Difference between Frame Rectangle and Alignment Rectangle

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Alignment Rectangles

As developers create complex views, they may introduce visual ornamentation such as shadows, exterior highlights, reflections, and engraving lines. As they do, these features are often drawn onto image art rather than being added through layers or subviews. Unlike frames, a view’s alignment rectangle should be limited to a core visual element. Its size should remain unaffected as new items are drawn onto the view. Consider the left side of Figure 1-11. It shows a view drawn with a shadow and a badge. When laying out this view, you want Auto Layout to focus on aligning just the core element―the blue rectangle―and not the ornamentation.

Figure 1-11 A view’s alignment rectangle (center) refers strictly to the core visual element to be aligned, without embellishments.

The center image in Figure 1-11 highlights the view’s alignment rectangle. This rectangle excludes all ornamentation, such as the drop shadow and badge. It’s the part of the view you want Auto Layout to consider when it does its work. Contrast this with the rectangle shown in the right image. This version includes all the visual ornamentation, extending the view’s frame beyond the area that should be considered for alignment.

The right-hand rectangle in Figure 1-11 encompasses all the view’s visual elements. It encompasses the shadow and badge. These ornaments could potentially throw off a view’s alignment features (for example, its center, bottom, and right) if they were considered during layout.

By working with alignment rectangles instead of frames, Auto Layout ensures that key information like a view’s edges and center are properly considered during layout. In Figure 1-12, the adorned view is perfectly aligned on the background grid. Its badge and shadow are not considered during placement.

Figure 1-12 Auto Layout only considers this view’s alignment rectangle when laying it out as centered in its superview. The shadow and badge don’t affect its placement.