Sempare Template - How to get contextual data in a utility class method?

[Edwin Yip 154]

@darnocian, according to the documents, since custom function for the template must be defined as static class methods, how do I access contextual data in those utility methods?

 

I can see you can use `class var` to pass contextual data to the utility class before calling `Template.Eval`, but in case of multi-threaded template rendering, it seems that locking is unnecessarily needed. But at least it's workable? I'll try but you might have a better answer? Thanks.

 

It'll be great if ITemplateFunctions.AddFunctions supports non-class methods, so that in the methods the object's fields are accessible.

[darnocian 83]

Hi @Edwin Yip

 

As functions are static, it means that you must pass all required data to them. This will further ensure they are in a way thread safe, unless you are referencing some data that is 'shared' across threads... So normal solutions apply:

1. get a copy before hand of whatever is required 

2. locking while the template is potentially referencing that shared data

 

doing 2. is probably the most expensive, but if you are referencing shared state, there are some as to the shared state changing or not. if it doesn't, well, then it shouldn't be a problem. This may be the case say with static config loaded at startup that may be referenced during rendering. if the shared state does change - say some service methods mutate the state, then you may need some locking to ensure the underlying structures are not corrupted. 

 

with the shared state,you  probably have a method that is used generally in anycase with locking for the multi threaded scenario. so exposing that function to the template is possible as well... so you may do the following:

1. pass in a reference to the static data when rendering the template

2. register a method with the template so that it can extract relevant information from the shared state

 

e.g.

type
   TMyProg = class
   private
      class var FLock:TCriticalSection;
      class constructor Create; // init flock
      class destructor Destroy;  // free flock
   public    
        class function getdata(const AKey:string; const AState:TDictionary<string, string>):string; static;
   end;

  TMyData = record
     SharedState : TDictionary<string,string>;
  end;

// -------------------

class function TMyProg.getdata(const AKey:string; const AState:TDictionary<string, string>):string;
begin
   FLock.Acquire;
   try
       if not AState.TryGetValue(akey, result) then
           exit('-');
   finally
      FLock.Release;
   end;
end;

// -------------------

var GState:=TDictionary<string, string>.Create();

var GTemplate := '<% getdata("somevalue", sharedstate) %>';
var GCtx := template.context; // context could be global as well, so you don't have to keep on creating it
GTtx.AddFunctions(TMyUtilities);

var GTpl := Template.Parse(GCtx, GTemplate); // template could be global as well

// local state
var LData : TMyData;
LData.SharedState := GState;

writeln(Template.Eval(GCtx, GTpl, LData); // this should be localised

I havn't thrown the above into an IDE, but it should work in principle... (code should be placed in relevant procedures and GState should be seen as global, etc...)

 

I hope the above makes sense...

Edited October 3, 2021 by darnocian

[Edwin Yip 154]

Oh, that make the template code complex, especially if you need third parties to write the template...

 

And actually I'm asking something else - What I mean is how to access relevant data in Delphi utility functions, like TMyUtilities.DoSomething in the document example...

[darnocian 83]

Hi @Edwin Yip

 

I'm not sure why 

 

'<% getdata("somevalue", sharedstate) %>';

would be difficult? from a user perspective (someone you may have creating a template), they may not have to know about the parameter types - they just need to know what parameters are available to be passed in...

 

I do understand however that you may have non technical users from which you may want to hide this detail. At this stage, the only way is to use class variables - which is what would be available to the class functions.

 

Maybe a potential enhancement would be to allow the template context to be param 1:

     class function dosomething(const ACtx:ITemplateContext;const AFirstArg:string):string;

From a usage perspective, the template engine could expect the following usage:

    <% dosomething('hello') %>

where it auto inserts the context.

 

The reason I mention the context is because it allows you to store data in there as well which can generally be accessed by the template.

 

 

 

 

[Edwin Yip 154]

As you said, "I do understand however that you may have non technical users from which you may want to hide this detail.", and that's what I wanted, just like DMustache allows you to use methods (non-class methods) to define custom functions.

 

Anyways,  I understand that's the current status of the template engine, I can live with that.

[darnocian 83]

I stumbled across this thread again and thought I'd just add a reference to https://dev.to/sempare/accessing-data-from-the-sempare-template-engine-for-delphi-5dg8

 

It illustrates various approaches to accessing data from the template engine. In the thread above, we discussed static methods, but in the article you can easily see how you can access data from data stored in the context, or by creating a custom object with methods on it, which can then be called.