Use RenderSliverPadding to inset SliverFillViewport (#45432)

Author: LongCatIsLooong

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

@@ -57,41 +57,6 @@ class RenderSliverFillViewport extends RenderSliverFixedExtentBoxAdaptor {
     _viewportFraction = value;
     markNeedsLayout();
   }
-
-  double get _padding => (1.0 - viewportFraction) * constraints.viewportMainAxisExtent * 0.5;
-
-  @override
-  double indexToLayoutOffset(double itemExtent, int index) {
-    return _padding + super.indexToLayoutOffset(itemExtent, index);
-  }
-
-  @override
-  int getMinChildIndexForScrollOffset(double scrollOffset, double itemExtent) {
-    return super.getMinChildIndexForScrollOffset(math.max(scrollOffset - _padding, 0.0), itemExtent);
-  }
-
-  @override
-  int getMaxChildIndexForScrollOffset(double scrollOffset, double itemExtent) {
-    return super.getMaxChildIndexForScrollOffset(math.max(scrollOffset - _padding, 0.0), itemExtent);
-  }
-
-  @override
-  double estimateMaxScrollOffset(
-    SliverConstraints constraints, {
-    int firstIndex,
-    int lastIndex,
-    double leadingScrollOffset,
-    double trailingScrollOffset,
-  }) {
-    final double padding = _padding;
-    return childManager.estimateMaxScrollOffset(
-      constraints,
-      firstIndex: firstIndex,
-      lastIndex: lastIndex,
-      leadingScrollOffset: leadingScrollOffset - padding,
-      trailingScrollOffset: trailingScrollOffset - padding,
-    ) + padding + padding;
-  }
 }
 
 /// A sliver that contains a single box child that fills the remaining space in

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

@@ -195,7 +195,7 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
     if (firstChild == null) {
       if (!addInitialChild(index: firstIndex, layoutOffset: indexToLayoutOffset(itemExtent, firstIndex))) {
         // There are either no children, or we are past the end of all our children.
-        // If it is the later, we will need to find the first available child.
+        // If it is the latter, we will need to find the first available child.
         double max;
         if (childManager.childCount != null) {
           max = computeMaxScrollOffset(constraints, itemExtent);

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

@@ -12,73 +12,27 @@ import 'debug.dart';
 import 'object.dart';
 import 'sliver.dart';
 
