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 Android教程網 >> Android技術 >> 關於Android編程 >> Android照片牆應用實現

Android照片牆應用實現

編輯:關於Android編程

照片牆這種功能現在應該算是挺常見了,在很多應用中你都可以經常看到照片牆的身影。它的設計思路其實也非常簡單,用一個GridView控件當作“牆”,然後隨著GridView的滾動將一張張照片貼在“牆”上,這些照片可以是手機本地中存儲的,也可以是從網上下載的。制作類似於這種的功能的應用,有一個非常重要的問題需要考慮,就是圖片資源何時應該釋放。因為隨著GridView的滾動,加載的圖片可能會越來越多,如果沒有一種合理的機制對圖片進行釋放,那麼當圖片達到一定上限時,程序就必然會崩潰。

今天我們照片牆應用的實現,重點也是放在了如何防止由於圖片過多導致程序崩潰上面。主要的核心算法使用了Android中提供的LruCache類,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。

那我們開始動手吧,新建一個Android項目,起名叫PhotoWallDemo,這裡我使用的是Android 4.0的API。

第一個要考慮的問題就是,我們從哪兒去收集這麼多的圖片呢?這裡我從谷歌官方提供的Demo裡將圖片源取了出來,我們就從這些網址中下載圖片,代碼如下所示:

public class Images {

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

圖片源已經有了,現在我們就該考慮在哪裡放置這些圖片了。新建或打開activity_main.xml作為程序的主布局,加入如下代碼:



    

可以看到,我們在這個布局文件中僅加入了一個GridView,這也就是我們程序中的“牆”,所有的圖片都將貼在這個“牆”上。
接著我們定義GridView中每一個子View的布局,新建一個photo_layout.xml布局,加入如下代碼:



    

在每一個子View中我們就簡單使用了一個ImageView來顯示一張圖片。這樣所有的布局就已經定義好了。
接下來新建PhotoWallAdapter做為GridView的適配器,代碼如下所示:

public class PhotoWallAdapter extends ArrayAdapter implements OnScrollListener {

    /**
     * 記錄所有正在下載或等待下載的任務。
     */
    private Set taskCollection;

    /**
     * 圖片緩存技術的核心類,用於緩存所有下載好的圖片,在程序內存達到設定值時會將最少最近使用的圖片移除掉。
     */
    private LruCache mMemoryCache;

    /**
     * GridView的實例
     */
    private GridView mPhotoWall;

    /**
     * 第一張可見圖片的下標
     */
    private int mFirstVisibleItem;

    /**
     * 一屏有多少張圖片可見
     */
    private int mVisibleItemCount;

    /**
     * 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。
     */
    private boolean isFirstEnter = true;

    public PhotoWallAdapter(Context context, int textViewResourceId, String[] objects,
            GridView photoWall) {
        super(context, textViewResourceId, objects);
        mPhotoWall = photoWall;
        taskCollection = new HashSet();
        // 獲取應用程序最大可用內存
        int maxMemory = (int) Runtime.getRuntime().maxMemory();
        int cacheSize = maxMemory / 8;
        // 設置圖片緩存大小為程序最大可用內存的1/8
        mMemoryCache = new LruCache(cacheSize) {
            @Override
            protected int sizeOf(String key, Bitmap bitmap) {
                return bitmap.getByteCount();
            }
        };
        mPhotoWall.setOnScrollListener(this);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        final String url = getItem(position);
        View view;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(R.layout.photo_layout, null);
        } else {
            view = convertView;
        }
        final ImageView photo = (ImageView) view.findViewById(R.id.photo);
        // 給ImageView設置一個Tag,保證異步加載圖片時不會亂序
        photo.setTag(url);
        setImageView(url, photo);
        return view;
    }

