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 Android教程網 >> Android技術 >> Android開發實例 >> Android使用LruCache和SD卡或手機緩存實現效果非常的流暢的異步加載圖片

Android使用LruCache和SD卡或手機緩存實現效果非常的流暢的異步加載圖片

編輯：Android開發實例

異步加載圖片的例子，網上也比較多，大部分用了HashMap<String, SoftReference<Drawable>> imageCache ，但是現在已經不再推薦使用這種方式了，因為從 Android 2.3 (API Level 9)開始，垃圾回收器會更傾向於回收持有軟引用或弱引用的對象，這讓軟引用和弱引用變得不再可靠。另外，Android 3.0 (API Level 11)中，圖片的數據會存儲在本地的內存當中，因而無法用一種可預見的方式將其釋放，這就有潛在的風險造成應用程序的內存溢出並崩潰，所以我這裡用得是LruCache來緩存圖片，當存儲Image的大小大於LruCache設定的值，系統自動釋放內存，這個類是3.1版本中提供的，如果你是在更早的Android版本中開發，則需要導入android-support-v4的jar包（這裡要注意咯）

為什麼寫這篇文章呢？
因為我之前做的項目中，也有異步加載圖片，那時候用得是Thread去下載圖片，每次下載圖片都要new Thread去下載，而且還是並發去下載，每次都new 一個線程浪費內存，老板說服務器承受不起這麼多的連接，叫我改成先獲取一張圖片之後再去獲取下一張，這樣子保存與服務器的連接為一個，服務器壓力小了，然後樓主就想到線程池，線程池很好的幫我們管理並發的問題，並發的問題解決了，可是後面又出問題了，圖片多了就出現OOM(OutOfMemory)異常，之後用了SoftReference，先用SoftReference中獲取圖片，SoftReference沒有就開線程去下載，老板說你為什麼不把圖片在手機上做個緩存呢，於是我用了手機緩存，大概思路就是先從SoftReference中獲取圖片，如果SoftReference沒有就去手機緩存中獲取，手機緩存中沒有就開啟先從去下載，然後成功的解決了OOM的問題，前些天老板要我重構下代碼，我也覺得之前寫的代碼耦合性太強，早就想改，然後之前http://www.fengfly.com/plus/view-214547-1.html這篇文章，LruCache和滑動過程中取消下載任務，停下來的時候才去下載這2點比較好，值得我學習，然後我就將我的項目異步加載這一塊改了下，發到這裡做個記錄吧，以後類似的異步加載圖片直接拷貝代碼，提交開發的效率

這篇文章做了哪些方面的優化
使用了線程池來管理下載任務
使用LruCache來緩存圖片
使用手機來緩存圖片
GridView滑動的時候取消下載任務，靜止的時候進行下載，GridView滑動更加的流暢
降低了代碼的耦合性，結構更加的清晰，便於以後重用
接下來我們先來看看項目的結構

 

FileUtils 文件操作的工具類，提供保存圖片，獲取圖片，判斷圖片是否存在，刪除圖片的一些方法，這個類比較簡單

  1. package com.example.asyncimageloader; 
  2.  
  3. import java.io.File; 
  4. import java.io.FileOutputStream; 
  5. import java.io.IOException; 
  6.  
  7. import android.content.Context; 
  8. import android.graphics.Bitmap; 
  9. import android.graphics.Bitmap.CompressFormat; 
  10. import android.graphics.BitmapFactory; 
  11. import android.os.Environment; 
  12.  
  13. public class FileUtils { 
  14.     /** 
  15.      * sd卡的根目錄 
  16.      */ 
  17.     private static String mSdRootPath = Environment.getExternalStorageDirectory().getPath(); 
  18.     /** 
  19.      * 手機的緩存根目錄 
  20.      */ 
  21.     private static String mDataRootPath = null; 
  22.     /** 
  23.      * 保存Image的目錄名 
  24.      */ 
  25.     private final static String FOLDER_NAME = "/AndroidImage"; 
  26.      
  27.      
  28.     public FileUtils(Context context){ 
  29.         mDataRootPath = context.getCacheDir().getPath(); 
  30.     } 
  31.      
  32.  
  33.     /** 
  34.      * 獲取儲存Image的目錄 
  35.      * @return 
  36.      */ 
  37.     private String getStorageDirectory(){ 
  38.         return Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED) ? 
  39.                 mSdRootPath + FOLDER_NAME : mDataRootPath + FOLDER_NAME; 
  40.     } 
  41.      
  42.     /** 
  43.      * 保存Image的方法，有sd卡存儲到sd卡，沒有就存儲到手機目錄 
  44.      * @param fileName  
  45.      * @param bitmap    
  46.      * @throws IOException 
  47.      */ 
  48.     public void savaBitmap(String fileName, Bitmap bitmap) throws IOException{ 
  49.         if(bitmap == null){ 
  50.             return; 
  51.         } 
  52.         String path = getStorageDirectory(); 
  53.         File folderFile = new File(path); 
  54.         if(!folderFile.exists()){ 
  55.             folderFile.mkdir(); 
  56.         } 
  57.         File file = new File(path + File.separator + fileName); 
  58.         file.createNewFile(); 
  59.         FileOutputStream fos = new FileOutputStream(file); 
  60.         bitmap.compress(CompressFormat.JPEG, 100, fos); 
  61.         fos.flush(); 
  62.         fos.close(); 
  63.     } 
  64.      
  65.     /** 
  66.      * 從手機或者sd卡獲取Bitmap 
  67.      * @param fileName 
  68.      * @return 
  69.      */ 
  70.     public Bitmap getBitmap(String fileName){ 
  71.         return BitmapFactory.decodeFile(getStorageDirectory() + File.separator + fileName); 
  72.     } 
  73.      
  74.     /** 
  75.      * 判斷文件是否存在 
  76.      * @param fileName 
  77.      * @return 
  78.      */ 
  79.     public boolean isFileExists(String fileName){ 
  80.         return new File(getStorageDirectory() + File.separator + fileName).exists(); 
  81.     } 
  82.      
  83.     /** 
  84.      * 獲取文件的大小 
  85.      * @param fileName 
  86.      * @return 
  87.      */ 
  88.     public long getFileSize(String fileName) { 
  89.         return new File(getStorageDirectory() + File.separator + fileName).length(); 
  90.     } 
  91.      
  92.      
  93.     /** 
  94.      * 刪除SD卡或者手機的緩存圖片和目錄 
  95.      */ 
  96.     public void deleteFile() { 
  97.         File dirFile = new File(getStorageDirectory()); 
  98.         if(! dirFile.exists()){ 
  99.             return; 
  100.         } 
  101.         if (dirFile.isDirectory()) { 
  102.             String[] children = dirFile.list(); 
  103.             for (int i = 0; i < children.length; i++) { 
  104.                 new File(dirFile, children[i]).delete(); 
  105.             } 
  106.         } 
  107.          
  108.         dirFile.delete(); 
  109.     } 
  110. } 

