Flutter自定义布局套路

开始

在Android中我们要实现一个布局需要继承ViewGroup, 重写其中的onLayout和onMeasure方法. 其中onLayout负责给子控件设置布局区域, onMeaseure度量子控件大小和自身大小. 今天我们就研究下Flutter是如何实现布局的.

Flutter布局

首先我们挑选一个Flutter控件去看源码, 我们就选Stack, 因为它足够简单. 从表象上讲它只要重叠摆放一组子控件即可. 先看下Stack的源码:

class Stack extends MultiChildRenderObjectWidget {
  Stack({
    Key key,
    this.alignment: AlignmentDirectional.topStart,
    this.textDirection,
    this.fit: StackFit.loose,
    this.overflow: Overflow.clip,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children);

  final AlignmentGeometry alignment;
  final StackFit fit;
  final Overflow overflow;

  @override
  RenderStack createRenderObject(BuildContext context) {
    return new RenderStack(
      alignment: alignment,
      textDirection: textDirection ?? Directionality.of(context),
      fit: fit,
      overflow: overflow,
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderStack renderObject) {
    renderObject
      ..alignment = alignment
      ..textDirection = textDirection ?? Directionality.of(context)
      ..fit = fit
      ..overflow = overflow;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DiagnosticsProperty<AlignmentGeometry>('alignment', alignment));
    properties.add(new EnumProperty<TextDirection>('textDirection', textDirection, defaultValue: null));
    properties.add(new EnumProperty<StackFit>('fit', fit));
    properties.add(new EnumProperty<Overflow>('overflow', overflow));
  }
}

Stack继承自MultiChildRenderObjectWidget, 重写了createRenderObject其返回了一个RenderStack对象, 实际的工作者. 而updateRenderObject则只是修改RenderStack对象的属性. debugFillProperties方法则是填充该类属性的参数值到DiagnosticPropertiesBuilder中.

我们看看Flex, 也是如此, 重写了createRenderObject其返回了一个RenderFlex对象, 实际的工作者. 而updateRenderObject则只是修改RenderFlex对象的属性.

所以我们接下来看看RenderStack, 精简代码如下:

class RenderStack extends RenderBox
    with 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;

  @override
  void performLayout() {
    _resolve();
    assert(_resolvedAlignment != null);
    _hasVisualOverflow = false;
    bool hasNonPositionedChildren = false;
    if (childCount == ) {
      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;
    }
  }

  @protected
  void paintStack(PaintingContext context, Offset offset) {
    defaultPaint(context, offset);
  }

  @override
  void 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 {
  ...
  @override
  bool 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);
  }

  @override
  void 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);
  }
  ...
}

重写了RenderStack的paintStack和hitTestChildren方法, 只绘制选中的子控件, 和接收事件.

总结

实现一个自定义布局, 我们需要先继承MultiChildRenderObjectWidget, 然后重写createRenderObject和updateRenderObject方法, 前者返回我们自定义的RenderBox的对象. 后者更新想要传递的属性. 然后需要我们继承RenderBox, 来扩展我们想要的功能特性.