_width="match_parent" 4 android:layout_height="match_parent" 5 > 6 <ImageView 7 android:id="@+id/iv_img" 8 android:layout_width="wrap_content" 9 android:layout_height="wrap_content" 10 android:src="@mipmap/ic_launcher" 11 android:layout_centerInParent="true"/> 12 <Button 13 android:id="@+id/btn_download" 14 android:layout_below="@+id/iv_img" 15 android:layout_centerHorizontal="true" 16 android:layout_width="wrap_content" 17 android:layout_height="wrap_content" 18 android:text="加载图片"/> 19 20 </RelativeLayout>
因为要从网络下载数据,还要存储到本地sd卡中,所以不要忘了为程序添加网络访问的权限、网络状态访问的权限和向外部存储设备写内容的权限:
1 <uses-permission android:name="android.permission.INTERNET" /> 2 <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" /> 3 <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
接着,创建一个 HttpUtils 工具类用于访问网络,代码如下:
1 package com.yztc.lx.cashimg; 2 3 import android.content.Context; 4 import android.net.ConnectivityManager; 5 import android.net.NetworkInfo; 6 7 import java.io.ByteArrayOutputStream; 8 import java.io.IOException; 9 import java.io.InputStream; 10 import java.net.HttpURLConnection; 11 import java.net.URL; 12 13 /**网络访问工具类 14 * Created by Lx on 2016/8/19. 15 */ 16 17 public class HttpUtils { 18 /** 19 * 判断网络连接是否通畅 20 * @param mContext 21 * @return 22 */ 23 public static boolean isNetConn(Context mContext) { 24 ConnectivityManager manager = (ConnectivityManager) mContext.getSystemService(Context.CONNECTIVITY_SERVICE); 25 NetworkInfo info = manager.getActiveNetworkInfo(); 26 if (info != null) { 27 return info.isConnected(); 28 } else { 29 return false; 30 } 31 } 32 33 /** 34 * 根据path下载网络上的数据 35 * @param path 路径 36 * @return 返回下载内容的byte数据形式 37 */ 38 public static byte[] getDateFromNet(String path) { 39 ByteArrayOutputStream baos = new ByteArrayOutputStream(); 40 try { 41 URL url = new URL(path); 42 HttpURLConnection conn = (HttpURLConnection) url.openConnection(); 43 conn.setRequestMethod("GET"); 44 conn.setConnectTimeout(5000); 45 conn.setDoInput(true); 46 conn.connect(); 47 if (conn.getResponseCode()==200) { 48 InputStream is = conn.getInputStream(); 49 byte b[] = new byte[1024]; 50 int len; 51 while ((len=is.read(b))!=-1) { 52 baos.write(b, 0, len); 53 } 54 return baos.toByteArray(); 55 } 56 } catch (IOException e) { 57 e.printStackTrace(); 58 } 59 return baos.toByteArray(); 60 } 61 62 }
还有操作外部存储的工具类:
1 package com.yztc.lx.cashimg; 2 3 import android.graphics.Bitmap; 4 import android.graphics.BitmapFactory; 5 import android.os.Environment; 6 7 import java.io.ByteArrayOutputStream; 8 import java.io.File; 9 import java.io.FileInputStream; 10 import java.io.FileOutputStream; 11 import java.io.IOException; 12 13 /** 14 * Created by Lx on 2016/8/20. 15 */ 16 17 public class ExternalStorageUtils { 18 /** 19 * 将传递过来的图片byte数组存储到sd卡中 20 * @param imgName 图片的名字 21 * @param buff byte数组 22 * @return 返回是否存储成功 23 */ 24 public static boolean storeToSDRoot(String imgName, byte buff[]) { 25 boolean b = false; 26 String basePath = Environment.getExternalStorageDirectory().getAbsolutePath(); 27 File file = new File(basePath, imgName); 28 try { 29 FileOutputStream fos = new FileOutputStream(file); 30 fos.write(buff); 31 fos.close(); 32 b = true; 33 } catch (IOException e) { 34 e.printStackTrace(); 35 } 36 return b; 37 } 38 39 /** 40 * 从本地内存中根据图片名字获取图片 41 * @param imgName 图片名字 42 * @return 返回图片的Bitmap格式 43 */ 44 public static Bitmap getImgFromSDRoot(String imgName) { 45 Bitmap bitmap = null; 46 String basePath = Environment.getExternalStorageDirectory().getAbsolutePath(); 47 File file = new File(basePath, imgName); 48 try { 49 FileInputStream fis = new FileInputStream(file); 50 ByteArrayOutputStream baos = new ByteArrayOutputStream(); 51 byte b[] = new byte[1024]; 52 int len; 53 while ((len = fis.read(b)) != -1) { 54 baos.write(b, 0, len); 55 } 56 byte buff[] = baos.toByteArray(); 57 if (buff != null && buff.length != 0) { 58 bitmap = BitmapFactory.decodeByteArray(buff, 0, buff.length); 59 } 60 } catch (IOException e) { 61 e.printStackTrace(); 62 } 63 return bitmap; 64 } 65 66 67 }
本例中将图片默认存在了sd卡根目录中。
然后是最主要的主函数了:
