Table of Contents

Dim.Auto Deep Dive

See Also

Overview

The Dim.Auto type is a specialized Dim class in Terminal.Gui v2 that enables automatic sizing of a View based on its content. This is particularly useful for dynamically sizing views to accommodate varying content such as text, subviews, or explicitly set content areas. Unlike other Dim types like Dim.Absolute or Dim.Fill, Dim.Auto calculates dimensions at runtime based on specified criteria, making it ideal for responsive UI design in terminal applications.

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 strategies that Dim.Auto can employ to size a view. The DimAutoStyle enum has the following values:

  • Text: The view is sized based on the Text property and TextFormatter settings. This considers the formatted text dimensions, constrained by any specified maximum dimensions.
  • Content: The view is sized based on either the value returned by View.GetContentSize() or the Subviews property. If the content size is explicitly set (via View.SetContentSize()), the view is sized based on that value. Otherwise, it considers the subview with the largest relevant dimension plus its position.
  • Auto: The view is sized based on both the text and content, whichever results in the larger dimension.

Using Dim.Auto

Dim.Auto is defined as:

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

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

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 Minimum 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.

View dialog = new ()
{
    Width = Dim.Auto(DimAutoStyle.Content, maximumContentDim: Dim.Percent(90)),
    Height = Dim.Auto(DimAutoStyle.Content, maximumContentDim: Dim.Percent(90)),
};

Technical Details

Calculation Logic

The Dim.Auto class calculates dimensions dynamically during the layout process. Here's how it works under the hood, based on the codebase analysis:

  • Text-Based Sizing (DimAutoStyle.Text): When using Text style, the dimension is determined by the formatted text size as computed by TextFormatter. For width, it uses ConstrainToWidth, and for height, it uses ConstrainToHeight. These values are set based on the formatted text size, constrained by any maximum dimensions provided.
  • Content-Based Sizing (DimAutoStyle.Content): For Content style, if ContentSizeTracksViewport is false and there are no subviews, it uses the explicitly set content size from GetContentSize(). Otherwise, it iterates through subviews to calculate the maximum dimension needed based on their positions and sizes.
  • Auto Sizing (DimAutoStyle.Auto): This combines both Text and Content strategies, taking the larger of the two calculated dimensions.

The calculation in DimAuto.Calculate method also respects minimumContentDim and maximumContentDim:

  • The final size is at least the minimum specified (if any), and at most the maximum specified (if any).
  • Adornments (like margins, borders, and padding) are added to the calculated content size to ensure the view's frame includes these visual elements.

Handling Subviews

When sizing based on subviews, Dim.Auto employs a sophisticated approach to handle dependencies:

  • It categorizes subviews based on their Pos and Dim types to manage layout dependencies. For instance, it processes subviews with absolute positions and dimensions first, then handles more complex cases like PosAnchorEnd or DimView.
  • This ensures that views dependent on other views' sizes or positions are calculated correctly, avoiding circular dependencies and ensuring accurate sizing.

Adornments Consideration

The size calculation includes the thickness of adornments (margin, border, padding) to ensure the view's total frame size accounts for these elements. This is evident in the code where adornmentThickness is added to the computed content size.

Limitations

Dim.Auto is not always the best choice for sizing a view. Consider the following limitations:

  • Performance Overhead: Dim.Auto can introduce performance overhead due to the dynamic calculation of sizes, especially with many subviews or complex text formatting. If the size is known and static, Dim.Absolute(n) might be more efficient.
  • Not Suitable for Full-Screen Layouts: If you want a view to fill the entire width or height of the superview, Dim.Fill() is more appropriate than Dim.Auto(DimAutoStyle.Content) as it directly uses the superview's dimensions without content-based calculations.
  • Dependency Complexity: When subviews themselves use Dim.Auto or other dependent Dim types, the layout process can become complex and may require multiple iterations to stabilize, potentially leading to unexpected results if not carefully managed.

Behavior of Other Pos/Dim Types When Used Within a Dim.Auto-Sized View

The table below describes the behavior of various Pos and Dim types when used by subviews of a view that uses Dim.Auto for its Width or Height. This reflects how these types influence the automatic sizing:

Type Impacts Dimension Notes
PosAlign Yes The subviews with the same GroupId will be aligned at the maximum 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 Impacts dimension if it includes a Pos type that affects dimension (like PosView or PosAnchorEnd).
PosAnchorEnd Yes The Dim.Auto dimension will be increased by the dimension of the subview to accommodate its anchored position.
PosCenter No Does not impact the dimension as it centers based on superview size, not content.
PosPercent No Does not impact dimension unless combined with other impacting types; based on superview size.
PosAbsolute Yes Impacts dimension if the absolute position plus subview dimension exceeds current content size.
PosFunc Yes Impacts dimension if the function returns a value that, combined with subview dimension, exceeds content size.
DimView Yes The dimension of subview.Target will contribute to the minimum Dim.Auto dimension.
DimCombine Yes Impacts dimension if it includes a Dim type that affects dimension (like DimView or DimAuto).
DimFill No Does not impact dimension as it fills remaining space, not contributing to content-based sizing.
DimPercent No Does not impact dimension as it is based on superview size, not content.
DimAuto Yes Contributes to dimension based on its own content or text sizing, potentially increasing the superview's size.
DimAbsolute Yes Impacts dimension if the absolute size plus position exceeds current content size.
DimFunc Yes Impacts dimension if the function returns a size that, combined with position, exceeds content size.

Building Dim.Auto Friendly Views

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

Slider is a good example of a sophisticated 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, sized based on the number of options, its 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 achieve 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 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.

Best Practices for Custom Views

  • Set Appropriate DimAutoStyle: Choose Text, Content, or Auto based on what drives the view's size. Use Text for text-driven views like labels, Content for container-like views with subviews or explicit content sizes, and Auto for mixed content.
  • Update Content Size Dynamically: If your view's content changes (e.g., text updates or subviews are added/removed), call SetContentSize() or ensure properties like Text are updated to trigger re-layout.
  • Consider Minimum and Maximum Constraints: Use minimumContentDim to prevent views from becoming too small to be usable, and maximumContentDim to prevent them from growing excessively large, especially in constrained terminal environments.
  • Handle Adornments: Be aware that Dim.Auto accounts for adornments in its sizing. If your view has custom adornments, ensure they are properly factored into the layout by the base View class.

Debugging Dim.Auto Issues

If you encounter unexpected sizing with Dim.Auto, consider the following debugging steps based on the codebase's diagnostic capabilities:

  • Enable Validation: Set ValidatePosDim to true on the view to enable runtime validation of Pos and Dim settings. This will throw exceptions if invalid configurations are detected, helping identify issues like circular dependencies or negative sizes.
  • Check Content Size: Verify if ContentSizeTracksViewport is behaving as expected. If set to false, ensure SetContentSize() is called with the correct dimensions. Use logging to track GetContentSize() outputs.
  • Review Subview Dependencies: Look for subviews with Pos or Dim types that impact dimension (like PosAnchorEnd or DimView). Ensure their target views are laid out before the current view to avoid incorrect sizing.
  • Inspect Text Formatting: For Text style, check TextFormatter settings and constraints (ConstrainToWidth, ConstrainToHeight). Ensure text is formatted correctly before sizing calculations.

By understanding the intricacies of Dim.Auto as implemented in Terminal.Gui v2, developers can create responsive and adaptive terminal UIs that automatically adjust to content changes, enhancing user experience and maintainability.

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