用Glide这么久了，我一直有个疑问，Glide该如何获取到指定的缓存图片？
原生Glide是没有提供任何Api用来获取缓存图片的，至少我是没找到。
翻看Glide源码（3.7），发现其中一个叫：EngineKey的类，Glide通过该类来查找对应的缓存文件。
该类构造方法参数多达10个，并且不是开放出来的，也就是说，通过自己构造EngineKey这条路是走不通的。
难道就没有办法了吗？仔细翻看EngineKey这个类，其中有一个叫getOriginalKey的方法：
public Key getOriginalKey() {
if (originalKey == null) {
originalKey = new OriginalKey(id, signature);
}
return originalKey;
}
对应OriginalKey只需要两个参数，并且类的内容也很简单：
class OriginalKey implements Key {
private final String id;
private final Key signature;
public OriginalKey(String id, Key signature) {
this.id = id;
this.signature = signature;
}
@Override
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null || getClass() != o.getClass()) {
return false;
}
OriginalKey that = (OriginalKey) o;
if (!id.equals(that.id)) {
return false;
}
if (!signature.equals(that.signature)) {
return false;
}
return true;
}
@Override
public int hashCode() {
int result = id.hashCode();
result = 31 * result + signature.hashCode();
return result;
}
@Override
public void updateDiskCacheKey(MessageDigest messageDigest) throws UnsupportedEncodingException {
messageDigest.update(id.getBytes(STRING_CHARSET_NAME));
signature.updateDiskCacheKey(messageDigest);
}
}
再查看getOriginalKey()方法的调用者，发现被一个DecodeJob的两个方法调用到了：
private Resource<T> cacheAndDecodeSourceData(A data) throws IOException {
long startTime = LogTime.getLogTime();
SourceWriter<A> writer = new SourceWriter<A>(loadProvider.getSourceEncoder(), data);
diskCacheProvider.getDiskCache().put(resultKey.getOriginalKey(), writer);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Wrote source to cache", startTime);
}
startTime = LogTime.getLogTime();
Resource<T> result = loadFromCache(resultKey.getOriginalKey());
if (Log.isLoggable(TAG, Log.VERBOSE) && result != null) {
logWithTimeAndKey("Decoded source from cache", startTime);
}
return result;
}
public Resource<Z> decodeSourceFromCache() throws Exception {
if (!diskCacheStrategy.cacheSource()) {
return null;
}
long startTime = LogTime.getLogTime();
Resource<T> decoded = loadFromCache(resultKey.getOriginalKey());
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Decoded source from cache", startTime);
}
return transformEncodeAndTranscode(decoded);
}
了解过Glide源码的人应该知道，这两个方法是在diskCacheStrategy设置为Source时，Glide缓存到本地和从本地取缓存的方法。
查看loadFromCache方法：
private Resource<T> loadFromCache(Key key) throws IOException {
File cacheFile = diskCacheProvider.getDiskCache().get(key);
if (cacheFile == null) {
return null;
}
Resource<T> result = null;
try {
result = loadProvider.getCacheDecoder().decode(cacheFile, width, height);
} finally {
if (result == null) {
diskCacheProvider.getDiskCache().delete(key);
}
}
return result;
}
get方法点进去，发现有两个实现：DiskCacheAdapter和DiskLruCacheWrapper，DiskCacheAdapter只是个空壳，那么只有DiskLruCacheWrapper了。
其中获取缓存文件的方法：
@Override
public File get(Key key) {
String safeKey = safeKeyGenerator.getSafeKey(key);
File result = null;
try {
//It is possible that the there will be a put in between these two gets. If so that shouldn't be a problem
//because we will always put the same value at the same key so our input streams will still represent
//the same data
final DiskLruCache.Value value = getDiskCache().get(safeKey);
if (value != null) {
result = value.getFile(0);
}
} catch (IOException e) {
if (Log.isLoggable(TAG, Log.WARN)) {
Log.w(TAG, "Unable to get from disk cache", e);
}
}
return result;
}
private synchronized DiskLruCache getDiskCache() throws IOException {
if (diskLruCache == null) {
diskLruCache = DiskLruCache.open(directory, APP_VERSION, VALUE_COUNT, maxSize);
}
return diskLruCache;
}
那就好办了！我们只需将diskCacheStrategy设置为Source，然后自己构造OriginalKey和DiskLruCache即可。
说干就干！
将Glide下的SafeKeyGenerator和OriginalKey类拷到自己的包下，这两个类也很简单：
/**
* A class that generates and caches safe and unique string file names from {@link com.bumptech.glide.load.Key}s.
