flutter · goderbauer · Nov 21, 2020 · Nov 16, 2020 · Nov 17, 2020 · Nov 18, 2020
diff --git a/packages/flutter/lib/rendering.dart b/packages/flutter/lib/rendering.dart
@@ -43,6 +43,7 @@ export 'src/rendering/flex.dart';
 export 'src/rendering/flow.dart';
 export 'src/rendering/image.dart';
 export 'src/rendering/layer.dart';
+export 'src/rendering/layout_helper.dart';
 export 'src/rendering/list_body.dart';
 export 'src/rendering/list_wheel_viewport.dart';
 export 'src/rendering/mouse_cursor.dart';

diff --git a/packages/flutter/lib/src/cupertino/action_sheet.dart b/packages/flutter/lib/src/cupertino/action_sheet.dart
@@ -721,40 +721,58 @@ class _RenderCupertinoAlert extends RenderBox {
     return 0.0;
   }
 
-  @override
-  void performLayout() {
-    final BoxConstraints constraints = this.constraints;
+  double _computeDividerThickness(BoxConstraints constraints) {
     final bool hasDivider = contentSection!.getMaxIntrinsicHeight(constraints.maxWidth) > 0.0
         && actionsSection!.getMaxIntrinsicHeight(constraints.maxWidth) > 0.0;
-    final double dividerThickness = hasDivider ? _dividerThickness : 0.0;
+    return hasDivider ? _dividerThickness : 0.0;
+  }
 
+  _AlertSizes _computeSizes({required BoxConstraints constraints, required ChildLayouter layoutChild, required double dividerThickness}) {
     final double minActionsHeight = actionsSection!.getMinIntrinsicHeight(constraints.maxWidth);
 
-    // Size alert content.
-    contentSection!.layout(
+    final Size contentSize = layoutChild(
+      contentSection!,
       constraints.deflate(EdgeInsets.only(bottom: minActionsHeight + dividerThickness)),
-      parentUsesSize: true,
     );
-    final Size contentSize = contentSection!.size;
 
-    // Size alert actions.
-    actionsSection!.layout(
+    final Size actionsSize = layoutChild(
+      actionsSection!,
       constraints.deflate(EdgeInsets.only(top: contentSize.height + dividerThickness)),
-      parentUsesSize: true,
     );
-    final Size actionsSize = actionsSection!.size;
 
-    // Calculate overall alert height.
     final double actionSheetHeight = contentSize.height + dividerThickness + actionsSize.height;
+    return _AlertSizes(
+      size: Size(constraints.maxWidth, actionSheetHeight),
+      contentHeight: contentSize.height,
+    );
+  }
 
-    // Set our size now that layout calculations are complete.
-    size = Size(constraints.maxWidth, actionSheetHeight);
+  @override
+  Size computeDryLayout(BoxConstraints constraints) {
+    return _computeSizes(
+      constraints: constraints,
+      layoutChild: ChildLayoutHelper.dryLayoutChild,
+      dividerThickness: _computeDividerThickness(constraints),
+    ).size;
+  }
+
+  @override
+  void performLayout() {
+    final BoxConstraints constraints = this.constraints;
+    final double dividerThickness = _computeDividerThickness(constraints);
+    final _AlertSizes alertSizes = _computeSizes(
+      constraints: constraints,
+      layoutChild: ChildLayoutHelper.layoutChild,
+      dividerThickness: dividerThickness,
+    );
+
+    size = alertSizes.size;
 
     // Set the position of the actions box to sit at the bottom of the alert.
     // The content box defaults to the top left, which is where we want it.
     assert(actionsSection!.parentData is MultiChildLayoutParentData);
     final MultiChildLayoutParentData actionParentData = actionsSection!.parentData! as MultiChildLayoutParentData;
-    actionParentData.offset = Offset(0.0, contentSize.height + dividerThickness);
+    actionParentData.offset = Offset(0.0, alertSizes.contentHeight + dividerThickness);
   }
 
   @override
@@ -806,6 +824,13 @@ class _RenderCupertinoAlert extends RenderBox {
   }
 }
 
+class _AlertSizes {
+  const _AlertSizes({required this.size, required this.contentHeight});
+
+  final Size size;
+  final double contentHeight;
+}
+
 // Visual components of an alert that need to be explicitly sized and
 // laid out at runtime.
 enum _AlertSections {
@@ -1264,8 +1289,17 @@ class _RenderCupertinoAlertActions extends RenderBox
     return heightAccumulation;
   }
 
+  @override
+  Size computeDryLayout(BoxConstraints constraints) {
+    return _performLayout(constraints, dry: true);
+  }
+
   @override
   void performLayout() {
+    size = _performLayout(constraints, dry: false);
+  }
+
+  Size _performLayout(BoxConstraints constraints, {bool dry = false}) {
     final BoxConstraints perButtonConstraints = constraints.copyWith(
       minHeight: 0.0,
       maxHeight: double.infinity,
@@ -1275,16 +1309,21 @@ class _RenderCupertinoAlertActions extends RenderBox
     int index = 0;
     double verticalOffset = 0.0;
     while (child != null) {
-      child.layout(
-        perButtonConstraints,
-        parentUsesSize: true,
-      );
-
-      assert(child.parentData is MultiChildLayoutParentData);
-      final MultiChildLayoutParentData parentData = child.parentData! as MultiChildLayoutParentData;
-      parentData.offset = Offset(0.0, verticalOffset);
+      final Size childSize;
+      if (!dry) {
+        child.layout(
+          perButtonConstraints,
+          parentUsesSize: true,
+        );
+        childSize = child.size;
+        assert(child.parentData is MultiChildLayoutParentData);
+        final MultiChildLayoutParentData parentData = child.parentData! as MultiChildLayoutParentData;
+        parentData.offset = Offset(0.0, verticalOffset);
+      } else {
+        childSize = child.getDryLayout(constraints);
+      }
 
-      verticalOffset += child.size.height;
+      verticalOffset += childSize.height;
       if (index < childCount - 1) {
         // Add a gap for the next divider.
         verticalOffset += dividerThickness;
@@ -1294,7 +1333,7 @@ class _RenderCupertinoAlertActions extends RenderBox
       child = childAfter(child);
     }
 
-    size = constraints.constrain(
+    return constraints.constrain(
       Size(constraints.maxWidth, verticalOffset)
     );
   }