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Android照片墙应用实现,再多的图片也不怕崩溃

 
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照片墙这种功能现在应该算是挺常见了,在很多应用中你都可以经常看到照片墙的身影。它的设计思路其实也非常简单,用一个GridView控件当作“墙”,然后随着GridView的滚动将一张张照片贴在“墙”上,这些照片可以是手机本地中存储的,也可以是从网上下载的。制作类似于这种的功能的应用,有一个非常重要的问题需要考虑,就是图片资源何时应该释放。因为随着GridView的滚动,加载的图片可能会越来越多,如果没有一种合理的机制对图片进行释放,那么当图片达到一定上限时,程序就必然会崩溃。

今天我们照片墙应用的实现,重点也是放在了如何防止由于图片过多导致程序崩溃上面。主要的核心算法使用了Android中提供的LruCache类,这个类是3.1版本中提供的,如果你是在更早的Android版本中开发,则需要导入android-support-v4的jar包。

关于LruCache用法的详细讲解,可以参考Android高效加载大图、多图方案,有效避免程序OOM

那我们开始动手吧,新建一个Android项目,起名叫PhotoWallDemo,这里我使用的是Android 4.0的API。

第一个要考虑的问题就是,我们从哪儿去收集这么多的图片呢?这里我从谷歌官方提供的Demo里将图片源取了出来,我们就从这些网址中下载图片,代码如下所示:

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

图片源已经有了,现在我们就该考虑在哪里放置这些图片了。新建或打开activity_main.xml作为程序的主布局,加入如下代码:

[html] view plain copy
  1. <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  
  2.     xmlns:tools="http://schemas.android.com/tools"  
  3.     android:layout_width="wrap_content"  
  4.     android:layout_height="wrap_content" >  
  5.       
  6.     <GridView   
  7.         android:id="@+id/photo_wall"  
  8.         android:layout_width="match_parent"  
  9.         android:layout_height="wrap_content"  
  10.         android:columnWidth="90dip"  
  11.         android:stretchMode="columnWidth"  
  12.         android:numColumns="auto_fit"  
  13.         android:verticalSpacing="10dip"  
  14.         android:gravity="center"  
  15.         ></GridView>  
  16.       
  17. </LinearLayout>  

 

可以看到,我们在这个布局文件中仅加入了一个GridView,这也就是我们程序中的“墙”,所有的图片都将贴在这个“墙”上。

接着我们定义GridView中每一个子View的布局,新建一个photo_layout.xml布局,加入如下代码:

[html] view plain copy
  1. <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  
  2.     xmlns:tools="http://schemas.android.com/tools"  
  3.     android:layout_width="wrap_content"  
  4.     android:layout_height="wrap_content" >  
  5.   
  6.     <ImageView   
  7.         android:id="@+id/photo"  
  8.         android:layout_width="90dip"  
  9.         android:layout_height="90dip"  
  10.         android:src="@drawable/empty_photo"  
  11.         android:layout_centerInParent="true"  
  12.         />  
  13.   
  14. </RelativeLayout>  

在每一个子View中我们就简单使用了一个ImageView来显示一张图片。这样所有的布局就已经定义好了。

 

接下来新建PhotoWallAdapter做为GridView的适配器,代码如下所示:

