Special case compute_identity edges to forward ref types (#5302)

* special case `compute_identity` edges to forward ref types

* update changelog

* fix format

---------

Co-authored-by: Simon <sdanisch@protonmail.com>

Author: ffreyer

CHANGELOG.md

@@ -7,6 +7,7 @@
 - Fixed `Symlog10` to work correctly with lower or upper thresholds smaller than 1, and adds a `linscale` argument [#5279](https://github.com/MakieOrg/Makie.jl/pull/5279)
 - Fixed `xlims!`/`ylims!` not fully propagating to linked axis [#5239](https://github.com/MakieOrg/Makie.jl/pull/5239)
 - Added support for plotting units with DynamicQuantities.jl [#5280](https://github.com/MakieOrg/Makie.jl/pull/5280)
+- Adjusted compute nodes to keep unspecialized types when transitioning from one graph to another [#5302](https://github.com/MakieOrg/Makie.jl/pull/5302)
 
 ## [0.24.6] - 2025-08-19
 

ComputePipeline/src/ComputePipeline.jl

@@ -736,6 +736,30 @@ end
 
 compute_identity(inputs, changed, cached) = values(inputs)
 
+function TypedEdge(edge::ComputeEdge, f::typeof(compute_identity), inputs)
+    if length(inputs) != length(edge.outputs)
+        error("A `compute_identity` callback requires the length of inputs and outputs to match.")
+    end
+
+    # use input refs as output refs so we don't even need to evaluate the callback
+    for i in eachindex(values(inputs))
+        edge.outputs[i].value = inputs[i]
+        edge.outputs[i].dirty = true
+    end
+
+    return TypedEdge(f, inputs, edge.inputs_dirty, inputs, edge.outputs)
+end
+
+function resolve!(edge::TypedEdge{IT, OT, typeof(compute_identity)}) where {IT, OT}
+    # outputs are identical to inputs, so just copy the input state. To be safe
+    # don't overwrite any `dirty = true` state with false (maybe a problem if
+    # the input gets resolved?)
+    for i in eachindex(edge.inputs_dirty)
+        edge.output_nodes[i].dirty |= edge.inputs_dirty[i]
+    end
+    return
+end
+
 """
     add_input!([callback], compute_graph, name::Symbol, node::Computed)
 

ComputePipeline/test/unit_tests.jl

@@ -799,3 +799,47 @@ end
     e3 = graph2.merged3.parent
     @test e3.inputs == [graph1.a1, graph2.a2]
 end
+
+@testset "compute_identity" begin
+    graph1 = ComputeGraph()
+    add_input!(graph1, :a1, Ref{Any}(1))
+    map!(x -> Ref{Any}(x), graph1, :a1, :b1)
+
+    graph2 = ComputeGraph()
+    add_input!(graph2, :b1, graph1.b1)
+    graph2.b1[]
+    @test graph2.b1.value isa Ref{Any}
+
+    edge = graph2.b1.parent
+    @test edge.callback == ComputePipeline.compute_identity
+    @test length(edge.inputs) == length(edge.outputs)
+    for (in, out) in zip(edge.inputs, edge.outputs)
+        @test in.value === out.value
+        @test !ComputePipeline.isdirty(in)
+        @test !ComputePipeline.isdirty(out)
+    end
+
+    update!(graph1, a1 = 5.0)
+
+    for (in, out) in zip(edge.inputs, edge.outputs)
+        @test in.value === out.value
+        @test ComputePipeline.isdirty(in)
+        @test ComputePipeline.isdirty(out)
+    end
+
+    graph1.b1[]
+
+    for (in, out) in zip(edge.inputs, edge.outputs)
+        @test in.value === out.value
+        @test !ComputePipeline.isdirty(in)
+        @test ComputePipeline.isdirty(out)
+    end
+
+    graph2.b1[]
+
+    for (in, out) in zip(edge.inputs, edge.outputs)
+        @test in.value === out.value
+        @test !ComputePipeline.isdirty(in)
+        @test !ComputePipeline.isdirty(out)
+    end
+end