Creating an expression from a string value is currently not supported - please create a new feature request in the PowerApps Ideas board if you believe it's a scenario we should support.
For your scenario, you can consider specifying the parameters in a generic collection that will be available to the screen where the menu navigates. For example, if you have this in the OnVisible property of your first (menu) screen:
ClearCollect(menuOptions,
{ menuLabel: "PowerApps RSS", screen: RSSReader, params:
Table({ name:"feedname",value:"PowerApps" }, {name:"feedurl",value:"https://powerapps.microsoft.com/blog/feed/" })},
{ menuLabel: "Flow RSS", screen: RSSReader, params:
Table({ name:"feedname",value:"Flow" }, {name:"feedurl",value:"https://us.flow.microsoft.com/blog/feed/" })},
{ menuLabel: "Other", screen: AnotherScreen, params:
Table({ name:"first",value:"First value" })})Then you can on the OnSelect property of the menu that navigates to that screen store the screen parameters in a collection:
ClearCollect(screenParameters, ThisItem.params);
Navigate(ThisItem.screen, ScreenTransition.Cover)
Now in the screens you can use a LookUp to retrieve the value that was passed to it. For example, in the RSSreader screen, the following two expressions will give you the value of the feedname and feedurl parameters:
LookUp(screenParameters, name = "feedname", value)
LookUp(screenParameters, name = "feedurl", value)
This is a very generic option that can be used for many scenarios.
Another alternative, if the parameters that are passed to the screens are always the same (which is the case in your example below), then we can use the parameters directly:
MenuScreen.OnVisible: ClearCollect(menuOptions,
{ menuLabel: "PowerApps RSS", screen: RSSReader, feedname:"PowerApps",feedurl:"https://powerapps.microsoft.com/blog/feed/"},
{ menuLabel: "Flow RSS", screen: RSSReader, feedname:"Flow",feedurl:"https://us.flow.microsoft.com/blog/feed/"})
MenuArrow.OnSelect:
Set(feedname, ThisItem.feedname);
Set(feedurl, ThisItem.feedurl);
Navigate(ThisItem.screen, ScreenTransition.Cover)And on the screens you can access the variables 'feedname' and 'feedurl' directly.
