fix isUrlCached(), add getCacheFilename()

Author: Wonday

README.md

@@ -147,7 +147,7 @@ import CachedImage from 'react-native-image-cache-wrapper';
 CachedImage.clearCache();
 ```
 
-**CachedImage.isUrlCached(url)**
+**CachedImage.isUrlCached(url,success=(cachFile)=>void,fail=(error)=>void))**
 
 check if a url is cached.
 
@@ -156,11 +156,22 @@ Example:
 import CachedImage from 'react-native-image-cache-wrapper';
 
 // check if a url is cached.
-if (CachedImage.isUrlCached(url)) {
-    // do something
-};
+CachedImage.isUrlCached(url,(exists)=>{
+    alert(exists);
+});
 ```
 
+**CachedImage.getCacheFilename(url)**
+
+make a cache filename.
+
+Example:
+```
+import CachedImage from 'react-native-image-cache-wrapper';
+
+// check if a url is cached.
+let cachedFilename = CachedImage.getCacheFilename(url);
+```
 
 **CachedImage.cacheDir**
 

index.js

@@ -46,11 +46,26 @@ export default class CachedImage extends Component {
     /**
      * check if a url is cached
      */
-    static isUrlCached = async (url) => {
+    static isUrlCached = (url: string, success: Function, failure: Function) => {
         const cacheFile = _getCacheFilename(url);
-        return !!(await RNFetchBlob.fs.stat(cacheFile));
+        RNFetchBlob.fs.exists(cacheFile)
+            .then((exists) => {
+                success && success(exists);
+            })
+            .catch((error) => {
+                failure && failure(error);
+            });
     };
 
+    /**
+     * make a cache filename
+     * @param url
+     * @returns {string}
+     */
+    static getCacheFilename = (url) => {
+        return _getCacheFilename(url);
+    }
+
     /**
      * Same as ReactNaive.Image.getSize only it will not download the image if it has a cached version
      * @param url
@@ -298,4 +313,4 @@ async function _saveCacheFile(url: string, success: Function, failure: Function)
     } catch (error) {
         failure && failure(error);
     }
-}
+}