[Blazor] API for interpreting RenderFragment contents.

[javiercn]

The idea is to allow interpreting a render fragment so that it can be transformed to change it before applying it.

[mrpmorris]

Cos you give a mock-up example of this, please! Thanks @javiercn

[Postlagerkarte]

Great! For what it is worth here are two more supporting issues for this:

#9493
#16737

[mrlife]

Related: #29648

[tstockwell]

The idea is to allow interpreting a render fragment so that it can be transformed to change it before applying it.

Seems like this would be possible right now...

// the original render fragment
void OriginalRenderFragment(RenderTreeBuillder builder) {
      // some rendering logic here
}

// a new render fragment that applies customizations to the output from OriginalRenderFragment
void RenderWithTransformations(RenderTreeBuillder builder) {
      var builder1= new RenderTreeBuilder();
      OriginalRenderFragment(builder1);
      var frames1= builder1.GetFrames();
      
      var frames2= ApplyMyTransformations(frames1);   
      RenderFrames(builder, frames2); 
}

// applies some transformations to an array of frames and returns a new array
ArrayRange<RenderTreeFrame> ApplyMyTransformations(ArrayRange<RenderTreeFrame> frames) {
       .....
}

// simply renders the given frames to a RenderTreeBuilder
void RenderFrames(RenderTreeBuilder builder, ArrayRange<RenderTreeFrame> frames) {
    ....
}

I'm aware that frames belong to an internal package, that's not a showstopping problem for me, but it would be nice if Blazor had a legit API that let me do something equivalent to this.

[Hecatron]

I recently ran into this problem with - #36976
One question is if there is a way to add an existing frame to a builder, since I didn't see an API for that
with the above this would be the part within RenderFrames

The way I've fudged it myself so far is to

// Add a dummy entry to the destination array in the builder, this increments the count
destbuilder.AddMarkupContent(i, "");

// Do a direct assign to the array to overwrite the struct.
var destframes = destbuilder.GetFrames();
destframes.Array[i] = srcframes.Array[i];

I'm sure this isn't right but for now it does work. Ideally we need an API for copying / assigning an existing frame from a different Array of frames.