[java] view plain copy
  1. public class PhotoWallAdapter extends ArrayAdapter<String> implements OnScrollListener {  
  2.   
  3.     /** 
  4.      * 记录所有正在下载或等待下载的任务。 
  5.      */  
  6.     private Set<BitmapWorkerTask> taskCollection;  
  7.   
  8.     /** 
  9.      * 图片缓存技术的核心类,用于缓存所有下载好的图片,在程序内存达到设定值时会将最少最近使用的图片移除掉。 
  10.      */  
  11.     private LruCache<String, Bitmap> mMemoryCache;  
  12.   
  13.     /** 
  14.      * GridView的实例 
  15.      */  
  16.     private GridView mPhotoWall;  
  17.   
  18.     /** 
  19.      * 第一张可见图片的下标 
  20.      */  
  21.     private int mFirstVisibleItem;  
  22.   
  23.     /** 
  24.      * 一屏有多少张图片可见 
  25.      */  
  26.     private int mVisibleItemCount;  
  27.   
  28.     /** 
  29.      * 记录是否刚打开程序,用于解决进入程序不滚动屏幕,不会下载图片的问题。 
  30.      */  
  31.     private boolean isFirstEnter = true;  
  32.   
  33.     public PhotoWallAdapter(Context context, int textViewResourceId, String[] objects,  
  34.             GridView photoWall) {  
  35.         super(context, textViewResourceId, objects);  
  36.         mPhotoWall = photoWall;  
  37.         taskCollection = new HashSet<BitmapWorkerTask>();  
  38.         // 获取应用程序最大可用内存  
  39.         int maxMemory = (int) Runtime.getRuntime().maxMemory();  
  40.         int cacheSize = maxMemory / 8;  
  41.         // 设置图片缓存大小为程序最大可用内存的1/8  
  42.         mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {  
  43.             @Override  
  44.             protected int sizeOf(String key, Bitmap bitmap) {  
  45.                 return bitmap.getByteCount();  
  46.             }  
  47.         };  
  48.         mPhotoWall.setOnScrollListener(this);  
  49.     }  
  50.   
  51.     @Override  
  52.     public View getView(int position, View convertView, ViewGroup parent) {  
  53.         final String url = getItem(position);  
  54.         View view;  
  55.         if (convertView == null) {  
  56.             view = LayoutInflater.from(getContext()).inflate(R.layout.photo_layout, null);  
  57.         } else {  
  58.             view = convertView;  
  59.         }  
  60.         final ImageView photo = (ImageView) view.findViewById(R.id.photo);  
  61.         // 给ImageView设置一个Tag,保证异步加载图片时不会乱序  
  62.         photo.setTag(url);  
  63.         setImageView(url, photo);  
  64.         return view;  
  65.     }  
  66.   
  67.     /** 
  68.      * 给ImageView设置图片。首先从LruCache中取出图片的缓存,设置到ImageView上。如果LruCache中没有该图片的缓存, 
  69.      * 就给ImageView设置一张默认图片。 
  70.      *  
  71.      * @param imageUrl 
  72.      *            图片的URL地址,用于作为LruCache的键。 
  73.      * @param imageView 
  74.      *            用于显示图片的控件。 
  75.      */  
  76.     private void setImageView(String imageUrl, ImageView imageView) {  
  77.         Bitmap bitmap = getBitmapFromMemoryCache(imageUrl);  
  78.         if (bitmap != null) {  
  79.             imageView.setImageBitmap(bitmap);  
  80.         } else {  
  81.             imageView.setImageResource(R.drawable.empty_photo);  
  82.         }  
  83.     }  
  84.   
  85.     /** 
  86.      * 将一张图片存储到LruCache中。 
  87.      *  
  88.      * @param key 
  89.      *            LruCache的键,这里传入图片的URL地址。 
  90.      * @param bitmap 
  91.      *            LruCache的键,这里传入从网络上下载的Bitmap对象。 
  92.      */  
  93.     public void addBitmapToMemoryCache(String key, Bitmap bitmap) {  
  94.         if (getBitmapFromMemoryCache(key) == null) {  
  95.             mMemoryCache.put(key, bitmap);  
  96.         }  
  97.     }  
  98.   
  99.     /** 
  100.      * 从LruCache中获取一张图片,如果不存在就返回null。 
  101.      *  
  102.      * @param key 
  103.      *            LruCache的键,这里传入图片的URL地址。 
  104.      * @return 对应传入键的Bitmap对象,或者null。 
  105.      */  
  106.     public Bitmap getBitmapFromMemoryCache(String key) {  
  107.         return mMemoryCache.get(key);  
  108.     }  
  109.   
  110.     @Override  
  111.     public void onScrollStateChanged(AbsListView view, int scrollState) {  
  112.         // 仅当GridView静止时才去下载图片,GridView滑动时取消所有正在下载的任务  
  113.         if (scrollState == SCROLL_STATE_IDLE) {  
  114.             loadBitmaps(mFirstVisibleItem, mVisibleItemCount);  
  115.         } else {  
  116.             cancelAllTasks();  
  117.         }  
  118.     }  
  119.   
  120.     @Override  
  121.     public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount,  
  122.             int totalItemCount) {  
  123.         mFirstVisibleItem = firstVisibleItem;  