1 package com.yztc.lx.cashimg; 2 3 import android.graphics.Bitmap; 4 import android.graphics.BitmapFactory; 5 import android.os.Bundle; 6 import android.os.Handler; 7 import android.os.Message; 8 import android.support.v7.app.AppCompatActivity; 9 import android.util.Log; 10 import android.util.LruCache; 11 import android.view.View; 12 import android.widget.Button; 13 import android.widget.ImageView; 14 import android.widget.Toast; 15 16 import java.lang.ref.SoftReference; 17 import java.util.LinkedHashMap; 18 19 public class MainActivity extends AppCompatActivity implements View.OnClickListener { 20 21 private Button btn_download; 22 private ImageView iv_img; 23 private MyLruCache myLruCache; 24 private LinkedHashMap<String, SoftReference<Bitmap>> cashMap = new LinkedHashMap<>(); 25 private static final String TAG = "MainActivity"; 26 private String imgPath = "http://www.3dmgame.com/UploadFiles/201212/Medium_20121217143424221.jpg"; 27 private Handler handler = new Handler() { 28 @Override 29 public void handleMessage(Message msg) { 30 Bitmap bitmap = (Bitmap) msg.obj; 31 iv_img.setImageBitmap(bitmap); 32 Toast.makeText(MainActivity.this, "从网络上下载图片", Toast.LENGTH_SHORT).show(); 33 } 34 }; 35 36 @Override 37 protected void onCreate(Bundle savedInstanceState) { 38 super.onCreate(savedInstanceState); 39 setContentView(R.layout.activity_main); 40 initView(); 41 int totalMemory = (int) Runtime.getRuntime().maxMemory(); 42 int size = totalMemory / 8; 43 myLruCache = new MyLruCache(size); 44 btn_download.setOnClickListener(this); 45 } 46 47 private void initView() { 48 btn_download = (Button) findViewById(R.id.btn_download); 49 iv_img = (ImageView) findViewById(R.id.iv_img); 50 } 51 52 @Override 53 public void onClick(View v) { 54 Bitmap b = getImgCache(); 55 if (b != null) { 56 iv_img.setImageBitmap(b); 57 } else { 58 new Thread(new Runnable() { 59 @Override 60 public void run() { 61 if (HttpUtils.isNetConn(MainActivity.this)) { 62 byte b[] = HttpUtils.getDateFromNet(imgPath); 63 if (b != null && b.length != 0) { 64 Bitmap bitmap = BitmapFactory.decodeByteArray(b, 0, b.length); 65 Message msg = Message.obtain(); 66 msg.obj = bitmap; 67 handler.sendMessage(msg); 68 myLruCache.put(imgPath, bitmap); 69 Log.d(TAG, "run: " + "缓存到强引用中成功"); 70 boolean bl = ExternalStorageUtils.storeToSDRoot("haha.jpg", b); 71 if (bl) { 72 Log.d(TAG, "run: " + "缓存到本地内存成功"); 73 } else { 74 Log.d(TAG, "run: " + "缓存到本地内存失败"); 75 } 76 } else { 77 Toast.makeText(MainActivity.this, "下载失败!", Toast.LENGTH_SHORT).show(); 78 } 79 80 } else { 81 Toast.makeText(MainActivity.this, "请检查你的网络!", Toast.LENGTH_SHORT).show(); 82 } 83 } 84 }).start(); 85 } 86 } 87 88 /** 89 * 从缓存中获取图片 90 * 91 * @return 返回获取到的Bitmap 92 */ 93 public Bitmap getImgCache() { 94 Bitmap bitmap = myLruCache.get(imgPath); 95 if (bitmap != null) { 96 Log.d(TAG, "getImgCache: " + "从LruCache获取图片"); 97 } else { 98 SoftReference<Bitmap> sr = cashMap.get(imgPath); 99 if (sr != null) { 100 bitmap = sr.get(); 101 myLruCache.put(imgPath, bitmap); 102 cashMap.remove(imgPath); 103 Log.d(TAG, "getImgCache: " + "从软引用获取图片"); 104 } else { 105 bitmap = ExternalStorageUtils.getImgFromSDRoot("haha.jpg"); 106 Log.d(TAG, "getImgCache: " + "从外部存储获取图片"); 107 } 108 } 109 return bitmap; 110 } 111 112 /** 113 * 自定义一个方法继承系统的LruCache方法 114 */ 115 public class MyLruCache extends LruCache<String, Bitmap> { 116 117 /** 118 * 必须重写的构造函数,定义强引用缓存区的大小 119 * @param maxSize for caches that do not override {@link #sizeOf}, this is 120 * the maximum number of entries in the cache. For all other caches, 121 * this is the maximum sum of the sizes of the entries in this cache. 122 */ 123 public MyLruCache(int maxSize) { 124 super(maxSize); 125 } 126 127 //返回每个图片的大小 128 @Override 129 protected int sizeOf(String key, Bitmap value) { 130 //获取当前变量每行的字节数和行高度(基本是固定写法,记不住给我背!) 131 return value.getRowBytes() * value.getHeight(); 132 } 133 134 /** 135 * 当LruCache中的数据被驱逐或是移除时回调的函数 136 * 137 * @param evicted 当LruCache中的数据被驱逐用来给新的value倒出空间的时候变化 138 * @param key 用来标示对象的键,一般put的时候传入图片的url地址 139 * @param oldValue 之前存储的旧的对象 140 * @param newValue 存储的新的对象 141 */ 142 @Override 143 protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue) { 144 if (evicted) { 145 /** 146 * 将旧的值存到软引用中,因为强引用中可能有多个值被驱逐, 147 * 所以创建一个LinkedHashMap<String, SoftReference<Bitmap>>来存储软引用 148 * 基本也是固定写法 149 */ 150 SoftReference<Bitmap> softReference = new SoftReference<Bitmap>(oldValue); 151 cashMap.put(key, softReference); 152 } 153 } 154 } 155 }
基本的思路都在代码注释中写的很详细了,主要就是要自定义一个类,来继承系统的LruCache,实现其中的两个主要的方法sizeOf()和entryRemoved(),还有就是必须重写它的构造函数。
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原文地址:http://www.cnblogs.com/RabbitLx/p/5792268.html