 

Images類，提供圖片資源的類，來自Android文檔中BitmapFun中http://developer.android.com/training/displaying-bitmaps/index.html，裡面一個靜態的圖片Url數組，這裡面比較坑爹，因為是圖片是google官網提供的，有時候會下載超時

  1. package com.example.asyncimageloader; 
  2.  
  3. public class Images { 
  4.  
  5.     public final static String[] imageThumbUrls = new String[] { 
  6.             "https://lh6.googleusercontent.com/-55osAWw3x0Q/URquUtcFr5I/AAAAAAAAAbs/rWlj1RUKrYI/s160-c/A%252520Photographer.jpg", 
  7.             "https://lh4.googleusercontent.com/--dq8niRp7W4/URquVgmXvgI/AAAAAAAAAbs/-gnuLQfNnBA/s160-c/A%252520Song%252520of%252520Ice%252520and%252520Fire.jpg", 
  8.             "https://lh5.googleusercontent.com/-7qZeDtRKFKc/URquWZT1gOI/AAAAAAAAAbs/hqWgteyNXsg/s160-c/Another%252520Rockaway%252520Sunset.jpg", 
  9.             "https://lh3.googleusercontent.com/--L0Km39l5J8/URquXHGcdNI/AAAAAAAAAbs/3ZrSJNrSomQ/s160-c/Antelope%252520Butte.jpg", 
  10.             "https://lh6.googleusercontent.com/-8HO-4vIFnlw/URquZnsFgtI/AAAAAAAAAbs/WT8jViTF7vw/s160-c/Antelope%252520Hallway.jpg", 
  11.             "https://lh4.googleusercontent.com/-WIuWgVcU3Qw/URqubRVcj4I/AAAAAAAAAbs/YvbwgGjwdIQ/s160-c/Antelope%252520Walls.jpg", 
  12.             "https://lh6.googleusercontent.com/-UBmLbPELvoQ/URqucCdv0kI/AAAAAAAAAbs/IdNhr2VQoQs/s160-c/Apre%2525CC%252580s%252520la%252520Pluie.jpg", 
  13.             "https://lh3.googleusercontent.com/-s-AFpvgSeew/URquc6dF-JI/AAAAAAAAAbs/Mt3xNGRUd68/s160-c/Backlit%252520Cloud.jpg", 
  14.             "https://lh5.googleusercontent.com/-bvmif9a9YOQ/URquea3heHI/AAAAAAAAAbs/rcr6wyeQtAo/s160-c/Bee%252520and%252520Flower.jpg", 
  15.             "https://lh5.googleusercontent.com/-n7mdm7I7FGs/URqueT_BT-I/AAAAAAAAAbs/9MYmXlmpSAo/s160-c/Bonzai%252520Rock%252520Sunset.jpg", 
  16.             "https://lh6.googleusercontent.com/-4CN4X4t0M1k/URqufPozWzI/AAAAAAAAAbs/8wK41lg1KPs/s160-c/Caterpillar.jpg", 
  17.             "https://lh3.googleusercontent.com/-rrFnVC8xQEg/URqufdrLBaI/AAAAAAAAAbs/s69WYy_fl1E/s160-c/Chess.jpg", 
  18.             "https://lh5.googleusercontent.com/-WVpRptWH8Yw/URqugh-QmDI/AAAAAAAAAbs/E-MgBgtlUWU/s160-c/Chihuly.jpg", 
  19.             "https://lh5.googleusercontent.com/-0BDXkYmckbo/URquhKFW84I/AAAAAAAAAbs/ogQtHCTk2JQ/s160-c/Closed%252520Door.jpg", 
  20.             "https://lh3.googleusercontent.com/-PyggXXZRykM/URquh-kVvoI/AAAAAAAAAbs/hFtDwhtrHHQ/s160-c/Colorado%252520River%252520Sunset.jpg", 
  21.             "https://lh3.googleusercontent.com/-ZAs4dNZtALc/URquikvOCWI/AAAAAAAAAbs/DXz4h3dll1Y/s160-c/Colors%252520of%252520Autumn.jpg", 
  22.             "https://lh4.googleusercontent.com/-GztnWEIiMz8/URqukVCU7bI/AAAAAAAAAbs/jo2Hjv6MZ6M/s160-c/Countryside.jpg", 
  23.             "https://lh4.googleusercontent.com/-bEg9EZ9QoiM/URquklz3FGI/AAAAAAAAAbs/UUuv8Ac2BaE/s160-c/Death%252520Valley%252520-%252520Dunes.jpg", 
  24.             "https://lh6.googleusercontent.com/-ijQJ8W68tEE/URqulGkvFEI/AAAAAAAAAbs/zPXvIwi_rFw/s160-c/Delicate%252520Arch.jpg", 
  25.             "https://lh5.googleusercontent.com/-Oh8mMy2ieng/URqullDwehI/AAAAAAAAAbs/TbdeEfsaIZY/s160-c/Despair.jpg", 
  26.             "https://lh5.googleusercontent.com/-gl0y4UiAOlk/URqumC_KjBI/AAAAAAAAAbs/PM1eT7dn4oo/s160-c/Eagle%252520Fall%252520Sunrise.jpg", 
  27.             "https://lh3.googleusercontent.com/-hYYHd2_vXPQ/URqumtJa9eI/AAAAAAAAAbs/wAalXVkbSh0/s160-c/Electric%252520Storm.jpg", 
  28.             "https://lh5.googleusercontent.com/-PyY_yiyjPTo/URqunUOhHFI/AAAAAAAAAbs/azZoULNuJXc/s160-c/False%252520Kiva.jpg", 