-/// Inset a [RenderSliver], applying padding on each side.
+/// Insets a [RenderSliver] by applying [resolvedPadding] on each side.
 ///
-/// A [RenderSliverPadding] object wraps the [SliverGeometry.layoutExtent] of
-/// its child. Any incoming [SliverConstraints.overlap] is ignored and not
+/// A [RenderSliverEdgeInsetsPadding] subclass wraps the [SliverGeometry.layoutExtent]
+/// of its child. Any incoming [SliverConstraints.overlap] is ignored and not
 /// passed on to the child.
 ///
+/// {@template flutter.rendering.sliverPadding.limitation}
 /// Applying padding to anything but the most mundane sliver is likely to have
-/// undesired effects. For example, wrapping a
-/// [RenderSliverPinnedPersistentHeader] will cause the app bar to overlap
-/// earlier slivers (contrary to the normal behavior of pinned app bars), and
-/// while the app bar is pinned, the padding will scroll away.
-class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<RenderSliver> {
-  /// Creates a render object that insets its child in a viewport.
-  ///
-  /// The [padding] argument must not be null and must have non-negative insets.
-  RenderSliverPadding({
-    @required EdgeInsetsGeometry padding,
-    TextDirection textDirection,
-    RenderSliver child,
-  }) : assert(padding != null),
-       assert(padding.isNonNegative),
-       _padding = padding,
-       _textDirection = textDirection {
-    this.child = child;
-  }
-
-  EdgeInsets _resolvedPadding;
-
-  void _resolve() {
-    if (_resolvedPadding != null)
-      return;
-    _resolvedPadding = padding.resolve(textDirection);
-    assert(_resolvedPadding.isNonNegative);
-  }
-
-  void _markNeedResolution() {
-    _resolvedPadding = null;
-    markNeedsLayout();
-  }
-
+/// undesired effects. For example, wrapping a [RenderSliverPinnedPersistentHeader]
+/// will cause the app bar to overlap earlier slivers (contrary to the normal
+/// behavior of pinned app bars), and while the app bar is pinned, the padding
+/// will scroll away.
+/// {@endtemplate}
+abstract class RenderSliverEdgeInsetsPadding extends RenderSliver with RenderObjectWithChildMixin<RenderSliver> {
   /// The amount to pad the child in each dimension.
   ///
-  /// If this is set to an [EdgeInsetsDirectional] object, then [textDirection]
-  /// must not be null.
-  EdgeInsetsGeometry get padding => _padding;
-  EdgeInsetsGeometry _padding;
-  set padding(EdgeInsetsGeometry value) {
-    assert(value != null);
-    assert(padding.isNonNegative);
-    if (_padding == value)
-      return;
-    _padding = value;
-    _markNeedResolution();
-  }
-
-  /// The text direction with which to resolve [padding].
+  /// The offsets are specified in terms of visual edges, left, top, right, and
+  /// bottom. These values are not affected by the [TextDirection].
   ///
-  /// This may be changed to null, but only after the [padding] has been changed
-  /// to a value that does not depend on the direction.
-  TextDirection get textDirection => _textDirection;
-  TextDirection _textDirection;
-  set textDirection(TextDirection value) {
-    if (_textDirection == value)
-      return;
-    _textDirection = value;
-    _markNeedResolution();
-  }
+  /// Must not be null or contain negative values when [performLayout] is called.
+  EdgeInsets get resolvedPadding;
 
   /// The padding in the scroll direction on the side nearest the 0.0 scroll direction.
   ///
@@ -88,16 +42,16 @@ class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<R
     assert(constraints != null);
     assert(constraints.axisDirection != null);
     assert(constraints.growthDirection != null);
-    assert(_resolvedPadding != null);
+    assert(resolvedPadding != null);
     switch (applyGrowthDirectionToAxisDirection(constraints.axisDirection, constraints.growthDirection)) {
       case AxisDirection.up:
-        return _resolvedPadding.bottom;
+        return resolvedPadding.bottom;
       case AxisDirection.right:
-        return _resolvedPadding.left;
+        return resolvedPadding.left;
       case AxisDirection.down:
-        return _resolvedPadding.top;
+        return resolvedPadding.top;
       case AxisDirection.left:
-        return _resolvedPadding.right;
+        return resolvedPadding.right;
     }
     return null;
   }
@@ -110,16 +64,16 @@ class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<R
     assert(constraints != null);
     assert(constraints.axisDirection != null);
     assert(constraints.growthDirection != null);
-    assert(_resolvedPadding != null);
+    assert(resolvedPadding != null);
     switch (applyGrowthDirectionToAxisDirection(constraints.axisDirection, constraints.growthDirection)) {
       case AxisDirection.up:
-        return _resolvedPadding.top;
+        return resolvedPadding.top;
       case AxisDirection.right:
-        return _resolvedPadding.right;
+        return resolvedPadding.right;
       case AxisDirection.down:
-        return _resolvedPadding.bottom;
+        return resolvedPadding.bottom;
       case AxisDirection.left:
-        return _resolvedPadding.left;
+        return resolvedPadding.left;
     }
     return null;
   }
@@ -133,8 +87,8 @@ class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<R
   double get mainAxisPadding {
     assert(constraints != null);
     assert(constraints.axis != null);
-    assert(_resolvedPadding != null);
-    return _resolvedPadding.along(constraints.axis);
+    assert(resolvedPadding != null);
+    return resolvedPadding.along(constraints.axis);
   }
 
