Dim.Auto Deep Dive

The Dim.Auto type is a type of Dim that automatically sizes the view based on its content. This is useful when you want to size a view based on the content it contains. That content can either be the Text, the SubViews, or something else defined by the view.

Like all Dim types, Dim.Auto is used to set the Width or Height of a view and can be combined with other Dim types using addition or subtraction (see. DimCombine).

The DimAutoStyle enum defines the different ways that Dim.Auto can be used to size a view. The DimAutoStyle enum has the following values:

Text - The view is sized based on the Text property and TextFormatter settings.
Content - The view is sized based on either the value returned by View.SetContentSize() or the Subviews property. If the content size is not explicitly set (via View.SetContentSize()), the view is sized based on the Subview with the largest relvant dimension plus location. If the content size is explicitly set, the view is sized based on the value returned by View.SetContentSize().
Auto - The view is sized based on both the text and content, whichever is larger.

Using Dim.Auto

Dim.Auto is defined as:

public static Dim Auto (DimAutoStyle style = DimAutoStyle.Auto, Dim minimumContentDim = null, Dim max = null)

To use Dim.Auto, set the Width or Height property of a view to Dim.Auto (DimAutoStyle.Text) or Dim.Auto (DimAutoStyle.Content).

For example, to create a View that is sized based on the Text property, you can do this:

View view = new ()
{
    Text = "Hello, World!",
    Width = Dim.Auto (DimAutoStyle.Text),
    Height = Dim.Auto (DimAutoStyle.Text),
};

Note, the built-in Label view class does precisely this in its constructor.

To create a View that is sized based on its Subviews, you can do this:

View view = new ()
{
    Width = Dim.Auto (DimAutoStyle.Content),
    Height = Dim.Auto (DimAutoStyle.Content),
};
view.Add (new Label () { Text = "Hello, World!" });

In this example, the View will be sized based on the size of the Label that is added to it.

Specifying a miniumum size

You can specify a minimum size by passing a Dim object to the minimumContentDim parameter. For example, to create a View that is sized based on the Text property, but has a minimum width of 10 columns, you can do this:

View view = new ()
{
    Text = "Hello, World!",
    Width = Dim.Auto (DimAutoStyle.Text, minimumContentDim: Dim.Absolute (10)), // Same as `minimumContentDim: 10`
    Height = Dim.Auto (DimAutoStyle.Text),
};

Sometimes it's useful to have the minimum size be dynamic. Use Dim.Func as follows:

View view = new ()
{
    Width = Dim.Auto (DimAutoStyle.Content, minimumContentDim: Dim.Func (GetDynamicMinSize)),
    Height = Dim.Auto (DimAutoStyle.Text),
};

int GetDynamicMinSize () 
{
    return someDynamicInt;
}

Specifying a maximum size

It is common to want to constrain how large a View can be sized. The maximumContentDim parameter to the Dim.Auto () method enables this. Like minimumContentDim it is of type Dim and thus can represent a dynamic value. For example, by default Dialog specifies maximumContentDim as Dim.Percent (90) to ensure a Dialog box is never larger than 90% of the screen.

Limitations

Dim.Auto is not always the best choice for sizing a view. For example, if you want a view to fill the entire width of the Superview, you should use Dim.Fill () instead of Dim.Auto (DimAutoStyle.Content).

Dim.Auto is also not always the most efficient way to size a view. If you know the size of the content ahead of time, you can set the Width and Height properties to Dim.Absolute (n) instead of using Dim.Auto.

Behavior of other Pos/Dim Types when used within a Dim.Auto-sized View

The table below descibes the behavior of the various Pos/Dim types when used by subviews of a View that uses Dim.Auto for it's Width or Height:

Building Dim.Auto friendly View

It is common to build View classes that have a natrual size based on their content. For example, the Label class is a view that is sized based on the Text property.

Slider is a good example of sophsticated Dim.Auto friendly view.

Developers using these views shouldn't need to know the details of how the view is sized, they should just be able to use the view and have it size itself correctly.

For example, a vertical Slider with 3 options may be created like this: which is size based on the number of options it has, it's orientation, etc...

List<object> options = new () { "Option 1", "Option 2", "Option 3" };
Slider slider = new (options)
{
    Orientation = Orientation.Vertical,
    Type = SliderType.Multiple,
};
view.Add (slider);

Note the developer does not need to specify the size of the Slider, it will size itself based on the number of options and the orientation.

Views like Slider do this by setting Width and Height to Dim.Auto (DimAutoStyle.Content) in the constructor and calling SetContentSize() whenever the desired content size changes. The View will then be sized to be big enough to fit the content.

Views that use Text for their content can just set Width and Height to Dim.Auto (DimAutoStyle.Text). It is recommended to use Height = Dim.Auto (DimAutoStyle.Text, minimumContentDim: 1) to ensure the View can show at least one line of text.

Type	Impacts Dimension	Notes
PosAlign	Yes	The subviews with the same `GroupId` will be aligned at the maximimum dimension to enable them to not be clipped. This dimension plus the group's position will determine the minimum `Dim.Auto` dimension.
PosView	Yes	The position plus the dimension of `subview.Target` will determine the minimum `Dim.Auto` dimension.
PosCombine	Yes
PosAnchorEnd	Yes	The `Dim.Auto` dimension will be increased by the dimension of the subview.
PosCenter	No
PosPercent	No
PosAbsolute	Yes
PosFunc	Yes
DimView	Yes	The position plus the dimension of `subview.Target` will determine the minimum `Dim.Auto` dimension.
DimCombine	Yes
DimFill	No
DimPercent	No
DimAuto	Yes
DimAbsolute	Yes
DimFunc	Yes

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