    /**
     * 給ImageView設置圖片。首先從LruCache中取出圖片的緩存,設置到ImageView上。如果LruCache中沒有該圖片的緩存,
     * 就給ImageView設置一張默認圖片。
     * 
     * @param imageUrl
     *            圖片的URL地址,用於作為LruCache的鍵。
     * @param imageView
     *            用於顯示圖片的控件。
     */
    private void setImageView(String imageUrl, ImageView imageView) {
        Bitmap bitmap = getBitmapFromMemoryCache(imageUrl);
        if (bitmap != null) {
            imageView.setImageBitmap(bitmap);
        } else {
            imageView.setImageResource(R.drawable.empty_photo);
        }
    }

    /**
     * 將一張圖片存儲到LruCache中。
     * 
     * @param key
     *            LruCache的鍵,這裡傳入圖片的URL地址。
     * @param bitmap
     *            LruCache的鍵,這裡傳入從網絡上下載的Bitmap對象。
     */
    public void addBitmapToMemoryCache(String key, Bitmap bitmap) {
        if (getBitmapFromMemoryCache(key) == null) {
            mMemoryCache.put(key, bitmap);
        }
    }

    /**
     * 從LruCache中獲取一張圖片,如果不存在就返回null。
     * 
     * @param key
     *            LruCache的鍵,這裡傳入圖片的URL地址。
     * @return 對應傳入鍵的Bitmap對象,或者null。
     */
    public Bitmap getBitmapFromMemoryCache(String key) {
        return mMemoryCache.get(key);
    }

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
        // 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
        if (scrollState == SCROLL_STATE_IDLE) {
            loadBitmaps(mFirstVisibleItem, mVisibleItemCount);
        } else {
            cancelAllTasks();
        }
    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount,
            int totalItemCount) {
        mFirstVisibleItem = firstVisibleItem;
        mVisibleItemCount = visibleItemCount;
        // 下載的任務應該由onScrollStateChanged裡調用,但首次進入程序時onScrollStateChanged並不會調用,
        // 因此在這裡為首次進入程序開啟下載任務。
        if (isFirstEnter && visibleItemCount > 0) {
            loadBitmaps(firstVisibleItem, visibleItemCount);
            isFirstEnter = false;
        }
    }

    /**
     * 加載Bitmap對象。此方法會在LruCache中檢查所有屏幕中可見的ImageView的Bitmap對象,
     * 如果發現任何一個ImageView的Bitmap對象不在緩存中,就會開啟異步線程去下載圖片。
     * 
     * @param firstVisibleItem
     *            第一個可見的ImageView的下標
     * @param visibleItemCount
     *            屏幕中總共可見的元素數
     */
    private void loadBitmaps(int firstVisibleItem, int visibleItemCount) {
        try {
            for (int i = firstVisibleItem; i < firstVisibleItem + visibleItemCount; i++) {
                String imageUrl = Images.imageThumbUrls[i];
                Bitmap bitmap = getBitmapFromMemoryCache(imageUrl);
                if (bitmap == null) {
                    BitmapWorkerTask task = new BitmapWorkerTask();
                    taskCollection.add(task);
                    task.execute(imageUrl);
                } else {
                    ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);
                    if (imageView != null && bitmap != null) {
                        imageView.setImageBitmap(bitmap);
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * 取消所有正在下載或等待下載的任務。
     */
    public void cancelAllTasks() {
        if (taskCollection != null) {
            for (BitmapWorkerTask task : taskCollection) {
                task.cancel(false);
            }
        }
    }

    /**
     * 異步下載圖片的任務。
     * 
     * @author guolin
     */
    class BitmapWorkerTask extends AsyncTask {

        /**
         * 圖片的URL地址
         */
        private String imageUrl;

        @Override
        protected Bitmap doInBackground(String... params) {
            imageUrl = params[0];
            // 在後台開始下載圖片
            Bitmap bitmap = downloadBitmap(params[0]);
            if (bitmap != null) {
                // 圖片下載完成後緩存到LrcCache中
                addBitmapToMemoryCache(params[0], bitmap);
            }
            return bitmap;
        }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            super.onPostExecute(bitmap);
            // 根據Tag找到相應的ImageView控件,將下載好的圖片顯示出來。
            ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);
            if (imageView != null && bitmap != null) {
                imageView.setImageBitmap(bitmap);
            }
            taskCollection.remove(this);
        }

