class RenderStack extends RenderBoxwith ContainerRenderObjectMixin<RenderBox, StackParentData>,RenderBoxContainerDefaultsMixin<RenderBox, StackParentData> {RenderStack({List<RenderBox> children,AlignmentGeometry alignment: AlignmentDirectional.topStart,TextDirection textDirection,StackFit fit: StackFit.loose,Overflow overflow: Overflow.clip,}) : assert(alignment != null),assert(fit != null),assert(overflow != null),_alignment = alignment,_textDirection = textDirection,_fit = fit,_overflow = overflow {addAll(children);}bool _hasVisualOverflow = false;@overridevoid performLayout() {_resolve();assert(_resolvedAlignment != null);_hasVisualOverflow = false;bool hasNonPositionedChildren = false;if (childCount == 0) {size = constraints.biggest;assert(size.isFinite);return;}double width = constraints.minWidth;double height = constraints.minHeight;BoxConstraints nonPositionedConstraints;assert(fit != null);switch (fit) {case StackFit.loose:nonPositionedConstraints = constraints.loosen();break;case StackFit.expand:nonPositionedConstraints = new BoxConstraints.tight(constraints.biggest);break;case StackFit.passthrough:nonPositionedConstraints = constraints;break;}assert(nonPositionedConstraints != null);RenderBox child = firstChild;while (child != null) {final StackParentData childParentData = child.parentData;if (!childParentData.isPositioned) {hasNonPositionedChildren = true;child.layout(nonPositionedConstraints, parentUsesSize: true);final Size childSize = child.size;width = math.max(width, childSize.width);height = math.max(height, childSize.height);}child = childParentData.nextSibling;}if (hasNonPositionedChildren) {size = new Size(width, height);assert(size.width == constraints.constrainWidth(width));assert(size.height == constraints.constrainHeight(height));} else {size = constraints.biggest;}assert(size.isFinite);child = firstChild;while (child != null) {final StackParentData childParentData = child.parentData;if (!childParentData.isPositioned) {childParentData.offset = _resolvedAlignment.alongOffset(size - child.size);} else {BoxConstraints childConstraints = const BoxConstraints();if (childParentData.left != null && childParentData.right != null)childConstraints = childConstraints.tighten(width: size.width - childParentData.right - childParentData.left);else if (childParentData.width != null)childConstraints = childConstraints.tighten(width: childParentData.width);if (childParentData.top != null && childParentData.bottom != null)childConstraints = childConstraints.tighten(height: size.height - childParentData.bottom - childParentData.top);else if (childParentData.height != null)childConstraints = childConstraints.tighten(height: childParentData.height);child.layout(childConstraints, parentUsesSize: true);double x;if (childParentData.left != null) {x = childParentData.left;} else if (childParentData.right != null) {x = size.width - childParentData.right - child.size.width;} else {x = _resolvedAlignment.alongOffset(size - child.size).dx;}if (x < 0.0 || x + child.size.width > size.width)_hasVisualOverflow = true;double y;if (childParentData.top != null) {y = childParentData.top;} else if (childParentData.bottom != null) {y = size.height - childParentData.bottom - child.size.height;} else {y = _resolvedAlignment.alongOffset(size - child.size).dy;}if (y < 0.0 || y + child.size.height > size.height)_hasVisualOverflow = true;childParentData.offset = new Offset(x, y);}assert(child.parentData == childParentData);child = childParentData.nextSibling;}}@protectedvoid paintStack(PaintingContext context, Offset offset) {defaultPaint(context, offset);}@overridevoid paint(PaintingContext context, Offset offset) {if (_overflow == Overflow.clip && _hasVisualOverflow) {context.pushClipRect(needsCompositing, offset, Offset.zero & size, paintStack);} else {paintStack(context, offset);}}
}

可以看出RenderStack接收了所有传递给Stack的参数, 毕竟RenderStack才是实际干活的^^. performLayout负责了所有布局相关的工作. performLayout首先分析StackFit参数, 该参数有3个值:

• StackFit.loose 按最小的来.
• StackFit.expand 按最大的来.
• StackFit.passthrough Stack上层为->Expanded->Row, 横向尽量大, 纵向尽量小.

得出BoxConstraints. 然后遍历所有子控件, 如果不是Positioned类型子控件, 则将BoxConstraints传给子控件让它根据父控件大小自己内部布局. 并且记录下所有子控件结合RenderStack自生大小得出的最大高度和宽度. 将其设置为当前控件大小.

接着再继续从头遍历子控件, 如果不是Positioned类型子控件, 根据alignment参数, 设置子控件在父控件中的偏移量, 比如Stack设置了居中, 上面计算出宽100, 高200, 而子控件宽30, 高30, 那么子控件需要偏移x=35, y=85. 如果是Positioned类型的子控件, 先将RenderStack的size大小, 减去Positioned属性里的大小. 再来计算便宜量.

这个里面有_hasVisualOverflow变量, 如果内容超出RenderStack大小, 其值为true. 也就是我们写布局时, 内容超过范围了, 报出来一个色块提示, 就是如此得出的.
_overflow属性则指定了子控件的绘制区域是否能超过父控件, 跟Android中的clipChildren属性很像.

另外我们再分析下IndexedStack, 该控件一次只能显示一个子控件. 其实际差异在RenderIndexedStack

class RenderIndexedStack extends RenderStack {...@overridebool hitTestChildren(HitTestResult result, { @required Offset position }) {if (firstChild == null || index == null)return false;assert(position != null);final RenderBox child = _childAtIndex();final StackParentData childParentData = child.parentData;return child.hitTest(result, position: position - childParentData.offset);}@overridevoid paintStack(PaintingContext context, Offset offset) {if (firstChild == null || index == null)return;final RenderBox child = _childAtIndex();final StackParentData childParentData = child.parentData;context.paintChild(child, childParentData.offset + offset);}...
}