  29.             "https://lh6.googleusercontent.com/-PYvLVdvXywk/URqunwd8hfI/AAAAAAAAAbs/qiMwgkFvf6I/s160-c/Fitzgerald%252520Streaks.jpg", 
  30.             "https://lh4.googleusercontent.com/-KIR_UobIIqY/URquoCZ9SlI/AAAAAAAAAbs/Y4d4q8sXu4c/s160-c/Foggy%252520Sunset.jpg", 
  31.             "https://lh6.googleusercontent.com/-9lzOk_OWZH0/URquoo4xYoI/AAAAAAAAAbs/AwgzHtNVCwU/s160-c/Frantic.jpg", 
  32.             "https://lh3.googleusercontent.com/-0X3JNaKaz48/URqupH78wpI/AAAAAAAAAbs/lHXxu_zbH8s/s160-c/Golden%252520Gate%252520Afternoon.jpg", 
  33.             "https://lh6.googleusercontent.com/-95sb5ag7ABc/URqupl95RDI/AAAAAAAAAbs/g73R20iVTRA/s160-c/Golden%252520Gate%252520Fog.jpg", 
  34.             "https://lh3.googleusercontent.com/-JB9v6rtgHhk/URqup21F-zI/AAAAAAAAAbs/64Fb8qMZWXk/s160-c/Golden%252520Grass.jpg", 
  35.             "https://lh4.googleusercontent.com/-EIBGfnuLtII/URquqVHwaRI/AAAAAAAAAbs/FA4McV2u8VE/s160-c/Grand%252520Teton.jpg", 
  36.             "https://lh4.googleusercontent.com/-WoMxZvmN9nY/URquq1v2AoI/AAAAAAAAAbs/grj5uMhL6NA/s160-c/Grass%252520Closeup.jpg", 
  37.             "https://lh3.googleusercontent.com/-6hZiEHXx64Q/URqurxvNdqI/AAAAAAAAAbs/kWMXM3o5OVI/s160-c/Green%252520Grass.jpg", 
  38.             "https://lh5.googleusercontent.com/-6LVb9OXtQ60/URquteBFuKI/AAAAAAAAAbs/4F4kRgecwFs/s160-c/Hanging%252520Leaf.jpg", 
  39.             "https://lh4.googleusercontent.com/-zAvf__52ONk/URqutT_IuxI/AAAAAAAAAbs/D_bcuc0thoU/s160-c/Highway%2525201.jpg", 
  40.             "https://lh6.googleusercontent.com/-H4SrUg615rA/URquuL27fXI/AAAAAAAAAbs/4aEqJfiMsOU/s160-c/Horseshoe%252520Bend%252520Sunset.jpg", 
  41.             "https://lh4.googleusercontent.com/-JhFi4fb_Pqw/URquuX-QXbI/AAAAAAAAAbs/IXpYUxuweYM/s160-c/Horseshoe%252520Bend.jpg", 
  42.             "https://lh5.googleusercontent.com/-UGgssvFRJ7g/URquueyJzGI/AAAAAAAAAbs/yYIBlLT0toM/s160-c/Into%252520the%252520Blue.jpg", 
  43.             "https://lh3.googleusercontent.com/-CH7KoupI7uI/URquu0FF__I/AAAAAAAAAbs/R7GDmI7v_G0/s160-c/Jelly%252520Fish%2525202.jpg", 
  44.             "https://lh4.googleusercontent.com/-pwuuw6yhg8U/URquvPxR3FI/AAAAAAAAAbs/VNGk6f-tsGE/s160-c/Jelly%252520Fish%2525203.jpg", 
  45.             "https://lh5.googleusercontent.com/-GoUQVw1fnFw/URquv6xbC0I/AAAAAAAAAbs/zEUVTQQ43Zc/s160-c/Kauai.jpg", 
  46.             "https://lh6.googleusercontent.com/-8QdYYQEpYjw/URquwvdh88I/AAAAAAAAAbs/cktDy-ysfHo/s160-c/Kyoto%252520Sunset.jpg", 
  47.             "https://lh4.googleusercontent.com/-vPeekyDjOE0/URquwzJ28qI/AAAAAAAAAbs/qxcyXULsZrg/s160-c/Lake%252520Tahoe%252520Colors.jpg", 
  48.             "https://lh4.googleusercontent.com/-xBPxWpD4yxU/URquxWHk8AI/AAAAAAAAAbs/ARDPeDYPiMY/s160-c/Lava%252520from%252520the%252520Sky.jpg", 
  49.             "https://lh3.googleusercontent.com/-897VXrJB6RE/URquxxxd-5I/AAAAAAAAAbs/j-Cz4T4YvIw/s160-c/Leica%25252050mm%252520Summilux.jpg", 
  50.             "https://lh5.googleusercontent.com/-qSJ4D4iXzGo/URquyDWiJ1I/AAAAAAAAAbs/k2pBXeWehOA/s160-c/Leica%25252050mm%252520Summilux.jpg", 
  51.             "https://lh6.googleusercontent.com/-dwlPg83vzLg/URquylTVuFI/AAAAAAAAAbs/G6SyQ8b4YsI/s160-c/Leica%252520M8%252520%252528Front%252529.jpg", 
  52.             "https://lh3.googleusercontent.com/-R3_EYAyJvfk/URquzQBv8eI/AAAAAAAAAbs/b9xhpUM3pEI/s160-c/Light%252520to%252520Sand.jpg", 
  53.             "https://lh3.googleusercontent.com/-fHY5h67QPi0/URqu0Cp4J1I/AAAAAAAAAbs/0lG6m94Z6vM/s160-c/Little%252520Bit%252520of%252520Paradise.jpg", 
