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Android照片墙应用实现,再多的图片也不怕崩溃

2016-02-01 15:13 411 查看
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
plaincopy

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", };

}

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
plaincopy

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="wrap_content"

android:layout_height="wrap_content" >

<GridView

android:id="@+id/photo_wall"

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:columnWidth="90dip"

android:stretchMode="columnWidth"

android:numColumns="auto_fit"

android:verticalSpacing="10dip"

android:gravity="center"

></GridView>

</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
plaincopy

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="wrap_content"

android:layout_height="wrap_content" >

<ImageView

android:id="@+id/photo"

android:layout_width="90dip"

android:layout_height="90dip"

android:src="@drawable/empty_photo"

android:layout_centerInParent="true"

/>

</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
plaincopy

public class PhotoWallAdapter extends ArrayAdapter<String> implements OnScrollListener {

/**

* 记录所有正在下载或等待下载的任务。

*/

private Set<BitmapWorkerTask> taskCollection;

/**

* 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。

*/

private LruCache<String, Bitmap> mMemoryCache;

/**

* GridView的实例

*/

private GridView mPhotoWall;

/**

* 第一张可见图片的下标

*/

private int mFirstVisibleItem;

/**

* 一屏有多少张图片可见

*/

private int mVisibleItemCount;

/**

* 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。

*/

private boolean isFirstEnter = true;

public PhotoWallAdapter(Context context, int textViewResourceId, String[] objects,

GridView photoWall) {

super(context, textViewResourceId, objects);

mPhotoWall = photoWall;

taskCollection = new HashSet<BitmapWorkerTask>();

// 获取应用程序最大可用内存

int maxMemory = (int) Runtime.getRuntime().maxMemory();

int cacheSize = maxMemory / 8;

// 设置图片缓存大小为程序最大可用内存的1/8

mMemoryCache = new LruCache<String, Bitmap>(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);

// 给ImageView设置一个Tag,保证异步加载图片时不会乱序

photo.setTag(url);

setImageView(url, photo);

return view;

}

/**

* 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,

* 就给ImageView设置一张默认图片。

*

* @param imageUrl

* 图片的URL地址,用于作为LruCache的键。

* @param imageView

* 用于显示图片的控件。

*/

private void setImageView(String imageUrl, ImageView imageView) {

Bitmap bitmap = getBitmapFromMemoryCache(imageUrl);

if (bitmap != null) {

imageView.setImageBitmap(bitmap);

} else {

imageView.setImageResource(R.drawable.empty_photo);

}

}

/**

* 将一张图片存储到LruCache中。

*

* @param key

* LruCache的键,这里传入图片的URL地址。

* @param bitmap

* LruCache的键,这里传入从网络上下载的Bitmap对象。

*/

public void addBitmapToMemoryCache(String key, Bitmap bitmap) {

if (getBitmapFromMemoryCache(key) == null) {

mMemoryCache.put(key, bitmap);

}

}

/**

* 从LruCache中获取一张图片,如果不存在就返回null。

*

* @param key

* LruCache的键,这里传入图片的URL地址。

* @return 对应传入键的Bitmap对象,或者null。

*/

public Bitmap getBitmapFromMemoryCache(String key) {

return mMemoryCache.get(key);

}

@Override

public void onScrollStateChanged(AbsListView view, int scrollState) {

// 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

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,

// 因此在这里为首次进入程序开启下载任务。

if (isFirstEnter && visibleItemCount > 0) {

loadBitmaps(firstVisibleItem, visibleItemCount);

isFirstEnter = false;

}

}

/**

* 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,

* 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。

*

* @param firstVisibleItem

* 第一个可见的ImageView的下标

* @param visibleItemCount

* 屏幕中总共可见的元素数

*/

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();

}

}

/**

* 取消所有正在下载或等待下载的任务。

*/

public void cancelAllTasks() {

if (taskCollection != null) {

for (BitmapWorkerTask task : taskCollection) {

task.cancel(false);

}

}

}

/**

* 异步下载图片的任务。

*

* @author guolin

*/

class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap> {

/**

* 图片的URL地址

*/

private String imageUrl;

@Override

protected Bitmap doInBackground(String... params) {

imageUrl = params[0];

// 在后台开始下载图片

Bitmap bitmap = downloadBitmap(params[0]);

if (bitmap != null) {

// 图片下载完成后缓存到LrcCache中

addBitmapToMemoryCache(params[0], bitmap);

}

return bitmap;

}

@Override

protected void onPostExecute(Bitmap bitmap) {

super.onPostExecute(bitmap);

// 根据Tag找到相应的ImageView控件,将下载好的图片显示出来。

ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);

if (imageView != null && bitmap != null) {

imageView.setImageBitmap(bitmap);

}

taskCollection.remove(this);

}

/**

* 建立HTTP请求,并获取Bitmap对象。

*

* @param imageUrl

* 图片的URL地址

* @return 解析后的Bitmap对象

*/

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);

con.setDoInput(true);

con.setDoOutput(true);

bitmap = BitmapFactory.decodeStream(con.getInputStream());

} catch (Exception e) {

e.printStackTrace();

} finally {

if (con != null) {

con.disconnect();

}

}

return bitmap;

}

}

}

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
plaincopy

public class MainActivity extends Activity {

/**

* 用于展示照片墙的GridView

*/

private GridView mPhotoWall;

/**

* GridView的适配器

*/

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();

// 退出程序时结束所有的下载任务

adapter.cancelAllTasks();

}

}

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。

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