Add support for edges to margins and margins to edges

Source/Constraint.swift

@@ -80,7 +80,42 @@ public class Constraint {
         
         for layoutFromAttribute in layoutFromAttributes {
             // get layout to attribute
-            let layoutToAttribute = (layoutToAttributes.count > 0) ? layoutToAttributes[0] : layoutFromAttribute
+            let layoutToAttribute: NSLayoutAttribute
+            if layoutToAttributes.count > 1 {
+                if self.from.attributes == .edges && self.to.attributes == .margins {
+                    switch layoutFromAttribute {
+                    case .left:
+                        layoutToAttribute = .leftMargin
+                    case .right:
+                        layoutToAttribute = .rightMargin
+                    case .top:
+                        layoutToAttribute = .topMargin
+                    case .bottom:
+                        layoutToAttribute = .bottomMargin
+                    default:
+                        fatalError()
+                    }
+                } else if self.from.attributes == .margins && self.to.attributes == .edges {
+                    switch layoutFromAttribute {
+                    case .leftMargin:
+                        layoutToAttribute = .left
+                    case .rightMargin:
+                        layoutToAttribute = .right
+                    case .topMargin:
+                        layoutToAttribute = .top
+                    case .bottomMargin:
+                        layoutToAttribute = .bottom
+                    default:
+                        fatalError()
+                    }
+                } else {
+                    layoutToAttribute = layoutToAttributes[0]
+                }
+            } else if layoutToAttributes.count == 1 {
+                layoutToAttribute = layoutToAttributes[0]
+            } else {
+                layoutToAttribute = layoutFromAttribute
+            }
             
             // get layout constant
             let layoutConstant: CGFloat = self.constant.constraintConstantTargetValueFor(layoutAttribute: layoutToAttribute)

Source/ConstraintMakerRelatable.swift

@@ -43,7 +43,9 @@ public class ConstraintMakerRelatable {
         if let other = other as? ConstraintItem {
             guard other.attributes == ConstraintAttributes.none ||
                   other.attributes.layoutAttributes.count <= 1 ||
-                  other.attributes.layoutAttributes == self.description.attributes.layoutAttributes else {
+                  other.attributes.layoutAttributes == self.description.attributes.layoutAttributes ||
+                  other.attributes == .edges && self.description.attributes == .margins ||
+                  other.attributes == .margins && self.description.attributes == .edges else {
                 fatalError("Cannot constraint to multiple non identical attributes. (\(file), \(line))");
             }
             

Tests/Tests.swift

@@ -418,4 +418,44 @@ class SnapKitTests: XCTestCase {
         XCTAssertEqual(constraints[0].identifier, identifier, "Identifier should be 'Test'")
     }
     
+    func testEdgesToMargins() {
+        var fromAttributes = Set<NSLayoutAttribute>()
+        var toAttributes = Set<NSLayoutAttribute>()
+        
+        let view = View()
+        self.container.addSubview(view)
+        
+        view.snp.remakeConstraints { (make) -> Void in
+            make.edges.equalTo(self.container.snp.margins)
+        }
+        
+        XCTAssertEqual(self.container.snp_constraints.count, 4, "Should have 4 constraints")
+        
+        for constraint in (container.snp_constraints as! [NSLayoutConstraint]) {
+            fromAttributes.insert(constraint.firstAttribute)
+            toAttributes.insert(constraint.secondAttribute)
+        }
+        
+        XCTAssert(fromAttributes == [.top, .left, .bottom, .right])
+        XCTAssert(toAttributes == [.topMargin, .leftMargin, .bottomMargin, .rightMargin])
+        
+        fromAttributes.removeAll()
+        toAttributes.removeAll()
+        
+        view.snp.remakeConstraints { (make) -> Void in
+            make.margins.equalTo(self.container.snp.edges)
+        }
+        
+        XCTAssertEqual(self.container.snp_constraints.count, 4, "Should have 4 constraints")
+        
+        for constraint in (container.snp_constraints as! [NSLayoutConstraint]) {
+            fromAttributes.insert(constraint.firstAttribute)
+            toAttributes.insert(constraint.secondAttribute)
+        }
+        
+        XCTAssert(toAttributes == [.top, .left, .bottom, .right])
+        XCTAssert(fromAttributes == [.topMargin, .leftMargin, .bottomMargin, .rightMargin])
+        
+    }
+    
 }