  54.             "https://lh5.googleusercontent.com/-TzF_LwrCnRM/URqu0RddPOI/AAAAAAAAAbs/gaj2dLiuX0s/s160-c/Lone%252520Pine%252520Sunset.jpg", 
  55.             "https://lh3.googleusercontent.com/-4HdpJ4_DXU4/URqu046dJ9I/AAAAAAAAAbs/eBOodtk2_uk/s160-c/Lonely%252520Rock.jpg", 
  56.             "https://lh6.googleusercontent.com/-erbF--z-W4s/URqu1ajSLkI/AAAAAAAAAbs/xjDCDO1INzM/s160-c/Longue%252520Vue.jpg", 
  57.             "https://lh6.googleusercontent.com/-0CXJRdJaqvc/URqu1opNZNI/AAAAAAAAAbs/PFB2oPUU7Lk/s160-c/Look%252520Me%252520in%252520the%252520Eye.jpg", 
  58.             "https://lh3.googleusercontent.com/-D_5lNxnDN6g/URqu2Tk7HVI/AAAAAAAAAbs/p0ddca9W__Y/s160-c/Lost%252520in%252520a%252520Field.jpg", 
  59.             "https://lh6.googleusercontent.com/-flsqwMrIk2Q/URqu24PcmjI/AAAAAAAAAbs/5ocIH85XofM/s160-c/Marshall%252520Beach%252520Sunset.jpg", 
  60.             "https://lh4.googleusercontent.com/-Y4lgryEVTmU/URqu28kG3gI/AAAAAAAAAbs/OjXpekqtbJ4/s160-c/Mono%252520Lake%252520Blue.jpg", 
  61.             "https://lh4.googleusercontent.com/-AaHAJPmcGYA/URqu3PIldHI/AAAAAAAAAbs/lcTqk1SIcRs/s160-c/Monument%252520Valley%252520Overlook.jpg", 
  62.             "https://lh4.googleusercontent.com/-vKxfdQ83dQA/URqu31Yq_BI/AAAAAAAAAbs/OUoGk_2AyfM/s160-c/Moving%252520Rock.jpg", 
  63.             "https://lh5.googleusercontent.com/-CG62QiPpWXg/URqu4ia4vRI/AAAAAAAAAbs/0YOdqLAlcAc/s160-c/Napali%252520Coast.jpg", 
  64.             "https://lh6.googleusercontent.com/-wdGrP5PMmJQ/URqu5PZvn7I/AAAAAAAAAbs/m0abEcdPXe4/s160-c/One%252520Wheel.jpg", 
  65.             "https://lh6.googleusercontent.com/-6WS5DoCGuOA/URqu5qx1UgI/AAAAAAAAAbs/giMw2ixPvrY/s160-c/Open%252520Sky.jpg", 
  66.             "https://lh6.googleusercontent.com/-u8EHKj8G8GQ/URqu55sM6yI/AAAAAAAAAbs/lIXX_GlTdmI/s160-c/Orange%252520Sunset.jpg", 
  67.             "https://lh6.googleusercontent.com/-74Z5qj4bTDE/URqu6LSrJrI/AAAAAAAAAbs/XzmVkw90szQ/s160-c/Orchid.jpg", 
  68.             "https://lh6.googleusercontent.com/-lEQE4h6TePE/URqu6t_lSkI/AAAAAAAAAbs/zvGYKOea_qY/s160-c/Over%252520there.jpg", 
  69.             "https://lh5.googleusercontent.com/-cauH-53JH2M/URqu66v_USI/AAAAAAAAAbs/EucwwqclfKQ/s160-c/Plumes.jpg", 
  70.             "https://lh3.googleusercontent.com/-eDLT2jHDoy4/URqu7axzkAI/AAAAAAAAAbs/iVZE-xJ7lZs/s160-c/Rainbokeh.jpg", 
  71.             "https://lh5.googleusercontent.com/-j1NLqEFIyco/URqu8L1CGcI/AAAAAAAAAbs/aqZkgX66zlI/s160-c/Rainbow.jpg", 
  72.             "https://lh5.googleusercontent.com/-DRnqmK0t4VU/URqu8XYN9yI/AAAAAAAAAbs/LgvF_592WLU/s160-c/Rice%252520Fields.jpg", 
  73.             "https://lh3.googleusercontent.com/-hwh1v3EOGcQ/URqu8qOaKwI/AAAAAAAAAbs/IljRJRnbJGw/s160-c/Rockaway%252520Fire%252520Sky.jpg", 
  74.             "https://lh5.googleusercontent.com/-wjV6FQk7tlk/URqu9jCQ8sI/AAAAAAAAAbs/RyYUpdo-c9o/s160-c/Rockaway%252520Flow.jpg", 
  75.             "https://lh6.googleusercontent.com/-6cAXNfo7D20/URqu-BdzgPI/AAAAAAAAAbs/OmsYllzJqwo/s160-c/Rockaway%252520Sunset%252520Sky.jpg", 
  76.             "https://lh3.googleusercontent.com/-sl8fpGPS-RE/URqu_BOkfgI/AAAAAAAAAbs/Dg2Fv-JxOeg/s160-c/Russian%252520Ridge%252520Sunset.jpg", 
  77.             "https://lh6.googleusercontent.com/-gVtY36mMBIg/URqu_q91lkI/AAAAAAAAAbs/3CiFMBcy5MA/s160-c/Rust%252520Knot.jpg", 
  78.             "https://lh6.googleusercontent.com/-GHeImuHqJBE/URqu_FKfVLI/AAAAAAAAAbs/axuEJeqam7Q/s160-c/Sailing%252520Stones.jpg", 
