文件下載cordovaFileTransfer：cordova.file.documentsDirectory is null

在Android平臺上使用：$cordovaFileTransfer進行文件下載時提示：cordova.file.documentsDirectory is null，查瞭如下文檔參照：http://ngcordova.com/docs/plugins/file/，

documentsDirectory是IOS平臺的用法，在Android平臺上沒有此方法因此獲取到null,解決方法：

經過使用：$cordovaDevice.getPlatform()獲取應用平臺，針對不一樣平臺進行文件下載處理：

IOS使用:cordova.file.documentsDirectory

Android使用:cordova.file.externalRootDirectory

iOS File System Layout

Device Path	cordova.file.*	iosExtraFileSystems	r/w?	persistent?	OS clears	sync	private
/var/mobile/Applications/<UUID>/	applicationStorageDirectory	-	r	N/A	N/A	N/A	Yes
appname.app/	applicationDirectory	bundle	r	N/A	N/A	N/A	Yes
www/	-	-	r	N/A	N/A	N/A	Yes
Documents/	documentsDirectory	documents	r/w	Yes	No	Yes	Yes
NoCloud/	-	documents-nosync	r/w	Yes	No	No	Yes
Library	-	library	r/w	Yes	No	Yes?	Yes
NoCloud/	dataDirectory	library-nosync	r/w	Yes	No	No	Yes
Cloud/	syncedDataDirectory	-	r/w	Yes	No	Yes	Yes
Caches/	cacheDirectory	cache	r/w	Yes*	Yes***	No	Yes
tmp/	tempDirectory	-	r/w	No**

Android File System Layout

Device Path	cordova.file.*	AndroidExtraFileSystems	r/w?	persistent?	OS clears	private
file:///android_asset/	applicationDirectory		r	N/A	N/A	Yes
/data/data/<app-id>/	applicationStorageDirectory	-	r/w	N/A	N/A	Yes
cache	cacheDirectory	cache	r/w	Yes	Yes*	Yes
files	dataDirectory	files	r/w	Yes	No	Yes
Documents		documents	r/w	Yes	No	Yes
<sdcard>/	externalRootDirectory	sdcard	r/w	Yes	No	No
Android/data/<app-id>/	externalApplicationStorageDirectory	-	r/w	Yes	No	No
cache	externalCacheDirectry	cache-external	r/w	Yes	No**	No
files	externalDataDirectory	files-external	r/w