在上一篇的文章中,讲解了Bitmap的高效加载,以及LruCache和DiskLruCache的基本用法。但是在实际的项目中,只使用内存缓存或者只使用磁盘缓存是有短板的。内存缓存速度快,但空间有限;磁盘缓存相对内存缓存来说,速度相对慢,但空间较充足。因此,如果将二者相结合到开发中,会有如虎添翼的效果。
下面还是以网络获取图片,并添加内存缓存和磁盘缓存为例,实现照片墙的效果。
首先,定义网络图片地址:
public class Images {
public final static String[] imageThumbUrls = new String[] {
"http://img.my.csdn.net/uploads/201407/26/1406383299_1976.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383291_6518.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383291_8239.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383290_9329.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383290_1042.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383275_3977.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383265_8550.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383264_3954.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383264_4787.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383264_8243.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383248_3693.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383243_5120.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383242_3127.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383242_9576.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383242_1721.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383219_5806.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383214_7794.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383213_4418.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383213_3557.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383210_8779.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383172_4577.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383166_3407.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383166_2224.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383166_7301.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383165_7197.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383150_8410.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383131_3736.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383130_5094.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383130_7393.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383129_8813.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383100_3554.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383093_7894.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383092_2432.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383092_3071.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383091_3119.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383059_6589.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383059_8814.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383059_2237.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383058_4330.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383038_3602.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382942_3079.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382942_8125.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382942_4881.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382941_4559.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382941_3845.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382924_8955.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382923_2141.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382923_8437.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382922_6166.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382922_4843.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382905_5804.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382904_3362.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382904_2312.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382904_4960.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382900_2418.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382881_4490.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382881_5935.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382880_3865.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382880_4662.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382879_2553.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382862_5375.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382862_1748.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382861_7618.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382861_8606.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382861_8949.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382841_9821.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382840_6603.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382840_2405.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382840_6354.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382839_5779.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382810_7578.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382810_2436.