  79.             "https://lh3.googleusercontent.com/-hBbYZjTOwGc/URqu_ycpIrI/AAAAAAAAAbs/nAdJUXnGJYE/s160-c/Seahorse.jpg", 
  80.             "https://lh3.googleusercontent.com/-Iwi6-i6IexY/URqvAYZHsVI/AAAAAAAAAbs/5ETWl4qXsFE/s160-c/Shinjuku%252520Street.jpg", 
  81.             "https://lh6.googleusercontent.com/-amhnySTM_MY/URqvAlb5KoI/AAAAAAAAAbs/pFCFgzlKsn0/s160-c/Sierra%252520Heavens.jpg", 
  82.             "https://lh5.googleusercontent.com/-dJgjepFrYSo/URqvBVJZrAI/AAAAAAAAAbs/v-F5QWpYO6s/s160-c/Sierra%252520Sunset.jpg", 
  83.             "https://lh4.googleusercontent.com/-Z4zGiC5nWdc/URqvBdEwivI/AAAAAAAAAbs/ZRZR1VJ84QA/s160-c/Sin%252520Lights.jpg", 
  84.             "https://lh4.googleusercontent.com/-_0cYiWW8ccY/URqvBz3iM4I/AAAAAAAAAbs/9N_Wq8MhLTY/s160-c/Starry%252520Lake.jpg", 
  85.             "https://lh3.googleusercontent.com/-A9LMoRyuQUA/URqvCYx_JoI/AAAAAAAAAbs/s7sde1Bz9cI/s160-c/Starry%252520Night.jpg", 
  86.             "https://lh3.googleusercontent.com/-KtLJ3k858eY/URqvC_2h_bI/AAAAAAAAAbs/zzEBImwDA_g/s160-c/Stream.jpg", 
  87.             "https://lh5.googleusercontent.com/-dFB7Lad6RcA/URqvDUftwWI/AAAAAAAAAbs/BrhoUtXTN7o/s160-c/Strip%252520Sunset.jpg", 
  88.             "https://lh5.googleusercontent.com/-at6apgFiN20/URqvDyffUZI/AAAAAAAAAbs/clABCx171bE/s160-c/Sunset%252520Hills.jpg", 
  89.             "https://lh4.googleusercontent.com/-7-EHhtQthII/URqvEYTk4vI/AAAAAAAAAbs/QSJZoB3YjVg/s160-c/Tenaya%252520Lake%2525202.jpg", 
  90.             "https://lh6.googleusercontent.com/-8MrjV_a-Pok/URqvFC5repI/AAAAAAAAAbs/9inKTg9fbCE/s160-c/Tenaya%252520Lake.jpg", 
  91.             "https://lh5.googleusercontent.com/-B1HW-z4zwao/URqvFWYRwUI/AAAAAAAAAbs/8Peli53Bs8I/s160-c/The%252520Cave%252520BW.jpg", 
  92.             "https://lh3.googleusercontent.com/-PO4E-xZKAnQ/URqvGRqjYkI/AAAAAAAAAbs/42nyADFsXag/s160-c/The%252520Fisherman.jpg", 
  93.             "https://lh4.googleusercontent.com/-iLyZlzfdy7s/URqvG0YScdI/AAAAAAAAAbs/1J9eDKmkXtk/s160-c/The%252520Night%252520is%252520Coming.jpg", 
  94.             "https://lh6.googleusercontent.com/-G-k7YkkUco0/URqvHhah6fI/AAAAAAAAAbs/_taQQG7t0vo/s160-c/The%252520Road.jpg", 
  95.             "https://lh6.googleusercontent.com/-h-ALJt7kSus/URqvIThqYfI/AAAAAAAAAbs/ejiv35olWS8/s160-c/Tokyo%252520Heights.jpg", 
  96.             "https://lh5.googleusercontent.com/-Hy9k-TbS7xg/URqvIjQMOxI/AAAAAAAAAbs/RSpmmOATSkg/s160-c/Tokyo%252520Highway.jpg", 
  97.             "https://lh6.googleusercontent.com/-83oOvMb4OZs/URqvJL0T7lI/AAAAAAAAAbs/c5TECZ6RONM/s160-c/Tokyo%252520Smog.jpg", 
  98.             "https://lh3.googleusercontent.com/-FB-jfgREEfI/URqvJI3EXAI/AAAAAAAAAbs/XfyweiRF4v8/s160-c/Tufa%252520at%252520Night.jpg", 
  99.             "https://lh4.googleusercontent.com/-vngKD5Z1U8w/URqvJUCEgPI/AAAAAAAAAbs/ulxCMVcU6EU/s160-c/Valley%252520Sunset.jpg", 
  100.             "https://lh6.googleusercontent.com/-DOz5I2E2oMQ/URqvKMND1kI/AAAAAAAAAbs/Iqf0IsInleo/s160-c/Windmill%252520Sunrise.jpg", 
  101.             "https://lh5.googleusercontent.com/-biyiyWcJ9MU/URqvKculiAI/AAAAAAAAAbs/jyPsCplJOpE/s160-c/Windmill.jpg", 
  102.             "https://lh4.googleusercontent.com/-PDT167_xRdA/URqvK36mLcI/AAAAAAAAAbs/oi2ik9QseMI/s160-c/Windmills.jpg", 
  103.             "https://lh5.googleusercontent.com/-kI_QdYx7VlU/URqvLXCB6gI/AAAAAAAAAbs/N31vlZ6u89o/s160-c/Yet%252520Another%252520Rockaway%252520Sunset.jpg", 
  104.             "https://lh4.googleusercontent.com/-e9NHZ5k5MSs/URqvMIBZjtI/AAAAAAAAAbs/1fV810rDNfQ/s160-c/Yosemite%252520Tree.jpg", }; 
  105.      
  106. } 

