開始

在Android中我們要實現一個佈局需要繼承ViewGroup, 重寫其中的onLayout和onMeasure方法. 其中onLayout負責給子控制元件設定佈局區域, onMeaseure度量子控制元件大小和自身大小. 今天我們就研究下Flutter是如何實現佈局的.

Flutter佈局

首先我們挑選一個Flutter控制元件去看原始碼, 我們就選Stack, 因為它足夠簡單. 從表象上講它只要重疊擺放一組子控制元件即可. 先看下Stack的原始碼:

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繼承自MultiChildRenderObjectWidget, 重寫了createRenderObject其返回了一個RenderStack物件, 實際的工作者. 而updateRenderObject則只是修改RenderStack物件的屬性. debugFillProperties方法則是填充該類屬性的引數值到DiagnosticPropertiesBuilder中.

我們看看Flex, 也是如此, 重寫了createRenderObject其返回了一個RenderFlex物件, 實際的工作者. 而updateRenderObject則只是修改RenderFlex物件的屬性.

所以我們接下來看看RenderStack, 精簡程式碼如下:

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 == 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;
    }
  }

  @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接收了所有傳遞給Stack的引數, 畢竟RenderStack才是實際幹活的^^. performLayout負責了所有佈局相關的工作. performLayout首先分析StackFit引數, 該引數有3個值:

• StackFit.loose 按最小的來.
• StackFit.expand 按最大的來.
• StackFit.passthrough Stack上層為->Expanded->Row, 橫向儘量大, 縱向儘量小.

得出BoxConstraints. 然後遍歷所有子控制元件, 如果不是Positioned型別子控制元件, 則將BoxConstraints傳給子控制元件讓它根據父控制元件大小自己內部佈局. 並且記錄下所有子控制元件結合RenderStack自生大小得出的最大高度和寬度. 將其設定為當前控制元件大小.

接著再繼續從頭遍歷子控制元件, 如果不是Positioned型別子控制元件, 根據alignment引數, 設定子控制元件在父控制元件中的偏移量, 比如Stack設定了居中, 上面計算出寬100, 高200, 而子控制元件寬30, 高30, 那麼子控制元件需要偏移x=35, y=85. 如果是Positioned型別的子控制元件, 先將RenderStack的size大小, 減去Positioned屬性裡的大小. 再來計算便宜量.

這個裡面有_hasVisualOverflow變數, 如果內容超出RenderStack大小, 其值為true. 也就是我們寫佈局時, 內容超過範圍了, 報出來一個色塊提示, 就是如此得出的.
_overflow屬性則指定了子控制元件的繪製區域是否能超過父控制元件, 跟Android中的clipChildren屬性很像.

另外我們再分析下IndexedStack, 該控制元件一次只能顯示一個子控制元件. 其實際差異在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);
  }
  ...
}

重寫了RenderStack的paintStack和hitTestChildren方法, 只繪製選中的子控制元件, 和接收事件.

總結

實現一個自定義佈局, 我們需要先繼承MultiChildRenderObjectWidget, 然後重寫createRenderObject和updateRenderObject方法, 前者返回我們自定義的RenderBox的物件. 後者更新想要傳遞的屬性. 然後需要我們繼承RenderBox, 來擴充套件我們想要的功能特性.