It is not possible to set an default onDismiss callback while using the directive

root: 
https://github.com/sachinchoolur/angular-flash/blob/master/src/angular-flash.js#L63-L68

When the directives link-function is called, there are 2 options: 
1) there is a callback on the scope and the default-config gets overridden (as should be expected)
2) there is no callback on the scope and the default-config still get's overridden since Flash.setOnDismiss is used there.
