Column Widget

A widget that displays its children in a vertical array.

The Column widget does not scroll (and in general it is considered an error to have more children in a Column than will fit in the available room). If you have a line of widgets and want them to be able to scroll if there is insufficient room, consider using a ListView.

Here is an example of using Column widget -

Column(
  children: <Widget>[
    Text('Deliver features faster'),
    Text('Craft beautiful UIs'),
    Expanded(
      child: FittedBox(
        fit: BoxFit.contain, // otherwise the logo will be tiny
        child: const FlutterLogo(),
      ),
    ),
  ],
)

When the incoming vertical constraints are unbounded

When a Column has one or more Expanded or Flexible children, and is placed in another Column, or in a ListView, or in some other context that does not provide a maximum height constraint for the Column, you will get an exception at runtime saying that there are children with non-zero flex but the vertical constraints are unbounded.

The reason is that using Flexible or Expanded means that the remaining space after laying out all the other children must be shared equally, but if the incoming vertical constraints are unbounded, there is infinite remaining space.

The key to solving this problem is usually to determine why the Column is receiving unbounded vertical constraints.

One common reason for this to happen is that the Column has been placed in another Column (without using Expanded or Flexible around the inner nested Column). When a Column lays out its non-flex children (those that have neither Expanded or Flexible around them), it gives them unbounded constraints so that they can determine their own dimensions (passing unbounded constraints usually signals to the child that it should shrink-wrap its contents). The solution in this case is typically to just wrap the inner column in an Expanded to indicate that it should take the remaining space of the outer column, rather than being allowed to take any amount of room it desires.

Another reason for this message to be displayed is nesting a Column inside a ListView or other vertical scrollable. In that scenario, there really is infinite vertical space (the whole point of a vertical scrolling list is to allow infinite space vertically). In such scenarios, it is usually worth examining why the inner Column should have an Expanded or Flexible child: what size should the inner children really be? The solution in this case is typically to remove the Expanded or Flexible widgets from around the inner children.

The yellow and black striped banner

When the contents of a Column exceed the amount of space available, the Column overflows, and the contents are clipped. In debug mode, a yellow and black striped bar is rendered at the overflowing edge to indicate the problem, and a message is printed below the Column saying how much overflow was detected.

The usual solution is to use a ListView rather than a Column, to enable the contents to scroll when vertical space is limited.

Layout algorithm

Layout for a Column proceeds in six steps:

• Layout each child a null or zero flex factor (e.g., those that are not Expanded) with unbounded vertical constraints and the incoming horizontal constraints. If the crossAxisAlignment is CrossAxisAlignment.stretch, instead use tight horizontal constraints that match the incoming max width.
• Divide the remaining vertical space among the children with non-zero flex factors (e.g., those that are Expanded) according to their flex factor. For example, a child with a flex factor of 2.0 will receive twice the amount of vertical space as a child with a flex factor of 1.0.
• Layout each of the remaining children with the same horizontal constraints as in step 1, but instead of using unbounded vertical constraints, use vertical constraints based on the amount of space allocated in step 2. Children with Flexible.fit properties that are FlexFit.tight are given tight constraints (i.e., forced to fill the allocated space), and children with Flexible.fit properties that are FlexFit.loose are given loose constraints (i.e., not forced to fill the allocated space).
• The width of the Column is the maximum width of the children (which will always satisfy the incoming horizontal constraints).
• The height of the Column is determined by the mainAxisSize property. If the mainAxisSize property is MainAxisSize.max, then the height of the Column is the max height of the incoming constraints. If the mainAxisSize property is MainAxisSize.min, then the height of the Column is the sum of heights of the children (subject to the incoming constraints).
• Determine the position for each child according to the mainAxisAlignment and the crossAxisAlignment. For example, if the mainAxisAlignment is MainAxisAlignment.spaceBetween, any vertical space that has not been allocated to children is divided evenly and placed between the children.

Properties

You can use crossAxisAlignment for horizontal positioning and mainAxisAlignment for vertical positioning and mainAxisSize for controlling the height of the column. To know more how to use these arguments, see Row Widget Section.