iOS AutoLayout

Auto Layout Guide

Solve a system of constraints to determine your layout.
Every constraint is of the form y = m*x + b, where y and x are attributes of views, and m and b are floating point values.
Use attributes leading and trailing instead of left and right to deal with RTL languages.
Constraints are cumulative, and do not override each other. You must remove the first constraint manually.
Intrinsic content size is when a leaf-level view, such as a button, knows more about its size than the code that positions it. The element will grow and shrink appropriately with different content.

In the visual format string, - denotes a standard Aqua space, which is 8.0 between sibling views and 20.0 between a view and its superview.
The NSDictionaryOfVariableBindings method creates a dictionary where the keys are the same as the corresponding value’s variable name.
If the bounds transform of a container changes, then a space like -80- scales too.

Auto Layout issues occur when there are too few constraints, or the system contains conflicting or ambiguous constraints.

Container views may use their content to determine their own size at runtime.
Placeholder constraints address this, such as a minimum width for a view. This placeholder is removed at build time, when your view defines its size.

Unlike standard views, custom views have no defined intrinsic content size.
Use a placeholder intrinsic content size to indicate the custom view’s content size. Again, this is removed at runtime.

Method _subtreeDescription of NSView creates a textual description of the view hierarchy.
Call method constraintsAffectingLayoutForAxis: to get constraints affecting a particular orientation. The debugger can print their details.
On OS X, you can visualize the constraints by calling visualizeConstraints:.

The size of the content inside of a scroll view is determined by the constraints of its descendants.
You must make sure you create constraints for all the subviews inside a scroll view.
There cannot be any missing constraints, starting from one edge of the scroll view to the other.

The height of the spacer views can be any value, including 0. However, you must create constraints for the height of the views.

A custom view class must provide enough information so that the Auto Layout system can make the correct calculations and satisfy the constraints.

The intrinsicContentSize method of a view tells the layout system that it contains content that the layout system doesn't understand, and provides to the layout system the intrinsic size of that content.
Method intrinsicContentSize can return absolute values for its width and height, or return NSViewNoInstrinsicMetric for either value to indicate no intrinsic metric for a dimension.
For example, a button returns an intrinsic size with a width and height large enough not to clip the text or image. A horizontal slider has an intrinsic height, but no intrinsic width.

If the content of a view changes, which in turn changes its intrinsic size, then it must call invalidateIntrinsicContentSize.

You cannot express a fixed aspect ratio using the visual format language; you must instead create a NSLayoutConstraint using constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:.

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Last modified 16 December 2024