   /// The total padding in the cross-axis direction. (In other words, for a
@@ -146,12 +100,12 @@ class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<R
   double get crossAxisPadding {
     assert(constraints != null);
     assert(constraints.axis != null);
-    assert(_resolvedPadding != null);
+    assert(resolvedPadding != null);
     switch (constraints.axis) {
       case Axis.horizontal:
-        return _resolvedPadding.vertical;
+        return resolvedPadding.vertical;
       case Axis.vertical:
-        return _resolvedPadding.horizontal;
+        return resolvedPadding.horizontal;
     }
     return null;
   }
@@ -164,8 +118,7 @@ class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<R
 
   @override
   void performLayout() {
-    _resolve();
-    assert(_resolvedPadding != null);
+    assert(resolvedPadding != null);
     final double beforePadding = this.beforePadding;
     final double afterPadding = this.afterPadding;
     final double mainAxisPadding = this.mainAxisPadding;
@@ -240,16 +193,16 @@ class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<R
     assert(constraints.growthDirection != null);
     switch (applyGrowthDirectionToAxisDirection(constraints.axisDirection, constraints.growthDirection)) {
       case AxisDirection.up:
-        childParentData.paintOffset = Offset(_resolvedPadding.left, calculatePaintOffset(constraints, from: _resolvedPadding.bottom + childLayoutGeometry.scrollExtent, to: _resolvedPadding.bottom + childLayoutGeometry.scrollExtent + _resolvedPadding.top));
+        childParentData.paintOffset = Offset(resolvedPadding.left, calculatePaintOffset(constraints, from: resolvedPadding.bottom + childLayoutGeometry.scrollExtent, to: resolvedPadding.bottom + childLayoutGeometry.scrollExtent + resolvedPadding.top));
         break;
       case AxisDirection.right:
-        childParentData.paintOffset = Offset(calculatePaintOffset(constraints, from: 0.0, to: _resolvedPadding.left), _resolvedPadding.top);
+        childParentData.paintOffset = Offset(calculatePaintOffset(constraints, from: 0.0, to: resolvedPadding.left), resolvedPadding.top);
         break;
       case AxisDirection.down:
-        childParentData.paintOffset = Offset(_resolvedPadding.left, calculatePaintOffset(constraints, from: 0.0, to: _resolvedPadding.top));
+        childParentData.paintOffset = Offset(resolvedPadding.left, calculatePaintOffset(constraints, from: 0.0, to: resolvedPadding.top));
         break;
       case AxisDirection.left:
-        childParentData.paintOffset = Offset(calculatePaintOffset(constraints, from: _resolvedPadding.right + childLayoutGeometry.scrollExtent, to: _resolvedPadding.right + childLayoutGeometry.scrollExtent + _resolvedPadding.left), _resolvedPadding.top);
+        childParentData.paintOffset = Offset(calculatePaintOffset(constraints, from: resolvedPadding.right + childLayoutGeometry.scrollExtent, to: resolvedPadding.right + childLayoutGeometry.scrollExtent + resolvedPadding.left), resolvedPadding.top);
         break;
     }
     assert(childParentData.paintOffset != null);
@@ -289,14 +242,14 @@ class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<R
     assert(constraints != null);
     assert(constraints.axisDirection != null);
     assert(constraints.growthDirection != null);
-    assert(_resolvedPadding != null);
+    assert(resolvedPadding != null);
     switch (applyGrowthDirectionToAxisDirection(constraints.axisDirection, constraints.growthDirection)) {
       case AxisDirection.up:
       case AxisDirection.down:
-        return _resolvedPadding.left;
+        return resolvedPadding.left;
       case AxisDirection.left:
       case AxisDirection.right:
-        return _resolvedPadding.top;
+        return resolvedPadding.top;
     }
     return null;
   }
@@ -346,6 +299,79 @@ class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<R
       return true;
     }());
   }
+}
+
+/// Insets a [RenderSliver], applying padding on each side.
+///
+/// A [RenderSliverPadding] object wraps the [SliverGeometry.layoutExtent] of
+/// its child. Any incoming [SliverConstraints.overlap] is ignored and not
+/// passed on to the child.
+///
+/// {@macro flutter.rendering.sliverPadding.limitation}
+class RenderSliverPadding extends RenderSliverEdgeInsetsPadding {
+  /// Creates a render object that insets its child in a viewport.
+  ///
+  /// The [padding] argument must not be null and must have non-negative insets.
+  RenderSliverPadding({
+    @required EdgeInsetsGeometry padding,
+    TextDirection textDirection,
+    RenderSliver child,
+  }) : assert(padding != null),
+       assert(padding.isNonNegative),
+       _padding = padding,
+       _textDirection = textDirection {
+    this.child = child;
+  }
+
+  @override
+  EdgeInsets get resolvedPadding => _resolvedPadding;
+  EdgeInsets _resolvedPadding;
+
+  void _resolve() {
+    if (resolvedPadding != null)
+      return;
+    _resolvedPadding = padding.resolve(textDirection);
+    assert(resolvedPadding.isNonNegative);
+  }
+
+  void _markNeedsResolution() {
+    _resolvedPadding = null;
+    markNeedsLayout();
+  }
+
+  /// The amount to pad the child in each dimension.
+  ///
+  /// If this is set to an [EdgeInsetsDirectional] object, then [textDirection]
+  /// must not be null.
+  EdgeInsetsGeometry get padding => _padding;
+  EdgeInsetsGeometry _padding;
+  set padding(EdgeInsetsGeometry value) {
+    assert(value != null);
+    assert(padding.isNonNegative);
+    if (_padding == value)
+      return;
+    _padding = value;
+    _markNeedsResolution();
+  }
+
+  /// The text direction with which to resolve [padding].
+  ///
+  /// This may be changed to null, but only after the [padding] has been changed
+  /// to a value that does not depend on the direction.
+  TextDirection get textDirection => _textDirection;
+  TextDirection _textDirection;
+  set textDirection(TextDirection value) {
+    if (_textDirection == value)
+      return;
+    _textDirection = value;
+    _markNeedsResolution();
+  }
+
+  @override
+  void performLayout() {
+    _resolve();
+    super.performLayout();
+  }
 
