How do I use CGAffineTransformMakeScale and Rotation at once?

Question

((UIImageView*)[dsry objectAtIndex:0]).transform = CGAffineTransformMakeRotation(1.57*2);
((UIImageView*)[dsry objectAtIndex:0]).transform = CGAffineTransformMakeScale(.5,.5);

Just one of these works at a time. How can I save a transformation and then apply another? Cheers

Answer 1

Just one of these works at a time.

Right, because you replaced the first one with the second.

How can I save a transformation and then apply another?

Concatenate them together, and assign the resulting matrix to the property.

You can store a transformation matrix in a variable of type CGAffineTransform; you can use that for intermediate steps in more complex transformations, or to make the code clearer (or both).

Answer 2

To expand upon what Peter said, you would want to use code like the following:

CGAffineTransform newTransform;
newTransform = CGAffineTransformMakeRotation(1.57*2);
((UIImageView*)[dsry objectAtIndex:0]).transform = CGAffineTransformScale(newTransform,.5,.5);

The CGAffineTransformMake... functions create new transforms from scratch, where the others concatenate transforms. Views and layers can only have one transform applied to them at a time, so this is how you create multiple scaling, rotation, and translation effects on a view at once.

You do need to be careful of the order in which transforms are concatenated in order to achieve the correct effect.

Answer 3

From the Apple Documentation:

CGAffineTransformConcat Returns an affine transformation matrix constructed by combining two existing affine transforms.

CGAffineTransform CGAffineTransformConcat (
   CGAffineTransform t1,
   CGAffineTransform t2
);

Parameters t1 The first affine transform.

t2 The second affine transform. This affine transform is concatenated to the first affine transform.

Return Value A new affine transformation matrix. That is, t’ = t1*t2.

Discussion Concatenation combines two affine transformation matrices by multiplying them together. You might perform several concatenations in order to create a single affine transform that contains the cumulative effects of several transformations.

Note that matrix operations are not commutative—the order in which you concatenate matrices is important. That is, the result of multiplying matrix t1 by matrix t2 does not necessarily equal the result of multiplying matrix t2 by matrix t1.