自动布局 Xcode

Question

I'm a beginner having some difficulty understanding Auto layout constraints. For each object placed in storyboard, do I have to specify width, height, along with both leading and trailing constraints to avoid an error message?

Answer 1

For any view to be rendered accurately, the system needs to know, unambiguously, it's - position and dimension.

Consider a simple scenario where you are trying to place a 'rectangle' of height 'Y' and width 'X' at the center of the screen, auto layout needs to know this intension. You convey this by setting 4 constraints:

1. Height constraint
2. Width constraint
3. Vertically in container constraint (or centerYAnchor)
4. Horizontally in container constraint (or centerXAnchor)

Any more or any less number of constraints would likely add ambiguity in terms of positioning or dimension.

In another example, where your 'rectangle' needs to be always 10 points from all edges of the parent view, you simply need to specify -

1. Leading Anchor of rectangle to be 10 points with respect to (wrt) leading anchor of the parent view
2. Trailing anchor of rectangle to be -10 points wrt the trailing anchor of the parent view
3. Top anchor of rectangle to be 10 points wrt the top anchor of the parent view
4. Bottom anchor of the rectangle to be -10 points wrt the bottom anchor of the parent view

Note that trailing anchor and bottom anchor are negative because you are placing the child view anchors before the respective origins of those parent anchors. Also, you don't need to specify height or width in this case because the given constraints implicitly specify it dynamically with respect to the device dimensions.

These are just 2 very simple scenarios but if you try rendering and playing around using this logic (programatically or via storyboard), it'll make things clearer.

But really, this just scratches the surface. To actually dive deep, I recommend digging in with this section in the auto layout documentaion - ' Anatomy of a constraint '