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {

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

@@ -345,8 +345,16 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
       forcePixels(getPixelsFromPage(oldPage));
   }
 
+  // The amount of offset that will be added to [minScrollExtent] and subtracted
+  // from [maxScrollExtent], such that every page will properly snap to the center
+  // of the viewport when viewportFraction is greater than 1.
+  //
+  // The value is 0 if viewportFraction is less than or equal to 1, larger than 0
+  // otherwise.
+  double get _initialPageOffset => math.max(0, viewportDimension * (viewportFraction - 1) / 2);
+
   double getPageFromPixels(double pixels, double viewportDimension) {
-    final double actual = math.max(0.0, pixels) / math.max(1.0, viewportDimension * viewportFraction);
+    final double actual = math.max(0.0, pixels - _initialPageOffset) / math.max(1.0, viewportDimension * viewportFraction);
     final double round = actual.roundToDouble();
     if ((actual - round).abs() < precisionErrorTolerance) {
       return round;
@@ -355,7 +363,7 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
   }
 
   double getPixelsFromPage(double page) {
-    return page * viewportDimension * viewportFraction;
+    return page * viewportDimension * viewportFraction + _initialPageOffset;
   }
 
   @override
@@ -396,6 +404,15 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
     return result;
   }
 
+  @override
+  bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
+    final double newMinScrollExtent = minScrollExtent + _initialPageOffset;
+    return super.applyContentDimensions(
+      newMinScrollExtent,
+      math.max(newMinScrollExtent, maxScrollExtent - _initialPageOffset),
+    );
+  }
+
   @override
   PageMetrics copyWith({
     double minScrollExtent,