Add engine attribute to Graph, and default to 'dot' layout

README.md

@@ -48,3 +48,23 @@ graph graphname {
  }
 """
 ```
+
+# Advanced Examples
+
+The default [layout](https://graphviz.org/docs/layouts/) used in this package in `dot`.
+To change the layout, you can access the `engine` attribute of the Graph **before** showing it, or using the constructor `GraphViz.Graph` with the additional keyword attribute `engine=...`.
+
+```
+G = GraphViz.Graph("""
+digraph graphname {
+    rankdir="LR";
+    x [label="{x = 3.0 | ∂x = 2.0 }", shape=record];
+    y [label="{y = 2.0 | ∂y = 3.0 }", shape=record];
+    z [label="{z = 6.0 | ∂z = 1.0 }", shape=record];
+    "z*" [label="*"];
+    x -> "z*";
+    y -> "z*";
+    "z*" -> z;
+}
+""", engine="dot")
+```

src/GraphViz.jl

@@ -59,8 +59,9 @@ module GraphViz
     mutable struct Graph
         handle::Ptr{Cvoid}
         didlayout::Bool
-        function Graph(p::Ptr{Cvoid})
-            this = new(p,false)
+        engine::String
+        function Graph(p::Ptr{Cvoid}, engine::String)
+            this = new(p,false,engine)
             finalizer(free, this)
             this
         end
@@ -73,23 +74,23 @@ module GraphViz
         g.handle = C_NULL
     end
 
-    function Graph(graph::IO)
+    function Graph(graph::IO; engine::String = "dot")
         iodisc = Ref(Agiodisc_s(
             @cfunction(jl_afread,Cint,(Any,Ptr{UInt8},Cint)),
             @cfunction(jl_putstr,Cint,(Any,Ptr{UInt8})),
             @cfunction(jl_flush,Cint,(Any,))))
         discs = Ref(Agdisc_s(cglobal((:AgMemDisc,libcgraph)),
             cglobal((:AgIdDisc,libcgraph)),
             Base.unsafe_convert(Ptr{Agiodisc_s}, iodisc)))
-        Graph(@GC.preserve iodisc ccall((:agread,libcgraph),Ptr{Cvoid},(Any,Ptr{Cvoid}),graph,discs))
+        Graph((@GC.preserve iodisc ccall((:agread,libcgraph),Ptr{Cvoid},(Any,Ptr{Cvoid}),graph,discs)), engine)
     end
-    Graph(graph::Vector{UInt8}) = Graph(IOBuffer(graph))
-    Graph(graph::String) = @GC.preserve graph Graph(unsafe_wrap(Vector{UInt8}, graph))
+    Graph(graph::Vector{UInt8}, args...; kwargs...) = Graph(IOBuffer(graph), args...; kwargs...)
+    Graph(graph::String, args...; kwargs...) = @GC.preserve graph Graph(unsafe_wrap(Vector{UInt8}, graph), args...; kwargs...)
 
     load(f::File{format"DOT"}) = open(Graph, f)
     load(io::IO) = Graph(io)
 
-    function layout!(g::Graph;engine="neato", context = default_context[])
+    function layout!(g::Graph;engine=g.engine, context = default_context[])
         @assert g.handle != C_NULL
         if ccall((:gvLayout,libgvc),Cint,(Ptr{Cvoid},Ptr{Cvoid},Ptr{UInt8}),context.handle,g.handle,engine) == 0
             g.didlayout = true
@@ -180,7 +181,7 @@ module GraphViz
 
     function Base.show(io::IO, ::MIME"image/svg+xml", x::Graph)
         if !x.didlayout
-            layout!(x,engine="neato")
+            layout!(x,engine=x.engine)
         end
         render(io,x)
     end

test/runtests.jl

@@ -1,17 +1,29 @@
 using GraphViz, Cairo, Test
 
-let g = dot"""
-graph graphname {
-     // The label attribute can be used to change the label of a node
-     a [label="Foo"];
-     // Here, the node shape is changed.
-     b [shape=box];
-     // These edges both have different line properties
-     a -- b -- c [color=blue];
-     b -- d [style=dotted];
- }
-"""
+@testset "Basic tests" begin
+    g = dot"""graph graphname {
+        a [label="Foo"];
+        b [shape=box];
+        a -- b -- c [color=blue];
+        b -- d [style=dotted];
+    }"""
+    @test_nowarn show(IOBuffer(), MIME"image/svg+xml"(), g)
+    @test_nowarn show(IOBuffer(), MIME"image/png"(), g)
+
+    dot_example = """graph graphname {
+        a [label="Foo"];
+        b [shape=box];
+        a -- b -- c [color=blue];
+        b -- d [style=dotted];
+    }"""
+
+    g = GraphViz.Graph(dot_example)
     @test_nowarn show(IOBuffer(), MIME"image/svg+xml"(), g)
     @test_nowarn show(IOBuffer(), MIME"image/png"(), g)
-end
 
+    @testset "Engine $engine" for engine in ["dot", "neato", "fdp", "circo", "twopi", "osage", "patchwork"]
+        g = GraphViz.Graph(dot_example, engine=engine)
+        @test_nowarn show(IOBuffer(), MIME"image/svg+xml"(), g)
+        @test_nowarn show(IOBuffer(), MIME"image/png"(), g)
+    end
+end