ImageDownLoader類，異步下載的核心類，保存圖片到手機緩存，將圖片加入LruCache中等等

  1. package com.example.asyncimageloader; 
  2.  
  3. import java.io.IOException; 
  4. import java.net.HttpURLConnection; 
  5. import java.net.URL; 
  6. import java.util.concurrent.ExecutorService; 
  7. import java.util.concurrent.Executors; 
  8.  
  9. import android.content.Context; 
  10. import android.graphics.Bitmap; 
  11. import android.graphics.BitmapFactory; 
  12. import android.os.Handler; 
  13. import android.os.Message; 
  14. import android.support.v4.util.LruCache; 
  15.  
  16. public class ImageDownLoader { 
  17.     /** 
  18.      * 緩存Image的類，當存儲Image的大小大於LruCache設定的值，系統自動釋放內存 
  19.      */ 
  20.     private LruCache<String, Bitmap> mMemoryCache; 
  21.     /** 
  22.      * 操作文件相關類對象的引用 
  23.      */ 
  24.     private FileUtils fileUtils; 
  25.     /** 
  26.      * 下載Image的線程池 
  27.      */ 
  28.     private ExecutorService mImageThreadPool = null; 
  29.      
  30.      
  31.     public ImageDownLoader(Context context){ 
  32.         //獲取系統分配給每個應用程序的最大內存，每個應用系統分配32M 
  33.         int maxMemory = (int) Runtime.getRuntime().maxMemory();   
  34.         int mCacheSize = maxMemory / 8; 
  35.         //給LruCache分配1/8 4M 
  36.         mMemoryCache = new LruCache<String, Bitmap>(mCacheSize){ 
  37.  
  38.             //必須重寫此方法，來測量Bitmap的大小 
  39.             @Override 
  40.             protected int sizeOf(String key, Bitmap value) { 
  41.                 return value.getRowBytes() * value.getHeight(); 
  42.             } 
  43.              
  44.         }; 
  45.          
  46.         fileUtils = new FileUtils(context); 
  47.     } 
  48.      
  49.      
  50.     /** 
  51.      * 獲取線程池的方法，因為涉及到並發的問題，我們加上同步鎖 
  52.      * @return 
  53.      */ 
  54.     public ExecutorService getThreadPool(){ 
  55.         if(mImageThreadPool == null){ 
  56.             synchronized(ExecutorService.class){ 
  57.                 if(mImageThreadPool == null){ 
  58.                     //為了下載圖片更加的流暢，我們用了2個線程來下載圖片 
  59.                     mImageThreadPool = Executors.newFixedThreadPool(2); 
  60.                 } 
  61.             } 
  62.         } 
  63.          
  64.         return mImageThreadPool; 
  65.          
  66.     } 
  67.      
  68.     /** 
  69.      * 添加Bitmap到內存緩存 
  70.      * @param key 
  71.      * @param bitmap 
  72.      */ 
  73.     public void addBitmapToMemoryCache(String key, Bitmap bitmap) {   
  74.         if (getBitmapFromMemCache(key) == null && bitmap != null) {   
  75.             mMemoryCache.put(key, bitmap);   
  76.         }   
  77.     }   
  78.       
  79.     /** 
  80.      * 從內存緩存中獲取一個Bitmap 
  81.      * @param key 
  82.      * @return 
  83.      */ 
  84.     public Bitmap getBitmapFromMemCache(String key) {   
  85.         return mMemoryCache.get(key);   
  86.     }  
  87.      
  88.     /** 
  89.      * 先從內存緩存中獲取Bitmap,如果沒有就從SD卡或者手機緩存中獲取，SD卡或者手機緩存 
  90.      * 沒有就去下載 
  91.      * @param url 
  92.      * @param listener 
  93.      * @return 
  94.      */ 
  95.     public Bitmap downloadImage(final String url, final onImageLoaderListener listener){ 
  96.         //替換Url中非字母和非數字的字符，這裡比較重要，因為我們用Url作為文件名，比如我們的Url 
  97.         //是Http://xiaanming/abc.jpg;用這個作為圖片名稱，系統會認為xiaanming為一個目錄， 
  98.         //我們沒有創建此目錄保存文件就會報錯 
  99.         final String subUrl = url.replaceAll("[^\\w]", ""); 
  100.         Bitmap bitmap = showCacheBitmap(subUrl); 
  101.         if(bitmap != null){ 
  102.             return bitmap; 
  103.         }else{ 
  104.              
  105.             final Handler handler = new Handler(){ 
  106.                 @Override 
  107.                 public void handleMessage(Message msg) { 
  108.                     super.handleMessage(msg); 
  109.                     listener.onImageLoader((Bitmap)msg.obj, url); 
  110.                 } 
  111.             }; 
  112.              
  113.             getThreadPool().execute(new Runnable() { 
  114.                  
  115.                 @Override 
  116.                 public void run() { 
  117.                     Bitmap bitmap = getBitmapFormUrl(url); 
  118.                     Message msg = handler.obtainMessage(); 
  119.                     msg.obj = bitmap; 
  120.                     handler.sendMessage(msg); 
  121.                      
  122.                     try { 
  123.                         //保存在SD卡或者手機目錄 
  124.                         fileUtils.savaBitmap(subUrl, bitmap); 
  125.                     } catch (IOException e) { 
  126.                         e.printStackTrace(); 
  127.                     } 
  128.                      
  129.                     //將Bitmap 加入內存緩存 
  130.                     addBitmapToMemoryCache(subUrl, bitmap); 
  131.                 } 
  132.             }); 
  133.         } 
  134.          
  135.         return null; 
  136.     } 
  137.      
  138.     /** 
  139.      * 獲取Bitmap, 內存中沒有就去手機或者sd卡中獲取，這一步在getView中會調用，比較關鍵的一步 
  140.      * @param url 
  141.      * @return 
  142.      */ 
  143.     public Bitmap showCacheBitmap(String url){ 
  144.         if(getBitmapFromMemCache(url) != null){ 
  145.             return getBitmapFromMemCache(url); 
  146.         }else if(fileUtils.isFileExists(url) && fileUtils.getFileSize(url) != 0){ 
  147.             //從SD卡獲取手機裡面獲取Bitmap 
  148.             Bitmap bitmap = fileUtils.getBitmap(url); 
  149.              
  150.             //將Bitmap 加入內存緩存 
  151.             addBitmapToMemoryCache(url, bitmap); 
  152.             return bitmap; 
  153.         } 
  154.          
  155.         return null; 
  156.     } 
  157.      
  158.      
  159.     /** 
  160.      * 從Url中獲取Bitmap 
  161.      * @param url 
  162.      * @return 
  163.      */ 
  164.     private Bitmap getBitmapFormUrl(String url) { 
  165.         Bitmap bitmap = null; 
  166.         HttpURLConnection con = null; 
  167.         try { 
  168.             URL mImageUrl = new URL(url); 
  169.             con = (HttpURLConnection) mImageUrl.openConnection(); 
  170.             con.setConnectTimeout(10 * 1000); 
  171.             con.setReadTimeout(10 * 1000); 
  172.             con.setDoInput(true); 
  173.             con.setDoOutput(true); 
  174.             bitmap = BitmapFactory.decodeStream(con.getInputStream()); 
  175.         } catch (Exception e) { 
  176.             e.printStackTrace(); 
  177.         } finally { 
  178.             if (con != null) { 
  179.                 con.disconnect(); 
  180.             } 
  181.         } 
  182.         return bitmap; 
  183.     } 
  184.      
  185.     /** 
  186.      * 取消正在下載的任務 
  187.      */ 
  188.     public synchronized void cancelTask() { 
  189.         if(mImageThreadPool != null){ 
  190.             mImageThreadPool.shutdownNow(); 
  191.             mImageThreadPool = null; 
  192.         } 
  193.     } 
  194.      
  195.      
  196.     /** 
  197.      * 異步下載圖片的回調接口 
  198.      * @author len 
  199.      * 
  200.      */ 
  201.     public interface onImageLoaderListener{ 
  202.         void onImageLoader(Bitmap bitmap, String url); 
  203.     } 
  204.      
  205. } 

