LruCache为GridView异步加载大量网络图片

MainActivity如下:

import android.os.Bundle;
import android.widget.GridView;
import android.app.Activity;
/**
* Demo描述:
* 在GridView中采用LruCache异步加载大量图片,避免OOM
*
* 学习资料:
* http://blog.csdn.net/guolin_blog/article/details/9526203
* Thank you very much
*/
public class MainActivity extends Activity {
private GridView mGridView;
private GridViewAdapter mGridViewAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
init();
}
private void init(){
mGridView = (GridView) findViewById(R.id.gridView);
mGridViewAdapter = new GridViewAdapter(this, 0, ImagesUrl.Urls, mGridView);
mGridView.setAdapter(mGridViewAdapter);
}
//取消所有的下载任务
@Override
protected void onDestroy() {
super.onDestroy();
mGridViewAdapter.cancelAllTasks();
}

GridViewAdapter如下:

import java.net.HttpURLConnection;
import java.net.URL;
import java.util.HashSet;
import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.support.v4.util.LruCache;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AbsListView;
import android.widget.AbsListView.OnScrollListener;
import android.widget.ArrayAdapter;
import android.widget.GridView;
import android.widget.ImageView;
/**
* LruCache的流程分析:
* 我们从第一次进入应用的情况下开始
* 1 依据图片的Url从LruCache缓存中取图片.
* 若图片存在缓存中,则显示该图片;否则显示默认图片
* 2 因为是第一次进入该界面所以会执行:
* loadBitmaps(firstVisibleItem, visibleItemCount);
* 我们从loadBitmaps()方法作为切入点,继续往下梳理
* 3 尝试从LruCache缓存中取图片.如果在显示即可,否则进入4
* 4 开启一个异步任务下载图片.下载完成后显示图片,并且将
* 该图片存入LruCache缓存中
*
* 在停止滑动时,会调用loadBitmaps(firstVisibleItem, visibleItemCount)
* 情况与上类似
*/
@SuppressLint("NewApi")
public class GridViewAdapter extends ArrayAdapter<String> {
private GridView mGridView;
//图片缓存类
private LruCache<String, Bitmap> mLruCache;
//记录所有正在下载或等待下载的任务
private HashSet<DownloadBitmapAsyncTask> mDownloadBitmapAsyncTaskHashSet;
//GridView中可见的第一张图片的下标
private int mFirstVisibleItem;
//GridView中可见的图片的数量
private int mVisibleItemCount;
//记录是否是第一次进入该界面
private boolean isFirstEnterThisActivity = true;
public GridViewAdapter(Context context, int textViewResourceId,String[] objects, GridView gridView) {
super(context, textViewResourceId, objects);
mGridView = gridView;
mGridView.setOnScrollListener(new ScrollListenerImpl());
mDownloadBitmapAsyncTaskHashSet = new HashSet<DownloadBitmapAsyncTask>();
// 获取应用程序最大可用内存
int maxMemory = (int) Runtime.getRuntime().maxMemory();
// 设置图片缓存大小为maxMemory的1/6
int cacheSize = maxMemory/6;
mLruCache = new LruCache<String, Bitmap>(cacheSize) {
@Override
protected int sizeOf(String key, Bitmap bitmap) {
return bitmap.getByteCount();
}
};
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
String url = getItem(position);
View view;
if (convertView == null) {
view = LayoutInflater.from(getContext()).inflate(R.layout.gridview_item, null);
} else {
view = convertView;
}
ImageView imageView = (ImageView) view.findViewById(R.id.imageView);
//为该ImageView设置一个Tag,防止图片错位
imageView.setTag(url);
//为该ImageView设置显示的图片
setImageForImageView(url, imageView);
return view;
}
/**
* 为ImageView设置图片(Image)
* 1 从缓存中获取图片
* 2 若图片不在缓存中则为其设置默认图片
*/
private void setImageForImageView(String imageUrl, ImageView imageView) {
Bitmap bitmap = getBitmapFromLruCache(imageUrl);
if (bitmap != null) {
imageView.setImageBitmap(bitmap);
} else {
imageView.setImageResource(R.drawable.default_image);
}
}
/**
* 将图片存储到LruCache
*/
public void addBitmapToLruCache(String key, Bitmap bitmap) {
if (getBitmapFromLruCache(key) == null) {
mLruCache.put(key, bitmap);
}
}
/**
* 从LruCache缓存获取图片
*/
