Size attributes (for example, Height and Width) and location attributes (for example, Leading, Left, and Top). However, not all attributes are compatible. You can define the space between views, align the edge of views, define the relative size of two views, or even define a view’s aspect ratio. The wide range of parameters and attributes available to these equations lets you create many different types of constraints. To see the full list of attributes, be sure you are looking at the correct platform’s documentation. Text items also have one or more baseline attributes.įor the complete list of attributes, see the NSLayoutAttribute enum.Īlthough both OS X and iOS use the NSLayoutAttribute enum, they define slightly different sets of values. In general, this includes the four edges (leading, trailing, top, and bottom), as well as the height, width, and vertical and horizontal centers. In Auto Layout, the attributes define a feature that can be constrained. When working with constant values, the second item is left blank, the second attribute is set to Not An Attribute, and the multiplier is set to 0.0. You can also assign constant values to an item’s height or width. Constraints can also define the relationship between two different attributes of a single item, for example, setting an aspect ratio between an item’s height and width. These items can represent either views or layout guides. Most constraints define a relationship between two items in our user interface. This value is added to the value of attribute 2. A constant, floating-point offset-in this case, 8.0. If the second item is left blank, this must be Not an Attribute.Ĭonstant. The attribute to be constrained on the second item-in this case, the blue view’s trailing edge. Unlike the first item, this can be left blank.Īttribute 2. The second item in the equation-in this case, the blue view. The value of attribute 2 is multiplied by this floating point number. In this case, the left and right side are equal. The relationship can have one of three values: equal, greater than or equal, or less than or equal. The relationship between the left and right sides. The attribute to be constrained on the first item-in this case, the red view’s leading edge. The item must be either a view or a layout guide.Īttribute 1. The first item in the equation-in this case, the red view. This constraint states that the red view’s leading edge must be 8.0 points after the blue view’s trailing edge. Your goal is to declare a series of equations that has one and only one possible solution. Each constraint represents a single equation. This is because there are implicit constraints added to the view.The layout of your view hierarchy is defined as a series of linear equations. You’d notice the view is placed where we expected in the simulator, even without any constraints. Place the view in the top left corner of the view. Open the main storyboard file in the sample project we created earlier and in the view controller, drag a new view into it. These constraints are not visible to us in the Xcode interface but they are there nonetheless. Whenever we add a new view to the view hierarchy, Xcode implicitly adds constraints for us. Implicit constraints are constraints that are automatically added to views when we add them to the view hierarchy.
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