 

ImageDownLoader中有幾個方法比較重要
首先我們需要重寫sizeOf(String key, Bitmap value)來計算圖片的大小，默認返回圖片的數量
downloadImage(final String url, final onImageLoaderListener listener)先去LruCache查看Image,沒有再去手機緩存中查看，在沒有則開啟線程下載，這裡我們提供了一個回調接口，回調方法中我們將Bitmap和圖片Url作為參數，String subUrl = url.replaceAll("[^\\w]", "") 我在代碼中注釋寫的比較清楚
showCacheBitmap(String url)方法，此方法在Adapter中的getView()當中調用，如果getView()中不調用此方法試試你就知道效果了

ImageAdapter GridView的適配器類，主要是GridView滑動的時候取消下載任務，靜止的時候去下載當前顯示的item的圖片，其他也沒什麼不同了

  1. package com.example.asyncimageloader; 
  2.  
  3. import android.content.Context; 
  4. import android.graphics.Bitmap; 
  5. import android.view.View; 
  6. import android.view.ViewGroup; 
  7. import android.widget.AbsListView; 
  8. import android.widget.AbsListView.OnScrollListener; 
  9. import android.widget.BaseAdapter; 
  10. import android.widget.GridView; 
  11. import android.widget.ImageView; 
  12.  
  13. import com.example.asyncimageloader.ImageDownLoader.onImageLoaderListener; 
  14.  
  15. public class ImageAdapter extends BaseAdapter implements OnScrollListener{ 
  16.     /** 
  17.      * 上下文對象的引用 
  18.      */ 
  19.     private Context context; 
  20.      
  21.     /** 
  22.      * Image Url的數組 
  23.      */ 
  24.     private String [] imageThumbUrls; 
  25.      
  26.     /** 
  27.      * GridView對象的應用 
  28.      */ 
  29.     private GridView mGridView; 
  30.      
  31.     /** 
  32.      * Image 下載器 
  33.      */ 
  34.     private ImageDownLoader mImageDownLoader; 
  35.      
  36.     /** 
  37.      * 記錄是否剛打開程序，用於解決進入程序不滾動屏幕，不會下載圖片的問題。 
  38.      * 參考http://blog.csdn.net/guolin_blog/article/details/9526203#comments 
  39.      */ 
  40.     private boolean isFirstEnter = true; 
  41.      
  42.     /** 
  43.      * 一屏中第一個item的位置 
  44.      */ 
  45.     private int mFirstVisibleItem; 
  46.      
  47.     /** 
  48.      * 一屏中所有item的個數 
  49.      */ 
  50.     private int mVisibleItemCount; 
  51.      
  52.      
  53.     public ImageAdapter(Context context, GridView mGridView, String [] imageThumbUrls){ 
  54.         this.context = context; 
  55.         this.mGridView = mGridView; 
  56.         this.imageThumbUrls = imageThumbUrls; 
  57.         mImageDownLoader = new ImageDownLoader(context); 
  58.         mGridView.setOnScrollListener(this); 
  59.     } 
  60.      
  61.     @Override 
  62.     public void onScrollStateChanged(AbsListView view, int scrollState) { 
  63.         //僅當GridView靜止時才去下載圖片，GridView滑動時取消所有正在下載的任務   
  64.         if(scrollState == AbsListView.OnScrollListener.SCROLL_STATE_IDLE){ 
  65.             showImage(mFirstVisibleItem, mVisibleItemCount); 
  66.         }else{ 
  67.             cancelTask(); 
  68.         } 
  69.          
  70.     } 
  71.  
  72.  
  73.     /** 
  74.      * GridView滾動的時候調用的方法，剛開始顯示GridView也會調用此方法 
  75.      */ 
  76.     @Override 
  77.     public void onScroll(AbsListView view, int firstVisibleItem, 
  78.             int visibleItemCount, int totalItemCount) { 
  79.         mFirstVisibleItem = firstVisibleItem; 
  80.         mVisibleItemCount = visibleItemCount; 
  81.         // 因此在這裡為首次進入程序開啟下載任務。  
  82.         if(isFirstEnter && visibleItemCount > 0){ 
  83.             showImage(mFirstVisibleItem, mVisibleItemCount); 
  84.             isFirstEnter = false; 
  85.         } 
  86.     } 
  87.      
  88.  
  89.     @Override 
  90.     public int getCount() { 
  91.         return imageThumbUrls.length; 
  92.     } 
  93.  
  94.     @Override 
  95.     public Object getItem(int position) { 
  96.         return imageThumbUrls[position]; 
  97.     } 
  98.  
  99.     @Override 
  100.     public long getItemId(int position) { 
  101.         return position; 
  102.     } 
  103.  
  104.     @Override 
  105.     public View getView(int position, View convertView, ViewGroup parent) { 
  106.         ImageView mImageView; 
  107.         final String mImageUrl = imageThumbUrls[position]; 
  108.         if(convertView == null){ 
  109.             mImageView = new ImageView(context); 
  110.         }else{ 
  111.             mImageView = (ImageView) convertView; 
  112.         } 
  113.          
  114.         mImageView.setLayoutParams(new GridView.LayoutParams(150, 150)); 
  115.         mImageView.setScaleType(ImageView.ScaleType.CENTER_INSIDE); 
  116.          
  117.         //給ImageView設置Tag,這裡已經是司空見慣了 
  118.         mImageView.setTag(mImageUrl); 
  119.          
  120.          
  121.         /*******************************去掉下面這幾行試試是什麼效果****************************/ 
  122.         Bitmap bitmap = mImageDownLoader.showCacheBitmap(mImageUrl.replaceAll("[^\\w]", "")); 
  123.         if(bitmap != null){ 
  124.             mImageView.setImageBitmap(bitmap); 
  125.         }else{ 
  126.             mImageView.setImageDrawable(context.getResources().getDrawable(R.drawable.ic_empty)); 
  127.         } 
  128.         /**********************************************************************************/ 
  129.          
  130.          
  131.         return mImageView; 
  132.     } 
  133.      
  134.     /** 
  135.      * 顯示當前屏幕的圖片，先會去查找LruCache，LruCache沒有就去sd卡或者手機目錄查找，在沒有就開啟線程去下載 
  136.      * @param firstVisibleItem 
  137.      * @param visibleItemCount 
  138.      */ 
  139.     private void showImage(int firstVisibleItem, int visibleItemCount){ 
  140.         Bitmap bitmap = null; 
  141.         for(int i=firstVisibleItem; i<firstVisibleItem + visibleItemCount; i++){ 
  142.             String mImageUrl = imageThumbUrls[i]; 
  143.             final ImageView mImageView = (ImageView) mGridView.findViewWithTag(mImageUrl); 
  144.             bitmap = mImageDownLoader.downloadImage(mImageUrl, new onImageLoaderListener() { 
  145.                  
  146.                 @Override 
  147.                 public void onImageLoader(Bitmap bitmap, String url) { 
  148.                     if(mImageView != null && bitmap != null){ 
  149.                         mImageView.setImageBitmap(bitmap); 
  150.                     } 
  151.                      
  152.                 } 
  153.             }); 
  154.              
  155.             //if(bitmap != null){ 
  156.             //  mImageView.setImageBitmap(bitmap); 
  157.             //}else{ 
  158.             //  mImageView.setImageDrawable(context.getResources().getDrawable(R.drawable.ic_empty)); 
  159.             //} 
  160.         } 
  161.     } 
  162.  
  163.     /** 
  164.      * 取消下載任務 
  165.      */ 
  166.     public void cancelTask(){ 
  167.         mImageDownLoader.cancelTask(); 
  168.     } 
  169.  
  170.  
  171. } 