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382809_3883.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382809_6269.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382808_4179.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382790_8326.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382789_7174.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382789_5170.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382789_4118.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382788_9532.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382767_3184.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382767_4772.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382766_4924.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382766_5762.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382765_7341.jpg"
};
}
在activity_main.xml中,需要通过GridView控件以网格形式显示图片:
<GridView
android:id="@+id/photo_wall"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:columnWidth="100dp"
android:gravity="center"
android:horizontalSpacing="2dp"
android:numColumns="auto_fit"
android:stretchMode="columnWidth"
android:verticalSpacing="2dp">
</GridView>
另外,需要定义GridView中每张图片的布局,新建photo_item.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
<ImageView
android:id="@+id/photo"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_centerInParent="true"
android:scaleType="fitXY"/>
</LinearLayout>
接下来为GridView创建适配器PhotoAdapter
public class PhotoAdapter extends ArrayAdapter<String> {
/*
* 内存缓存
*/
private LruCache<String, Bitmap> mMemoryCache;
/*
* 磁盘缓存
*/
private DiskLruCache mDiskCache;
private Context mContext;
private GridView mGridView;
/*
* 存放图片下载队列
*/
private Set<downBitmapTask> mHttpTask;
/*
* 图片item的默认高度
*/
private int mHeight = 100;
public PhotoAdapter(@NonNull Context context, @LayoutRes int resource, @IdRes int textViewResourceId, @NonNull String[] objects, GridView gridbiew) {
super(context, resource, textViewResourceId, objects);
this.mContext = context;
this.mGridView = gridbiew;
mHttpTask = new HashSet<downBitmapTask>();
int maxMemory = (int) Runtime.getRuntime().maxMemory();
int cacheSize = maxMemory / 8;
mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
@Override
protected int sizeOf(String key, Bitmap value) {
return value.getByteCount();
}
};
File file = getDiskCacheDir(mContext, "bitmap");
if (!file.exists()) {
file.mkdirs();
}
try {
mDiskCache = DiskLruCache.open(file, getAppVersion(mContext), 1, 100 * 1024 * 1024);
} catch (IOException e) {
e.printStackTrace();
}
}
@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
final String url = getItem(position);
View view;
if (convertView == null) {
view = LayoutInflater.from(mContext).inflate(R.layout.photo_item, null);
} else {
view = convertView;
}
final ImageView imageView = view.findViewById(R.id.photo);
if (imageView.getLayoutParams().height != mHeight) {
imageView.getLayoutParams().height = mHeight;
}
imageView.setImageResource(R.mipmap.cat);
imageView.setTag(url);
loadBitmaps(imageView, url);
return view;
}
/*
* 加载bitmap,首先从内存缓存获取,
* 如果当前没有缓存,则网络获取图片
*/
private void loadBitmaps(ImageView imageView, String url) {
Bitmap bitmap = getBitmapFormMemory(url);
if (bitmap == null) {
downBitmapTask task = new downBitmapTask();
mHttpTask.add(task);
task.execute(url);
} else {
imageView.setImageBitmap(bitmap);
}
}
//写入缓存
private void addBitmapToMemory(String key, Bitmap bitmap) {
if (getBitmapFormMemory(key) == null) {
mMemoryCache.put(key, bitmap);
}
}
//读取缓存
private Bitmap getBitmapFormMemory(String key) {
return mMemoryCache.get(key);
}
//异步网络获取图片
class downBitmapTask extends AsyncTask<String, Void, Bitmap> {
String imageUrl;
@Override
protected Bitmap doInBackground(String... strings) {
imageUrl = strings[0];
/*
* 1、判断磁盘缓存是否存在,Y:从缓存获取
* 2、N:从网络获取,并写入缓存
*/
FileDescriptor filedes = null;
FileInputStream fileis = null;
final String key = hashKeyForDisk(imageUrl);
try {
DiskLruCache.Snapshot snapshot = mDiskCache.get(key);
if (snapshot == null) {
DiskLruCache.Editor edit = mDiskCache.edit(key);
if (edit != null) {
OutputStream os = edit.newOutputStream(0);
if (downloadUrlToStream(imageUrl, os)) {
edit.commit();
} else {
edit.abort();
}
}
snapshot = mDiskCache.get(key);
}
if (snapshot != null) {
fileis = (FileInputStream) snapshot.getInputStream(0);
filedes = fileis.getFD();
}
Bitmap bitmap = BitmapFactory.decodeFileDescriptor(filedes);
if (bitmap != null) {
addBitmapToMemory(strings[0], bitmap);
}
return bitmap;
} catch (IOException e) {
e.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
fileis.close();
} catch (IOException e) {
e.printStackTrace();
}
}
return null;
}
@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 = mGridView.findViewWithTag(imageUrl);
if (bitmap != null && imageview != null) {
imageview.setImageBitmap(bitmap);
}
imageview.setImageBitmap(bitmap);
mHttpTask.remove(this);
}
}
/*
* 网络获取图片,并写入输出流
*/
private boolean downloadUrlToStream(String urlstring, OutputStream outputstream) {
HttpURLConnection httpconnection = null;
