Add method to define stubs

spec/spectator/mocks/double_spec.cr

@@ -209,4 +209,41 @@ Spectator.describe Spectator::Double do
       end
     end
   end
+
+  describe "#_spectator_define_stub" do
+    subject(dbl) { FooBarDouble.new }
+    let(stub3) { Spectator::ValueStub.new(:foo, 3) }
+    let(stub5) { Spectator::ValueStub.new(:foo, 5) }
+    let(stub7) { Spectator::ValueStub.new(:foo, 7, Spectator::Arguments.capture(:lucky)) }
+
+    it "overrides an existing method" do
+      expect { dbl._spectator_define_stub(stub3) }.to change { dbl.foo }.from(42).to(3)
+    end
+
+    it "replaces an existing stub" do
+      dbl._spectator_define_stub(stub3)
+      expect { dbl._spectator_define_stub(stub5) }.to change { dbl.foo }.from(3).to(5)
+    end
+
+    it "doesn't affect other methods" do
+      expect { dbl._spectator_define_stub(stub5) }.to_not change { dbl.bar }
+    end
+
+    it "picks the correct stub based on arguments" do
+      dbl._spectator_define_stub(stub5)
+      dbl._spectator_define_stub(stub7)
+      aggregate_failures do
+        expect(dbl.foo).to eq(5)
+        expect(dbl.foo(:lucky)).to eq(7)
+      end
+    end
+
+    it "only uses a stub if an argument constraint is met" do
+      dbl._spectator_define_stub(stub7)
+      aggregate_failures do
+        expect(dbl.foo).to eq(42)
+        expect(dbl.foo(:lucky)).to eq(7)
+      end
+    end
+  end
 end

src/spectator/dsl.cr

@@ -9,6 +9,6 @@ module Spectator
   module DSL
     # Keywords that cannot be used in specs using the DSL.
     # These are either problematic or reserved for internal use.
-    RESERVED_KEYWORDS = %i[initialize _spectator_stubbed_name _spectator_find_stub]
+    RESERVED_KEYWORDS = %i[initialize _spectator_stubbed_name _spectator_find_stub _spectator_define_stub]
   end
 end

src/spectator/mocks/double.cr

@@ -37,6 +37,10 @@ module Spectator
     def initialize(@stubs : Array(Stub) = [] of Stub)
     end
 
+    protected def _spectator_define_stub(stub : Stub) : Nil
+      @stubs.unshift(stub)
+    end
+
     private def _spectator_find_stub(call) : Stub?
       Log.debug { "Finding stub for #{call}" }
       stub = @stubs.find &.===(call)