public Bitmap getBitmapFromLruCache(String key) {
return mLruCache.get(key);
}
/**
* 为GridView的item加载图片
*
* @param firstVisibleItem
* GridView中可见的第一张图片的下标
*
* @param visibleItemCount
* GridView中可见的图片的数量
*
*/
private void loadBitmaps(int firstVisibleItem, int visibleItemCount) {
try {
for (int i = firstVisibleItem; i < firstVisibleItem + visibleItemCount; i++) {
String imageUrl = ImagesUrl.Urls[i];
Bitmap bitmap = getBitmapFromLruCache(imageUrl);
if (bitmap == null) {
DownloadBitmapAsyncTask downloadBitmapAsyncTask = new DownloadBitmapAsyncTask();
mDownloadBitmapAsyncTaskHashSet.add(downloadBitmapAsyncTask);
downloadBitmapAsyncTask.execute(imageUrl);
} else {
//依据Tag找到对应的ImageView显示图片
ImageView imageView = (ImageView) mGridView.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 (mDownloadBitmapAsyncTaskHashSet != null) {
for (DownloadBitmapAsyncTask task : mDownloadBitmapAsyncTaskHashSet) {
task.cancel(false);
}
}
}
private class ScrollListenerImpl implements OnScrollListener{
/**
*
* 我们的本意是通过onScrollStateChanged获知:每次GridView停止滑动时加载图片
* 但是存在一个特殊情况:
* 当第一次入应用的时候,此时并没有滑动屏幕的操作即不会调用onScrollStateChanged,但应该加载图片.
* 所以在此处做一个特殊的处理.
* 即代码:
* if (isFirstEnterThisActivity && visibleItemCount > 0) {
* loadBitmaps(firstVisibleItem, visibleItemCount);
* isFirstEnterThisActivity = false;
* }
*
* ------------------------------------------------------------
*
* 其余的都是正常情况.
* 所以我们需要不断保存:firstVisibleItem和visibleItemCount
* 从而便于中在onScrollStateChanged()判断当停止滑动时加载图片
*
*/
@Override
public void onScroll(AbsListView view, int firstVisibleItem,int visibleItemCount, int totalItemCount) {
mFirstVisibleItem = firstVisibleItem;
mVisibleItemCount = visibleItemCount;
if (isFirstEnterThisActivity && visibleItemCount > 0) {
loadBitmaps(firstVisibleItem, visibleItemCount);
isFirstEnterThisActivity = false;
}
}
/**
* GridView停止滑动时下载图片
* 其余情况下取消所有正在下载或者等待下载的任务
*/
@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
if (scrollState == SCROLL_STATE_IDLE) {
loadBitmaps(mFirstVisibleItem, mVisibleItemCount);
} else {
cancelAllTasks();
}
}
}
/**
* 下载图片的异步任务
*/
class DownloadBitmapAsyncTask extends AsyncTask<String, Void, Bitmap> {
private String imageUrl;
@Override
protected Bitmap doInBackground(String... params) {
imageUrl = params[0];
Bitmap bitmap = downloadBitmap(params[0]);
if (bitmap != null) {
//下载完后,将其缓存到LrcCache
addBitmapToLruCache(params[0], bitmap);
}
return bitmap;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
super.onPostExecute(bitmap);
//下载完成后,找到其对应的ImageView显示图片
ImageView imageView = (ImageView) mGridView.findViewWithTag(imageUrl);
if (imageView != null && bitmap != null) {
imageView.setImageBitmap(bitmap);
}
mDownloadBitmapAsyncTaskHashSet.remove(this);
}
}
// 获取Bitmap
private Bitmap downloadBitmap(String imageUrl) {
Bitmap bitmap = null;
HttpURLConnection httpURLConnection = null;
try {
URL url = new URL(imageUrl);
httpURLConnection = (HttpURLConnection) url.openConnection();
httpURLConnection.setConnectTimeout(5 * 1000);
httpURLConnection.setReadTimeout(10 * 1000);
httpURLConnection.setDoInput(true);
httpURLConnection.setDoOutput(true);
bitmap = BitmapFactory.decodeStream(httpURLConnection.getInputStream());
} catch (Exception e) {
e.printStackTrace();
} finally {
if (httpURLConnection != null) {
httpURLConnection.disconnect();
}
}
return bitmap;
}
}

ImagesUrl如下:

public class ImagesUrl {
public final static String[] Urls = 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"｝；

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