NKContentVerticalAlignment.fill vs NKContentVerticalAlignment.fit in FrameLayoutKit: A Complete Comparison

NKContentVerticalAlignment.fill forces the target view to stretch and occupy the full height of the FrameLayout container, while NKContentVerticalAlignment.fit preserves the view's intrinsic content height and centers it vertically within the available space.

FrameLayoutKit by kennic provides a lightweight yet powerful layout system for iOS that relies on alignment enums to determine how child views are positioned and sized. Understanding the difference between NKContentVerticalAlignment.fill and NKContentVerticalAlignment.fit is critical for controlling whether a view expands to fill its container or maintains its natural dimensions. These alignment behaviors are implemented in the core layout engine within FrameLayout.swift.

How Vertical Alignment Controls Sizing in FrameLayout

The NKContentVerticalAlignment enum determines how the target view calculates its frame rectangle during the layout pass. According to the source code in kennic/framelayoutkit, the layout engine evaluates this alignment setting to derive both the targetFrame.size.height and targetFrame.origin.y values. The distinction between .fill and .fit fundamentally changes whether the view respects its intrinsic content size or yields to the container's boundary constraints.

NKContentVerticalAlignment.fill: Stretch to Container Height

When you configure alignment.vertical = .fill, the layout engine ignores the target view's intrinsic size and forces it to occupy the entire vertical space of the FrameLayout. This behavior is defined in FrameLayout.swift at lines 529-532:

case .fill:
    targetFrame.origin.y = containerFrame.minY
    targetFrame.size.height = containerFrame.height
    break

The implementation pins the view's top edge to the container's minimum Y boundary and sets the height equal to containerFrame.height. Use this alignment for background views, separators, or any element that must expand to match its parent's full vertical bounds regardless of content.

NKContentVerticalAlignment.fit: Respect Intrinsic Height

The .fit alignment allows the target view to maintain its natural content height while remaining vertically centered inside the container. As implemented in FrameLayout.swift at lines 534-543, the sizing logic respects the allowContentVerticalGrowing property:

case .fit:
    if allowContentVerticalGrowing {
        targetFrame.size.height = max(containerFrame.height, contentSize.height)
    } else {
        targetFrame.size.height = min(containerFrame.height, contentSize.height)
    }
    targetFrame.origin.y = containerFrame.minY + (containerFrame.height - targetFrame.height) / 2
    break

By default, the height becomes the smaller of the container height and the content's intrinsic height, though enabling allowContentVerticalGrowing permits expansion. The view is always mathematically centered using the calculation (containerFrame.height - targetFrame.height) / 2. This alignment is ideal for UILabels, UIImageViews, or buttons that should retain their content-driven sizing.

Practical Code Examples

The following examples demonstrate how to apply these alignments in real FrameLayout configurations.

Filling the Container Height

let backgroundView = UIView()
backgroundView.backgroundColor = .systemIndigo

let layout = FrameLayout()
layout.targetView = backgroundView
layout.alignment.vertical = .fill  // Stretches to match container height
layout.padding = .zero

Fitting and Centering Content

let messageLabel = UILabel()
messageLabel.text = "Hello World"
messageLabel.sizeToFit()

let layout = FrameLayout()
layout.targetView = messageLabel
layout.alignment.vertical = .fit   // Keeps intrinsic height, centers vertically
layout.padding = UIEdgeInsets(top: 12, left: 20, bottom: 12, right: 20)

Summary

• NKContentVerticalAlignment.fill stretches the target view to match the container's full height by setting targetFrame.size.height = containerFrame.height and aligning the top edge to containerFrame.minY.
• NKContentVerticalAlignment.fit calculates height as the minimum (or maximum if growing is allowed) of the container and content sizes, then centers the view vertically using (containerFrame.height - targetFrame.height) / 2.
• Both behaviors are defined in the switch-case logic starting at line 529 of FrameLayout.swift in the kennic/framelayoutkit repository.
• Use .fill for decorative backgrounds or dividers, and .fit for text, images, or controls that require natural sizing with vertical centering.

Frequently Asked Questions

When should I choose .fill over .fit for vertical alignment?

Choose NKContentVerticalAlignment.fill when the view must expand to occupy all available vertical space regardless of its content, such as background layers or separator lines. Choose NKContentVerticalAlignment.fit when the view has an intrinsic content size—like a UILabel or UIButton—and you want it to maintain that natural height while being centered within the layout container.

Does .fit allow content to grow larger than the container?

Yes. When the allowContentVerticalGrowing property is set to true, the .fit alignment uses max(containerFrame.height, contentSize.height) to determine the target height, allowing the view to expand beyond the container if its content requires more space. By default, this property is false, which clamps the height to the smaller of the two values.

How is the vertical position calculated differently between these two alignments?

With .fill, the targetFrame.origin.y is set directly to containerFrame.minY, pinning the view to the top of the container. With .fit, the origin is calculated as containerFrame.minY + (containerFrame.height - targetFrame.height) / 2, which positions the view so that equal space exists above and below it, achieving vertical centering.

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