  124.         mVisibleItemCount = visibleItemCount;  
  125.         // 下载的任务应该由onScrollStateChanged里调用,但首次进入程序时onScrollStateChanged并不会调用,  
  126.         // 因此在这里为首次进入程序开启下载任务。  
  127.         if (isFirstEnter && visibleItemCount > 0) {  
  128.             loadBitmaps(firstVisibleItem, visibleItemCount);  
  129.             isFirstEnter = false;  
  130.         }  
  131.     }  
  132.   
  133.     /** 
  134.      * 加载Bitmap对象。此方法会在LruCache中检查所有屏幕中可见的ImageView的Bitmap对象, 
  135.      * 如果发现任何一个ImageView的Bitmap对象不在缓存中,就会开启异步线程去下载图片。 
  136.      *  
  137.      * @param firstVisibleItem 
  138.      *            第一个可见的ImageView的下标 
  139.      * @param visibleItemCount 
  140.      *            屏幕中总共可见的元素数 
  141.      */  
  142.     private void loadBitmaps(int firstVisibleItem, int visibleItemCount) {  
  143.         try {  
  144.             for (int i = firstVisibleItem; i < firstVisibleItem + visibleItemCount; i++) {  
  145.                 String imageUrl = Images.imageThumbUrls[i];  
  146.                 Bitmap bitmap = getBitmapFromMemoryCache(imageUrl);  
  147.                 if (bitmap == null) {  
  148.                     BitmapWorkerTask task = new BitmapWorkerTask();  
  149.                     taskCollection.add(task);  
  150.                     task.execute(imageUrl);  
  151.                 } else {  
  152.                     ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);  
  153.                     if (imageView != null && bitmap != null) {  
  154.                         imageView.setImageBitmap(bitmap);  
  155.                     }  
  156.                 }  
  157.             }  
  158.         } catch (Exception e) {  
  159.             e.printStackTrace();  
  160.         }  
  161.     }  
  162.   
  163.     /** 
  164.      * 取消所有正在下载或等待下载的任务。 
  165.      */  
  166.     public void cancelAllTasks() {  
  167.         if (taskCollection != null) {  
  168.             for (BitmapWorkerTask task : taskCollection) {  
  169.                 task.cancel(false);  
  170.             }  
  171.         }  
  172.     }  
  173.   
  174.     /** 
  175.      * 异步下载图片的任务。 
  176.      *  
  177.      * @author guolin 
  178.      */  
  179.     class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap> {  
  180.   
  181.         /** 
  182.          * 图片的URL地址 
  183.          */  
  184.         private String imageUrl;  
  185.   
  186.         @Override  
  187.         protected Bitmap doInBackground(String... params) {  
  188.             imageUrl = params[0];  
  189.             // 在后台开始下载图片  
  190.             Bitmap bitmap = downloadBitmap(params[0]);  
  191.             if (bitmap != null) {  
  192.                 // 图片下载完成后缓存到LrcCache中  
  193.                 addBitmapToMemoryCache(params[0], bitmap);  
  194.             }  
  195.             return bitmap;  
  196.         }  
  197.   
  198.         @Override  
  199.         protected void onPostExecute(Bitmap bitmap) {  
  200.             super.onPostExecute(bitmap);  
  201.             // 根据Tag找到相应的ImageView控件,将下载好的图片显示出来。  
  202.             ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);  
  203.             if (imageView != null && bitmap != null) {  
  204.                 imageView.setImageBitmap(bitmap);  
  205.             }  
  206.             taskCollection.remove(this);  
  207.         }  
  208.   
  209.         /** 
  210.          * 建立HTTP请求,并获取Bitmap对象。 
  211.          *  
  212.          * @param imageUrl 
  213.          *            图片的URL地址 
  214.          * @return 解析后的Bitmap对象 
  215.          */  
  216.         private Bitmap downloadBitmap(String imageUrl) {  
  217.             Bitmap bitmap = null;  
  218.             HttpURLConnection con = null;  
  219.             try {  
  220.                 URL url = new URL(imageUrl);  
  221.                 con = (HttpURLConnection) url.openConnection();  
  222.                 con.setConnectTimeout(5 * 1000);  
  223.                 con.setReadTimeout(10 * 1000);  
  224.                 bitmap = BitmapFactory.decodeStream(con.getInputStream());  
  225.             } catch (Exception e) {  
  226.                 e.printStackTrace();  
  227.             } finally {  
  228.                 if (con != null) {  
  229.                     con.disconnect();  
  230.                 }  
  231.             }  
  232.             return bitmap;  
  233.         }  
  234.   
  235.     }  
  236.   
  237. }  