BufferedOutputStream out = null;
BufferedInputStream in = null;
try {
final URL url = new URL(urlstring);
httpconnection = (HttpURLConnection) url.openConnection();
in = new BufferedInputStream(httpconnection.getInputStream(), 8 * 1024);
out = new BufferedOutputStream(outputstream, 8 * 1024);
int b;
if ((b = in.read()) != -1) {
out.write(b);
}
return true;
} catch (IOException e) {
e.printStackTrace();
} finally {
if (httpconnection != null) {
httpconnection.disconnect();
}
try {
if (out != null) {
out.close();
}
if (in != null) {
in.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
return false;
}
/**
* 获取当前存储路径
*/
public File getDiskCacheDir(Context context, String uniqueName) {
String cachePath;
if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())
|| !Environment.isExternalStorageRemovable()) {
cachePath = context.getExternalCacheDir().getPath();
} else {
cachePath = context.getCacheDir().getPath();
}
return new File(cachePath + File.separator + uniqueName);
}
/**
* MD5转换图片url key
*/
private String hashKeyForDisk(String key) {
String cacheKey = null;
try {
MessageDigest md = MessageDigest.getInstance("MD5");
md.update(key.getBytes());
cacheKey = bytesToHexString(md.digest());
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
}
return cacheKey;
}
private String bytesToHexString(byte[] bytes) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < bytes.length; i++) {
String hex = Integer.toHexString(0xFF & bytes[i]);
if (hex.length() == 1) {
sb.append('0');
}
sb.append(hex);
}
return sb.toString();
}
public void setHeight(int height) {
if (this.mHeight != height) {
this.mHeight = height;
}
notifyDataSetChanged();
}
public void clearTask() {
if (mHttpTask != null) {
for (downBitmapTask task : mHttpTask) {
task.cancel(false);
}
}
}
/**
* 将缓存记录同步到journal文件中。
*/
public void fluchCache() {
if (mDiskCache != null) {
try {
mDiskCache.flush();
} catch (IOException e) {
e.printStackTrace();
}
}
}
/**
* 获取当前应用程序的版本号。
*/
public int getAppVersion(Context context) {
try {
PackageInfo info = context.getPackageManager().getPackageInfo(context.getPackageName(),
0);
return info.versionCode;
} catch (PackageManager.NameNotFoundException e) {
e.printStackTrace();
}
return 1;
}
/*
* 得到图片字节流 数组大小
*/
public static byte[] readStream(InputStream inStream) throws Exception {
ByteArrayOutputStream outStream = new ByteArrayOutputStream();
byte[] buffer = new byte[1024];
int len = 0;
while ((len = inStream.read(buffer)) != -1) {
outStream.write(buffer, 0, len);
}
outStream.close();
inStream.close();
return outStream.toByteArray();
}
}
上面关于适配器adapter的代码里注释的都比较清楚了,这里就只讲讲具体实现的思路:
在构造方法中,进行相关的初始化,获取应用可用的最大内存,设置LruCache的最大缓存为最大内存的1/8;获取当前的存储路径,通过DiskLruCache的open方法创建磁盘缓存对象。
在getView方法中,首先为ImageView设置默认图片,添加唯一的Tag,以便后面能够准确的找到该View,避免在异步加载图片时出现乱序的情况。接着通过loadBitmaps方法加载图片,如果内存缓存存在获取对象,则直接从缓存里获取,反之,开启异步线程downBitmapTask。
downBitmapTask中,通过doInBackground方法开启后台线程,首先判断磁盘缓存是否存在,如果不存在则通过downloadUrlToStpianream获取网络图片并写入缓存,最后从缓存里获取bitmap对象交给ImageView显示。当然,最后也将其写入内存缓存里。
通过异步线程返回的bitmap对象,将在onPostExecute方法里被引用,该方法在主线程中执行,因此可以通过ImageView直接更新UI。
上面的适配器准备完成后,那么回到主Activity,
public class MainActivity extends AppCompatActivity {
private GridView mGridView;
private PhotoAdapter mAdapter;
private final static int mWidth = 100;
private final static int mMargin = 2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mGridView = (GridView) findViewById(R.id.photo_wall);
mAdapter = new PhotoAdapter(this,0,0,Images.imageThumbUrls,mGridView);
mGridView.setAdapter(mAdapter);
mGridView.getViewTreeObserver().addOnGlobalLayoutListener(
new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
int count = (int) Math.floor(mGridView.getWidth() / (mWidth + mMargin));
if (count > 0){
int width = (mGridView.getWidth() / count) - mMargin;
mAdapter.setHeight(width);
mGridView.getViewTreeObserver().removeOnGlobalLayoutListener(this);
}
}
}
);
}
@Override
protected void onPause() {
super.onPause();
mAdapter.fluchCache();
}
@Override
protected void onDestroy() {
super.onDestroy();
// 退出程序时结束所有的下载任务
mAdapter.clearTask();
}
}
在onCreat方法中,初始化PhotoAdapter对象,并绑定GridView,通过getViewTreeObserver()的方式监听View的布局事件,当布局完成以后,重新修改一下GridView中子View的高度,以保证子View的宽度和高度可以保持一致。
值得一提的是,关于缓存机制,涉及到文件的读写操作,因此应该添加对应的权限:
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
另外,在Android 6.0后,需要动态获取sd卡的读写权限:
private static final String[] PERMISSION_EXTERNAL_STORAGE = new String[] {
Manifest.permission.WRITE_EXTERNAL_STORAGE, Manifest.permission.READ_EXTERNAL_STORAGE};
private static final int REQUEST_EXTERNAL_STORAGE = 100;
private void verifyStoragePermissions(Activity activity) {
int permissionWrite = ActivityCompat.checkSelfPermission(activity,
Manifest.permission.WRITE_EXTERNAL_STORAGE);
if(permissionWrite != PackageManager.PERMISSION_GRANTED) {
ActivityCompat.requestPermissions(activity, PERMISSION_EXTERNAL_STORAGE,
REQUEST_EXTERNAL_STORAGE);
}
}
好了,关于LruCache和DiskLruCache相结合的缓存机制就介绍到这里了,在实际的开发过程中,一般还需将Bitmap的高效加载也融入其中,这样即使在处理大量图片时,我们的应用也会保持流畅,具有良好的交互性。