Cleanup axis-aligned padding (#4845)

We now have an EdgeInsets.along function that projects the padding onto the
given Axis rather than repeating this function in several places.

packages/flutter/lib/src/painting/edge_insets.dart

@@ -6,6 +6,15 @@ import 'dart:ui' as ui show lerpDouble, WindowPadding;
 
 import 'basic_types.dart';
 
+/// The two cardinal directions in two dimensions.
+enum Axis {
+  /// Left and right
+  horizontal,
+
+  /// Up and down
+  vertical,
+}
+
 /// An immutable set of offsets in each of the four cardinal directions.
 ///
 /// Typically used for an offset from each of the four sides of a box. For
@@ -56,6 +65,18 @@ class EdgeInsets {
   /// The total offset in the horizontal direction.
   double get vertical => top + bottom;
 
+  /// The total offset in the given direction.
+  double along(Axis axis) {
+    assert(axis != null);
+    switch (axis) {
+      case Axis.horizontal:
+        return horizontal;
+      case Axis.vertical:
+        return vertical;
+    }
+    return null;
+  }
+
   /// The size that this EdgeInsets would occupy with an empty interior.
   Size get collapsedSize => new Size(horizontal, vertical);
 

packages/flutter/lib/src/rendering/box.dart

@@ -19,15 +19,6 @@ class _DebugSize extends Size {
   final bool _canBeUsedByParent;
 }
 
-/// The two cardinal directions in two dimensions.
-enum Axis {
-  /// Left and right
-  horizontal,
-
-  /// Up and down
-  vertical,
-}
-
 /// Immutable layout constraints for box layout.
 ///
 /// A size respects a BoxConstraints if, and only if, all of the following
@@ -130,10 +121,10 @@ class BoxConstraints extends Constraints {
   BoxConstraints deflate(EdgeInsets edges) {
     assert(edges != null);
     assert(debugAssertIsValid());
-    double horizontal = edges.left + edges.right;
-    double vertical = edges.top + edges.bottom;
-    double deflatedMinWidth = math.max(0.0, minWidth - horizontal);
-    double deflatedMinHeight = math.max(0.0, minHeight - vertical);
+    final double horizontal = edges.horizontal;
+    final double vertical = edges.vertical;
+    final double deflatedMinWidth = math.max(0.0, minWidth - horizontal);
+    final double deflatedMinHeight = math.max(0.0, minHeight - vertical);
     return new BoxConstraints(
       minWidth: deflatedMinWidth,
       maxWidth: math.max(deflatedMinWidth, maxWidth - horizontal),
@@ -1144,7 +1135,7 @@ abstract class RenderBox extends RenderObject {
   /// The caller is responsible for transforming [position] into the local
   /// coordinate space of the callee.  The callee is responsible for checking
   /// whether the given position is within its bounds.
-  bool hitTest(HitTestResult result, { Point position }) {
+  bool hitTest(HitTestResult result, { @required Point position }) {
     assert(() {
       if (needsLayout) {
         throw new FlutterError(

packages/flutter/lib/src/rendering/list.dart

@@ -75,17 +75,6 @@ class RenderList extends RenderVirtualViewport<ListParentData> {
       child.parentData = new ListParentData();
   }
 
-  double get _scrollAxisPadding {
-    switch (mainAxis) {
-      case Axis.vertical:
-        return padding.vertical;
-      case Axis.horizontal:
-        return padding.horizontal;
-    }
-    assert(mainAxis != null);
-    return null;
-  }
-
   double get _preferredExtent {
     if (itemExtent == null)
       return double.INFINITY;
@@ -94,7 +83,7 @@ class RenderList extends RenderVirtualViewport<ListParentData> {
       return double.INFINITY;
     double extent = itemExtent * count;
     if (padding != null)
-      extent += _scrollAxisPadding;
+      extent += padding.along(mainAxis);
     return extent;
   }
 

packages/flutter/lib/src/rendering/viewport.dart

@@ -274,7 +274,7 @@ class RenderViewport extends RenderViewportBase with RenderObjectWithChildMixin<
 
   @override
   void performLayout() {
-    ViewportDimensions oldDimensions = dimensions;
+    final ViewportDimensions oldDimensions = dimensions;
     if (child != null) {
       child.layout(_getInnerConstraints(constraints), parentUsesSize: true);
       size = constraints.constrain(child.size);

packages/flutter/lib/src/widgets/lazy_block.dart

@@ -315,18 +315,7 @@ class LazyBlockViewport extends RenderObjectWidget {
   /// See [LazyBlockDelegate] for details.
   final LazyBlockDelegate delegate;
 
-  double get _mainAxisPadding {
-    if (padding == null)
-      return 0.0;
-    switch (mainAxis) {
-      case Axis.horizontal:
-        return padding.horizontal;
-      case Axis.vertical:
-        return padding.vertical;
-    }
-    assert(mainAxis != null);
-    return null;
-  }
+  double get _mainAxisPadding => padding == null ? 0.0 : padding.along(mainAxis);
 
   @override
   _LazyBlockElement createElement() => new _LazyBlockElement(this);