网站建设开发怎么样,太原网络营销外包,网站开发动态,农机局网站建设方案照片墙这种功能现在应该算是挺常见了#xff0c;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#xff0c;用一个GridView控件当作“墙”#xff0c;然后随着GridView的滚动将一张张照片贴在“墙”上#xff0c;这些照片可以是手机本地中存储的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, 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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:androidhttp://schemas.android.com/apk/res/android xmlns:toolshttp://schemas.android.com/tools android:layout_widthwrap_content android:layout_heightwrap_content GridView android:idid/photo_wall android:layout_widthmatch_parent android:layout_heightwrap_content android:columnWidth90dip android:stretchModecolumnWidth android:numColumnsauto_fit android:verticalSpacing10dip android:gravitycenter /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:androidhttp://schemas.android.com/apk/res/android xmlns:toolshttp://schemas.android.com/tools android:layout_widthwrap_content android:layout_heightwrap_content ImageView android:idid/photo android:layout_width90dip android:layout_height90dip android:srcdrawable/empty_photo android:layout_centerInParenttrue / /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 ArrayAdapterString implements OnScrollListener { /** * 记录所有正在下载或等待下载的任务。 */ private SetBitmapWorkerTask 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 LruCacheString, 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 HashSetBitmapWorkerTask(); // 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 LruCacheString, 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 AsyncTaskString, 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 [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。 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。 本文转自农夫山泉别墅博客园博客原文链接http://www.cnblogs.com/yaowen/p/6347857.html如需转载请自行联系原作者