PhotoWallAdapter是整个照片墙程序中最关键的一个类了,这里我来重点给大家讲解一下。首先在PhotoWallAdapter的构造函数中,我们初始化了LruCache类,并设置了最大缓存容量为程序最大可用内存的1/8,接下来又为GridView注册了一个滚动监听器。然后在getView()方法中,我们为每个ImageView设置了一个唯一的Tag,这个Tag的作用是为了后面能够准确地找回这个ImageView,不然异步加载图片会出现乱序的情况。之后调用了setImageView()方法为ImageView设置一张图片,这个方法首先会从LruCache缓存中查找是否已经缓存了这张图片,如果成功找到则将缓存中的图片显示在ImageView上,否则就显示一张默认的空图片。

 

看了半天,那到底是在哪里下载图片的呢?这是在GridView的滚动监听器中进行的,在onScrollStateChanged()方法中,我们对GridView的滚动状态进行了判断,如果当前GridView是静止的,则调用loadBitmaps()方法去下载图片,如果GridView正在滚动,则取消掉所有下载任务,这样可以保证GridView滚动的流畅性。在loadBitmaps()方法中,我们为屏幕上所有可见的GridView子元素开启了一个线程去执行下载任务,下载成功后将图片存储到LruCache当中,然后通过Tag找到相应的ImageView控件,把下载好的图片显示出来。

由于我们使用了LruCache来缓存图片,所以不需要担心内存溢出的情况,当LruCache中存储图片的总大小达到容量上限的时候,会自动把最近最少使用的图片从缓存中移除。

最后新建或打开MainActivity作为程序的主Activity,代码如下所示:

[java] view plain copy
  1. public class MainActivity extends Activity {  
  2.   
  3.     /** 
  4.      * 用于展示照片墙的GridView 
  5.      */  
  6.     private GridView mPhotoWall;  
  7.   
  8.     /** 
  9.      * GridView的适配器 
  10.      */  
  11.     private PhotoWallAdapter adapter;  
  12.   
  13.     @Override  
  14.     protected void onCreate(Bundle savedInstanceState) {  
  15.         super.onCreate(savedInstanceState);  
  16.         setContentView(R.layout.activity_main);  
  17.         mPhotoWall = (GridView) findViewById(R.id.photo_wall);  
  18.         adapter = new PhotoWallAdapter(this0, Images.imageThumbUrls, mPhotoWall);  
  19.         mPhotoWall.setAdapter(adapter);  
  20.     }  
  21.   
  22.     @Override  
  23.     protected void onDestroy() {  
  24.         super.onDestroy();  
  25.         // 退出程序时结束所有的下载任务  
  26.         adapter.cancelAllTasks();  
  27.     }  
  28.   
  29. }  

MainActivity中的代码非常简单,没什么需要说明的了,在Activity被销毁时取消掉了所有的下载任务,避免程序在后台耗费流量。另外由于我们使用了网络功能,别忘了在AndroidManifest.xml中加入网络权限的声明。

 

现在可以运行一下程序了,效果如下图所示:

                                          

可以看到,滚动照片墙,会异步加载图片到相应的ImageView上。随着加载图片的增多,会释放掉一些之前加载过的图片,你多滚动几次就可以看得出了。另外为了能让大家明显看出图片的释放情况,我在这个程序中没有使用本地缓存,所有被释放掉的图片再次显示需要从网络上再下载一遍。在实际的项目中配合适当的本地缓存效果会更好。

打开DDMS,我们可以发现,由于有LruCache帮我们管理图片缓存,不管如何滚动照片墙,程序内存始终会保持在一个合理的范围内。

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