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What is priority in constraints?

Ans : 

Constant priority is a number to determine how important is that constraint. The number can range from 1 to 1000, the higher the number goes, the more important a constraint is. Lower priority in screen seen like dashed blue line.

This is useful when two constraint make conflict.

Example :

Let's take one UIView and place them in main view.

Set Leading and Trailing constraint is 40,40.
Set width and height is 240,240.

Now if iPhone SE is selected, then all goes fine. No red line will show about conflicting constraints due to iPhone SE width is 320. So if it takes 40,40 as leading and constraints, it will calculate view's width as 240. And we also set width constraint as 240. So there is no conflicting.

But if iPhone 8 is selected then view's width constraint is conflicting with leading, trailing constraints.
iPhone 8 width = 375 - 40 - 40 = 295 But we have set width constraint as 240. So here conflicting occurs.

Solution : 

If we set width constraint priority 900 then its priority is lower than leading and trailing priority(1000) So width constraint will be ignored here. So when we do some auto layout and we feel conflicting constraint, we can lower priority of constraint according to requirement.

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