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Monday, September 22, 2014

UIView Frame vs. Bounds

The UIView class on iOS has several properties related to size and position, and some of them seem to overlap in functionality. It may not be obvious at first which ones to use when or whether there is even a difference, and as a result I have seen many people get confused by how these properties fit together, and many examples of them being used improperly. This post will try to explain how these properties work.

Coordinate systems

First, let's look at each of the properties that are available on UIView for size and position:

  • RectangleF Frame (CGRect in the new unified API1)
  • RectangleF Bounds (CGRect)
  • PointF Center (CGSize)
  • CGAffineTransform Transform

To understand how each of these relate to each other you first need to understand the concept of a coordinate system. A coordinate system is what defines how to interpret an x,y coordinate. What does it mean to say that a view is located at coordinate (10, 20)? If a view has a width of 30 then how big is it? Without any context these numbers mean nothing. The coordinate system gives us the context we need to interpret the number.

In iOS, as in many GUI systems, there are actually multiple coordinate systems. The screen has its own coordinate system, but also each view has its own coordinate system. When a view wants to position its subviews it describes their size and position (within itself) using its own coordinate system, which is different from the coordinate system of its parent. To explore this further let's consider an example:

test

In the above diagram we have two views, Parent and Child. In case it's not obvious, Child is a subview of Parent. Parent has a size of (100, 100) and Child has a size of (50, 50). We want Child to be centered within Parent. To accomplish this we need the top-left of Child to be at the location (25, 25) within Parent.

Consider the very center of the image. What coordinate is that point? There is no single answer to this question. It depends on whose coordinate system you want to use. It could be (50, 50) if you are looking at it from the perspective of Parent, or it could be (25, 25) if you are looking at it from the perspective of Child. This is the first conceptual hurdle to overcome when thinking about layout. Every coordinate and size is only meaningful in the context of a coordinate system (usually defined by a view), and it is important to carefully consider whose coordinate system you are in and to avoid mixing them (more on that later).

As a teaser, let's take a look at the most common mistake I see people make in iOS layout code:

public override void ViewDidLayoutSubviews()
{
    ChildView.Frame = new RectangleF { Width = View.Frame.Width, Height = ChildViewHeight };
    // Also wrong:
    // ChildView.Frame = View.Frame;
}

Keep this code in mind as you read on and see if you can figure out what the bug is. In the meantime, let's consider some approaches we could take for creating the example view layout form the diagram above.

Option 1: Frame

The first option is to set the Frame property of Child. Frame is in the coordinate system of the view's parent so we would set it like this:

Child.Frame = new RectangleF { X = 25, Y = 25, Width = 50, Height = 50 };

For me this is by far the easiest to understand and easiest to code so I almost always do layout using Frame. However, there are some cases where this doesn't work, and we'll discuss those later.

Option 2: Bounds and Center

The second option is to set both Bounds and Center. These two properties work together to do the same thing as Frame. That may sounds stupid. Why would you want to use two separate properties when one would do the same thing? Again, I'll discuss this more shortly. For now let's look at how you use these two properties.

Bounds is in the coordinate system of the view itself (not its parent). It is a rectangle, but typically it is only used to set the size of a view. That is, the X and Y are almost always left at (0, 0). Setting the X and Y of Bounds does not change the position of the view relative to its parent. To set the position of a view within its parent (aside from using Frame) you use Center.

Center is in the coordinate system of the view's parent. That is, this property defines the location in the parent's coordinate system where this view's center should be placed. Putting these two properties together we can set our view's size and position like this:

Child.Bounds = new RectangleF { Width = 50, Height = 50 };
Child.Center = new PointF { X = 50, Y = 50 };

This code has exactly the same effect as the code above that used Frame instead. In fact, if you examined Frame after running this code you would find that it has the same value as we set in the other code. Likewise, if you use the code that sets Frame and then examine Bounds and Center you will find that they have these same values we set here.

Transforms

Earlier I mentioned that there was a case when you would want to use Bounds and Center instead of Frame. That case is when you also use Transform.

Transform is a property that is used to modify the size, position, or even rotation of a view within its parent. This property uses a type called CGAffineTransform, which is a matrix. The Transform is used to convert points in one coordinate system into points in another.

As an example, let's say we wanted our Child view to appear as if it were sized at (50, 50), but the actual view is (100, 100). That is, we want to shrink the view to half its size. To accomplish this we use a combination of Bounds, Center, and Transform:

Child.Bounds = new RectangleF { Width = 100, Height = 100 };
Child.Center = new PointF { X = 50, Y = 50 };
Child.Transform = CGAffineTransform.MakeScale(0.5f, 0.5f);

Here things start to get complicated. First, now that we have a non-identity Transform applied the value of Frame is undefined. It no longer has any meaning. Frame should never be used when you have a non-identity Transform. That is why it sometimes makes sense to use Bounds and Center instead of Frame.

It is also now harder to reason about how to convert from a coordinate within Child to a coordinate in Parent (or vice versa). Let's say, for whatever reason, you needed to convert a coordinate in one view into a coordinate in its parent. You might be tempted to write code like this:

var xInParent = x + Frame.X;
var yInParent = y + Frame.Y;

Intuitively this code seems correct, but it is not. It is making an assumption about the relationship between the parent's coordinate system and the child's coordinate system. Basically, it is assuming that the transform is the identity transform. If that assumption turns out to be false then this code will get the wrong answer.

