Android照片墙应用实现,再多的图片也不怕崩溃

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][/java] view plaincopy

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

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][/html] view plaincopy

  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.     <GridView
  6.         android:id=”@+id/photo_wall”
  7.         android:layout_width=”match_parent”
  8.         android:layout_height=”wrap_content”
  9.         android:columnWidth=”90dip”
  10.         android:stretchMode=”columnWidth”
  11.         android:numColumns=”auto_fit”
  12.         android:verticalSpacing=”10dip”
  13.         android:gravity=”center”
  14.         ></GridView>
  15. </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][/html] view plaincopy

  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.     <ImageView
  6.         android:id=”@+id/photo”
  7.         android:layout_width=”90dip”
  8.         android:layout_height=”90dip”
  9.         android:src=”@drawable/empty_photo”
  10.         android:layout_centerInParent=”true”
  11.         />
  12. </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][/java] view plaincopy

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

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][/java] view plaincopy

  1. public class MainActivity extends Activity {
  2.     /**
  3.      * 用于展示照片墙的GridView
  4.      */
  5.     private GridView mPhotoWall;
  6.     /**
  7.      * GridView的适配器
  8.      */
  9.     private PhotoWallAdapter adapter;
  10.     @Override
  11.     protected void onCreate(Bundle savedInstanceState) {
  12.         super.onCreate(savedInstanceState);
  13.         setContentView(R.layout.activity_main);
  14.         mPhotoWall = (GridView) findViewById(R.id.photo_wall);
  15.         adapter = new PhotoWallAdapter(this, 0, Images.imageThumbUrls, mPhotoWall);
  16.         mPhotoWall.setAdapter(adapter);
  17.     }
  18.     @Override
  19.     protected void onDestroy() {
  20.         super.onDestroy();
  21.         // 退出程序时结束所有的下载任务
  22.         adapter.cancelAllTasks();
  23.     }
  24. }

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。

好了,今天的讲解到此结束,有疑问的朋友请在下面留言。

源码下载,请点击这里

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