效果如下图所示：
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：
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", 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"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", 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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。
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。