然后，视图的宽度和结束边界位置都可以根据父视图的尺寸计算出来，其它约束也是。
Both the width and trailing edge's position can then be calculated based on the superview's size and the other constraints.
视图拥有位置，表现为一对左和上坐标，以及两个尺寸，表现为宽度和高度。
A view has a location, expressed as a pair of left and top coordinates, and two dimensions, expressed as a width and a height.
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