Flutter中的一个小技巧

///打印日志的类
class Log {
  Log._();

  static void printMsg(Object obj) {
    assert(() {
      print(obj.toString());
      return true;
    }());
  }
}

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系统代码中，有大量的这种形式，因为断言只在调试模式执行，这样就可以将一些只需要在调试模式下执行的代码放到断言里，release的时候是不执行的，比如我们新手阶段常见的这个：

拿RenderFlex来举例，就是在它的paint方法里有这样的代码：

@override
  void paint(PaintingContext context, Offset offset) {
    if (_overflow <= 0.0) {
      defaultPaint(context, offset);
      return;
    }

    // There's no point in drawing the children if we're empty.
    if (size.isEmpty)
      return;

    // We have overflow. Clip it.
    context.pushClipRect(needsCompositing, offset, Offset.zero & size, defaultPaint);

    assert(() {
      // Only set this if it's null to save work. It gets reset to null if the
      // _direction changes.
      final String debugOverflowHints =
        'The overflowing $runtimeType has an orientation of $_direction.\n'
        'The edge of the $runtimeType that is overflowing has been marked '
        'in the rendering with a yellow and black striped pattern. This is '
        'usually caused by the contents being too big for the $runtimeType. '
        'Consider applying a flex factor (e.g. using an Expanded widget) to '
        'force the children of the $runtimeType to fit within the available '
        'space instead of being sized to their natural size.\n'
        'This is considered an error condition because it indicates that there '
        'is content that cannot be seen. If the content is legitimately bigger '
        'than the available space, consider clipping it with a ClipRect widget '
        'before putting it in the flex, or using a scrollable container rather '
        'than a Flex, like a ListView.';

      // Simulate a child rect that overflows by the right amount. This child
      // rect is never used for drawing, just for determining the overflow
      // location and amount.
      Rect overflowChildRect;
      switch (_direction) {
        case Axis.horizontal:
          overflowChildRect = Rect.fromLTWH(0.0, 0.0, size.width + _overflow, 0.0);
          break;
        case Axis.vertical:
          overflowChildRect = Rect.fromLTWH(0.0, 0.0, 0.0, size.height + _overflow);
          break;
      }
      paintOverflowIndicator(context, offset, Offset.zero & size, overflowChildRect, overflowHints: debugOverflowHints);
      return true;
    }());
  }
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