Understanding convertPoint:toView:

Question

I do not quite understand the method convertPoint:toView:.

In Apple's documentation it is written that

convertPoint:toView:

Converts a point from the receiver’s coordinate system to that of the specified view.

- (CGPoint)convertPoint:(CGPoint)point toView:(UIView *)view 

But what does converting a point from one to the other actually mean?

Does it imply that the points in both bounds have different units? Or just different values?

If it is the latter, why is there such a method when we can simply assign the value of a's contentOffset to b's ?

CGPoint a = [a contentOffset]; [b setContentOffset:a]; 

How is convertPoint:toView: different from simply assigning contentOffset? Or did I misunderstand the entire concept? What does converting points actually do? When should this method be used?

Answer 1

Every UIView has its own coordinates system. So if you have a UIView_1 that contains another UIView_2, they both have a point (10,10) within them.

convertPoint:toView: allows the developer to take a point in one view and convert the point to another view coordinate system.

Example: view1 contains view2. The top left corner of view2 is located at view1 point (10,10), or better to say view2.frame.orgin = {10,10}. That {10,10} is based in the view1 coordinate system. So far so good.

The user touches the view2 at point {20,20} inside the view2. Now those coordinates are in the view2 coordinate system. You can now use covertPoint:toView: to convert {20,20} into the coordinate system of view1. touchPoint = {20,20}

CGPoint pointInView1Coords = [view2 convertPoint:touchPoint toView:view1]; 

So now pointInView1Coords should be {30,30} in the view1 coordinate systems. Now that was just simple math on this example, but there are all sorts of things that contribute to the conversion. Transforms and scaling come to mind.

Read about UIView frame, bounds, and center. They are all related and they deal with coordinate systems for a view. Its confusing until you start doing stuff with them. Remember this frame and center are in the parent's coordinate system. Bounds is in the view's coordinate system.

John