        /**
         * 建立HTTP請求,並獲取Bitmap對象。
         * 
         * @param imageUrl
         *            圖片的URL地址
         * @return 解析後的Bitmap對象
         */
        private Bitmap downloadBitmap(String imageUrl) {
            Bitmap bitmap = null;
            HttpURLConnection con = null;
            try {
                URL url = new URL(imageUrl);
                con = (HttpURLConnection) url.openConnection();
                con.setConnectTimeout(5 * 1000);
                con.setReadTimeout(10 * 1000);
                bitmap = BitmapFactory.decodeStream(con.getInputStream());
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                if (con != null) {
                    con.disconnect();
                }
            }
            return bitmap;
        }

    }

}

PhotoWallAdapter是整個照片牆程序中最關鍵的一個類了,這裡我來重點給大家講解一下。首先在PhotoWallAdapter的構造函數中,我們初始化了LruCache類,並設置了最大緩存容量為程序最大可用內存的1/8,接下來又為GridView注冊了一個滾動監聽器。然後在getView()方法中,我們為每個ImageView設置了一個唯一的Tag,這個Tag的作用是為了後面能夠准確地找回這個ImageView,不然異步加載圖片會出現亂序的情況。之後調用了setImageView()方法為ImageView設置一張圖片,這個方法首先會從LruCache緩存中查找是否已經緩存了這張圖片,如果成功找到則將緩存中的圖片顯示在ImageView上,否則就顯示一張默認的空圖片。
看了半天,那到底是在哪裡下載圖片的呢?這是在GridView的滾動監聽器中進行的,在onScrollStateChanged()方法中,我們對GridView的滾動狀態進行了判斷,如果當前GridView是靜止的,則調用loadBitmaps()方法去下載圖片,如果GridView正在滾動,則取消掉所有下載任務,這樣可以保證GridView滾動的流暢性。在loadBitmaps()方法中,我們為屏幕上所有可見的GridView子元素開啟了一個線程去執行下載任務,下載成功後將圖片存儲到LruCache當中,然後通過Tag找到相應的ImageView控件,把下載好的圖片顯示出來。
由於我們使用了LruCache來緩存圖片,所以不需要擔心內存溢出的情況,當LruCache中存儲圖片的總大小達到容量上限的時候,會自動把最近最少使用的圖片從緩存中移除。
最後新建或打開MainActivity作為程序的主Activity,代碼如下所示:

public class MainActivity extends Activity {

    /**
     * 用於展示照片牆的GridView
     */
    private GridView mPhotoWall;

    /**
     * GridView的適配器
     */
    private PhotoWallAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mPhotoWall = (GridView) findViewById(R.id.photo_wall);
        adapter = new PhotoWallAdapter(this, 0, Images.imageThumbUrls, mPhotoWall);
        mPhotoWall.setAdapter(adapter);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // 退出程序時結束所有的下載任務
        adapter.cancelAllTasks();
    }

}

MainActivity中的代碼非常簡單,沒什麼需要說明的了,在Activity被銷毀時取消掉了所有的下載任務,避免程序在後台耗費流量。另外由於我們使用了網絡功能,別忘了在AndroidManifest.xml中加入網絡權限的聲明。

現在可以運行一下程序了,效果如下圖所示:

 

這裡寫圖片描述

 

可以看到,滾動照片牆,會異步加載圖片到相應的ImageView上。隨著加載圖片的增多,會釋放掉一些之前加載過的圖片,你多滾動幾次就可以看得出了。另外為了能讓大家明顯看出圖片的釋放情況,我在這個程序中沒有使用本地緩存,所有被釋放掉的圖片再次顯示需要從網絡上再下載一遍。在實際的項目中配合適當的本地緩存效果會更好。

打開DDMS,我們可以發現,由於有LruCache幫我們管理圖片緩存,不管如何滾動照片牆,程序內存始終會保持在一個合理的范圍內。

 

這裡寫圖片描述
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