This may seem like a contrived example, but there is a common situation in which it might become an issue. In iOS 7.x2 and earlier each UIWindow's coordinate system is pinned. The window ignores the interface orientation. In order to make the UI face the right direction the RootViewController's view has a rotation transform applied to it. This means that the coordinate system of the window is totally different from the coordinate system of the view controller's view. If you try to insert a view directly into the window then you have to very carefully convert between these two coordinate systems.The same situation can arise if you have a scaling transform applied, which would happen if you use UIScrollView's built-in zooming capability.

If you want to write view code that works properly regardless of any transforms that might be applied to it you should carefully consider which coordinate system each coordinate and rectangle is associated with and convert between coordinate systems when necessary. This is why the UIPopoverController and UIMenuController APIs take both a rectangle and and UIView. The view that you give it is used to determine the coordinate system that the rectangle is in.

Converting between coordinate systems

Converting between coordinate systems with transforms involved could be pretty complex. Fortunately, UIKit provides some useful functions that make this easy:

  • PointF ConvertPointFromView(PointF point, UIView fromView)
  • PointF ConvertPointToView(PointF point, UIView toView)
  • RectangleF ConvertRectangleFromView(RectangleF point, UIView fromView)
  • RectangleF ConvertRectToView(RectangleF point, UIView toView)

In iOS 8 these were generalized a bit into these methods, which work on UIScreen as well:

  • PointF ConvertPointFromCoordinateSpace(PointF point, IUICoordinateSpace coordinateSpace)
  • PointF ConvertPointToCoordinateSpace(PointF point, IUICoordinateSpace coordinateSpace)
  • RectangleF ConvertRectFromCoordinateSpace(RectangleF point, IUICoordinateSpace coordinateSpace)
  • RectangleF ConvertRectToCoordinateSpace(RectangleF point, IUICoordinateSpace coordinateSpace)

These methods are useful whenever you need to place or size something in one view such that it matches the location or size of something else in a different view. Using these methods you can easily convert from one coordinate system to another even if there are transforms involved.

Avoiding common pitfalls

Let's go back to the first example code shown above:

public override void ViewDidLayoutSubviews()
{
    ChildView.Frame = new RectangleF { Width = View.Frame.Width, Height = ChildViewHeight };
    // Also wrong:
    // ChildView.Frame = View.Frame;
}

As mentioned previously, this code demonstrates the most common mistake in layout code that I come across. To see what's wrong consider what each variable represents:

  • View.Frameis the size and location of View in its parent's coordinate system.
  • ChildView.Frame is the size and location of ChildView in its parent's coordinate system (that is, in View's coordinate system).

These are two different coordinate systems, but they are being mixed without conversion. What happens if View has a non-identity transform applied? Frame will not have expected value (in fact, it is undefined), and so ChildView will not have the correct size.

The commented out code (ChildView.Frame = View.Frame) has a more basic problem: View.Frame may have a location (relative to View's parent) that doesn't make sense to apply to ChildView. Never assume that a view controller's view has a location of (0, 0). That will not always be true.

So what is the fix? It's simple: use View.Bounds instead. This works because View.Bounds is in the coordinate system of View, and ChildView.Frame is also in that same coordinate system. Here is the fixed code:

public override void ViewDidLayoutSubviews()
{
    ChildView.Frame = new RectangleF { Width = View.Bounds.Width, Height = ChildViewHeight };
    // Also valid:
    // ChildView.Frame = View.Bounds;
}

The same mistake can be made for a custom UIView:

public override void LayoutSubviews()
{
    // Wrong!
    ChildView.Frame = Frame;
}

The correct code would be this:

public override void LayoutSubviews()
{
    // Right!
    ChildView.Frame = Bounds;
}

In order to avoid these kinds of pitfalls here are some rules to live by whenever writing layout code:

  1. When looking at the size of your own view always use this.Bounds for a UIView and View.Bounds for a UIViewController.
  2. When looking at the size of a child view you can use either Frame or Bounds and Center.
  3. Always use Bounds and Center when there are transforms involved.

The first rule can be stated another way: if you see View.Frame in a UIViewController or this.Frame in a custom UIView then that is almost certainly a bug. I can't think of any legitimate reasons for a view to look at its own Frame. Use Bounds instead.

Summary

Here are the most important things to remember from this post:

  1. Each view has its own coordinate system.
  2. Frame is in the coordinate system of the parent.
  3. Bounds is in the coordinate system of the view itself.
  4. Center is in the coordinate system of the parent.
  5. Whenever a view has a non-identity transform applied you should always use Bounds and Center because Frame is undefined.
  6. Avoid mixing coordinates or rectangles/sizes from different coordinate systems. Use the provided conversion methods if necessary.

  1. Xamarin.Mac and Xamarin.iOS unified API 

  2. In iOS 8 this story has changed a bit. If you link against the iOS 8 SDK (or later) and your app is running in iOS 8 (or later) the UIScreen changes its bounds to match the interface orientation, and each window and its RootViewController takes on the same bounds. No rotation transform is applied to the root view controller. You should still not make any assumptions about this because the same app running on iOS 7.1 or earlier would still have the rotation transform, and you never know when Apple might change how this works again. 

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