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Android实现照片墙效果的实例代码

来源:互联网 收集:自由互联 发布时间:2021-08-03
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

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、多图解决方案 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:

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", 
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 "https://lh6.googleusercontent.com/-PYvLVdvXywk/URqunwd8hfI/AAAAAAAAAbs/qiMwgkFvf6I/s160-c/Fitzgerald%252520Streaks.jpg", 
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 "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", 
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 "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:

<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:

<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:

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); 
 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:

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。

本篇文章的重点在于如何对图片进行更好的回收,因此照片墙只是简单地使用GridView进行了展示,想要看更酷更炫的照片墙效果的朋友,可以参考我后面的一篇文章Android瀑布流照片墙实现,体验不规则排列的美感。

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

以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持自由互联。

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