MainActivity 裡面一個GridView，然後提供一個系統菜單來刪除手機上的緩存圖片，直接上代碼，比較簡單所以裡面也沒有注釋

  1. package com.example.asyncimageloader; 
  2.  
  3. import android.app.Activity; 
  4. import android.os.Bundle; 
  5. import android.view.Menu; 
  6. import android.view.MenuItem; 
  7. import android.widget.GridView; 
  8. import android.widget.Toast; 
  9.  
  10. public class MainActivity extends Activity { 
  11.     private GridView mGridView; 
  12.     private String [] imageThumbUrls = Images.imageThumbUrls;  
  13.     private ImageAdapter mImageAdapter; 
  14.     private FileUtils fileUtils; 
  15.  
  16.     @Override 
  17.     protected void onCreate(Bundle savedInstanceState) { 
  18.         super.onCreate(savedInstanceState); 
  19.         setContentView(R.layout.activity_main); 
  20.         fileUtils = new FileUtils(this); 
  21.         mGridView = (GridView) findViewById(R.id.gridView); 
  22.         mImageAdapter = new ImageAdapter(this, mGridView, imageThumbUrls); 
  23.         mGridView.setAdapter(mImageAdapter); 
  24.     } 
  25.  
  26.     @Override 
  27.     protected void onDestroy() { 
  28.         mImageAdapter.cancelTask(); 
  29.         super.onDestroy(); 
  30.     } 
  31.  
  32.     @Override 
  33.     public boolean onCreateOptionsMenu(Menu menu) { 
  34.         super.onCreateOptionsMenu(menu); 
  35.         menu.add("刪除手機中圖片緩存"); 
  36.         return super.onCreateOptionsMenu(menu); 
  37.     } 
  38.  
  39.     @Override 
  40.     public boolean onOptionsItemSelected(MenuItem item) { 
  41.         switch (item.getItemId()) { 
  42.         case 0: 
  43.             fileUtils.deleteFile(); 
  44.             Toast.makeText(getApplication(), "清空緩存成功", Toast.LENGTH_SHORT).show(); 
  45.             break; 
  46.         } 
  47.         return super.onOptionsItemSelected(item); 
  48.     } 
  49.  
  50. } 

 

MainActivity的布局文件

  1. <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
  2.     xmlns:tools="http://schemas.android.com/tools" 
  3.     android:layout_width="match_parent" 
  4.     android:layout_height="match_parent" 
  5.     tools:context=".MainActivity" > 
  6.  
  7.     <GridView 
  8.         android:id="@+id/gridView" 
  9.         android:layout_width="match_parent" 
  10.         android:layout_height="match_parent" 
  11.         android:stretchMode="columnWidth"  
  12.         android:columnWidth="90dip"  
  13.         android:verticalSpacing="10dip" 
  14.         android:horizontalSpacing="10dip" 
  15.         android:cacheColorHint="@android:color/transparent" 
  16.         android:numColumns="auto_fit" > 
  17.     </GridView> 
  18.  
  19. </RelativeLayout> 

因為有網絡操作和對sd卡的操作，所以我們必須加上相對應的權限

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

好了，代碼就全部介紹完了，我們運行下程序看看效果，這裡的效果是用一個線程下載圖片的效果，可能有一些不完整，想看看效果自己動手試試

 

好了，今天的講解到此結束，有疑問的朋友請在下面留言,源碼裡面GridView的布局文件 columnWedth設置了120dip,有點丑，大家可以改成90dip

源碼下載

 

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