Flutter Stack Tutorial with Examples

Stack
children
alignment
textDirection
fit
overflow
clipBehavior

1. Stack

In Flutter, Stack is a container that allows placing its child widget on top of each other, the first child widget will be placed in the bottom. Stack is a solution to save space of the application. You can change the order of the child widgets to create a simple animation.

Stack stacks its children according to the principle: the first child widget will be placed on the bottom, the newest child widget will be placed on the top. When you change order of the child widgets, Stack will be redrawn. If the number and order of child widgets changes continuously, each child widget needs to be provided with a specific and unique Key value, helping Stack to efficiently manage child widgets.

IndexedStack is a subclass of Stack. Unlike Stack, IndexedStack only displays at most one widget at a time, and other widgets will be hidden. You can specify which child widget will be displayed through the property index, if the value of index is null, no child widget will be displayed.

Flutter IndexedStack

Stack Constructor

Stack(
    {Key key,
    List<Widget> children: const <Widget>[],
    AlignmentGeometry alignment: AlignmentDirectional.topStart,
    TextDirection textDirection,
    StackFit fit: StackFit.loose,
    Overflow overflow: Overflow.clip,
    Clip clipBehavior: Clip.hardEdge
    }
)

Here is a simple example, a Stack with 3 child widgets, by default they are located at the top right corner of the Stack, you can see them overlap each other:

(ex1)

Stack(
  children: <Widget>[
    Container(
      width: 290,
      height: 190,
      color: Colors.green,
    ),
    Container(
      width: 250,
      height: 170,
      color: Colors.red,
    ),
    Container(
      width: 220,
      height: 150,
      color: Colors.yellow,
    ),
  ],
)

Basically, Stack size is as small as possible, and try to be larger than all of its child widgets (Except child widgets such as Positioned or Transform, see more overflow property).

Let's see an example: A Stack with a child widget of maximum size.

(ex2)

Stack(
  children: <Widget>[
    Container( // First child (child 1)
      width: double.infinity,
      height: double.infinity,
      color: Colors.green,
      margin: EdgeInsets.all(20)
    ),
    Container(
      width: 250,
      height: 170,
      color: Colors.red,
    ),
    Container(
      width: 220,
      height: 150,
      color: Colors.yellow,
    ),
  ],
)

If you want to align position of a child widget (of Stack), put it in a Align.

Note: When widthFactor and heightFactor are not specified, but the child is specified, Align will be as large as possible.

(ex3)

Stack(
  children: <Widget>[
    Container(
        width: double.infinity,
        height: double.infinity,
        color: Colors.green,
        margin: EdgeInsets.all(20)
    ),
    Align (
      alignment: Alignment.centerRight,
      child: Container(
        width: 250,
        height: 170,
        color: Colors.red,
      ),
    ),
    Container(
      width: 220,
      height: 150,
      color: Colors.yellow,
    ),
  ],
)

Flutter Align

You can also align the position of a child widget (of Stack) by placing it in a Positioned.

Flutter Positioned

2. children

children - is a list of child widget of a Stack.

List<Widget> children: const <Widget>[]

3. alignment

alignment property is used to align child widgets other than Positioned. Its default value is AlignmentDirectional.topStart.

AlignmentGeometry alignment: AlignmentDirectional.topStart

Flutter AlignmentDirectional

Example below shows alignment property of Stack has no effect on the child widget which is Positioned.

SizedBox (
   width: double.infinity,
   height: double.infinity,
   child: Stack(
     alignment: Alignment.centerLeft,
     children: <Widget>[
       Container(
         width: 290,
         height: 190,
         color: Colors.green,
       ),
       Container(
         width: 220,
         height: 130,
         color: Colors.yellow,
       ),
       Positioned (
         bottom: 10,
         right: 10,
         child: Container(
           width: 150,
           height: 90,
           color: Colors.red,
         ),
       ),
     ],
   )
)

4. textDirection

textDirection property is used to set text direction. Its value affects behavior of alignment property.

TextDirection textDirection

5. fit

fit property shows "How to define size non-Positioned child widgets of IndexedStack". Its default value is StackFit.loose.

StackFit fit: StackFit.loose

// Enum:
 StackFit.expand
 StackFit.loose
 StackFit.passthrough

Flutter StackFit

6. overflow

overflow property is used to specify how Stack treats those child widgets that overflow from it. The treatment could be to clip the overflow, or allow the overflow to show. Default value of overflow is Overflow.clip.

Overflow overflow: Overflow.clip

// Enum:
Overflow.clip
Overflow.visible

Test results show that overflow:Overflow.visible only works with some types of widget (eg Positioned or Transform).

Flutter Positioned
Flutter Transform

Example:

overflow (ex1)

Container (
    width: 250,
    height: 250,
    color: Colors.blueGrey,
    margin: EdgeInsets.all(20),
    child: Stack (
      overflow: Overflow.visible,
      children: <Widget>[
        Positioned(
          top: 50,
          left: 50,
          child: Container(
            width: 290,
            height: 100,
            color: Colors.green,
          ),
        ),
        Positioned(
          top: 70,
          left: 70,
          child: Container(
            width: 120,
            height: 230,
            color: Colors.yellow,
          ),
        )
      ],
    )
)

Example: A Transform is skewed along Y axis, it can be overflow from Stack. The overflow along Y axis will be displayed, while that of X axis will be clipped.

overflow (ex2)

Container (
    width: 250,
    height: 250,
    margin: EdgeInsets.all(20),
    color: Colors.blueGrey,
    child: Stack (
      overflow: Overflow.visible,
      children: <Widget>[
        Container(
          height: 100,
          width: 300,
          color: Colors.green,
        ),
        Transform(
          alignment: FractionalOffset.topLeft,
          transform: Matrix4.skewY(0.7), // skew will not go out of y bounds
          child: Container(
            height: 100,
            width: 300,
            color: Colors.red,
          ),
        ),
      ],
    )
)

Flutter Transform

7. clipBehavior

Clip clipBehavior: Clip.hardEdge

Flutter Clip clipBehavior