*/
class SafeKeyGenerator {
private final LruCache<Key, String> loadIdToSafeHash = new LruCache<Key, String>(1000);
public String getSafeKey(Key key) {
String safeKey;
synchronized (loadIdToSafeHash) {
safeKey = loadIdToSafeHash.get(key);
}
if (safeKey == null) {
try {
MessageDigest messageDigest = MessageDigest.getInstance("SHA-256");
key.updateDiskCacheKey(messageDigest);
safeKey = Util.sha256BytesToHex(messageDigest.digest());
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
synchronized (loadIdToSafeHash) {
loadIdToSafeHash.put(key, safeKey);
}
}
return safeKey;
}
}
//OriginalKey 在上面！
然后就可以缓存文件获取了，如果你没有自己构造Signature和修改磁盘缓存位置，那用下面这个方法即可：
//3.7
public File getCacheFile(String url) {
OriginalKey originalKey = new OriginalKey(url, EmptySignature.obtain());
SafeKeyGenerator safeKeyGenerator = new SafeKeyGenerator();
String safeKey = safeKeyGenerator.getSafeKey(originalKey);
try {
DiskLruCache diskLruCache = DiskLruCache.open(new File(getCacheDir(), DiskCache.Factory.DEFAULT_DISK_CACHE_DIR), 1, 1, DiskCache.Factory.DEFAULT_DISK_CACHE_SIZE);
DiskLruCache.Value value = diskLruCache.get(safeKey);
if (value != null) {
return value.getFile(0);
}
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
效果：
当然，如果不是Glide3.7，而是4.x呢？额，亲测在4.0上只需要改一个类即可，将OriginalKey换为DataCacheKey即可：
/**
* A cache key for original source data + any requested signature.
*/
public class DataCacheKey implements Key {
private final Key sourceKey;
private final Key signature;
public DataCacheKey(Key sourceKey, Key signature) {
this.sourceKey = sourceKey;
this.signature = signature;
}
public Key getSourceKey() {
return sourceKey;
}
@Override
public boolean equals(Object o) {
if (o instanceof DataCacheKey) {
DataCacheKey other = (DataCacheKey) o;
return sourceKey.equals(other.sourceKey) && signature.equals(other.signature);
}
return false;
}
@Override
public int hashCode() {
int result = sourceKey.hashCode();
result = 31 * result + signature.hashCode();
return result;
}
@Override
public String toString() {
return "DataCacheKey{"
+ "sourceKey=" + sourceKey
+ ", signature=" + signature
+ '}';
}
@Override
public void updateDiskCacheKey(MessageDigest messageDigest) {
sourceKey.updateDiskCacheKey(messageDigest);
signature.updateDiskCacheKey(messageDigest);
}
}
对应的方法：
//4.0
public File getCacheFile2(String url) {
DataCacheKey dataCacheKey = new DataCacheKey(new GlideUrl(url), EmptySignature.obtain());
SafeKeyGenerator safeKeyGenerator = new SafeKeyGenerator();
String safeKey = safeKeyGenerator.getSafeKey(dataCacheKey);
try {
int cacheSize = 100 * 1000 * 1000;
DiskLruCache diskLruCache = DiskLruCache.open(new File(getCacheDir(), DiskCache.Factory.DEFAULT_DISK_CACHE_DIR), 1, 1, cacheSize);
DiskLruCache.Value value = diskLruCache.get(safeKey);
if (value != null) {
return value.getFile(0);
}
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
写了一个非常简单的Demo ： https://github.com/HuangShengHuan/GlideCache