Now, I understand the coordinate transformation method of UIView (Swift)

Introduction

In rare cases, you may want to convert the coordinates using the convert method of UIView, but which one should you specify? I will forget it, so I will summarize it.

View preparation

Let's perform coordinate conversion with the following configuration.

The ScrollView is placed in the same size as the ViewController's View, and the contentView (green) is placed in it with the same width as the ScrollView and 1200 pt (StackView is for the leftmost memory). Inside the contentView, the 120 pt vertical and horizontal targetView (white) is placed at the top 900 pt of the contentView, and the horizontal is centered, and the button is placed in the center of the targetView (white).

Coordinate transformation

The following method is provided in UIView for coordinate conversion, and it is converted using this method.

Both methods require the target View to be in the same Window. If the argument view is nil, it will be converted to the Window coordinate system. convert(_: to:) point specifies the coordinate system of the receiver, and view specifies the view of the coordinate system you want to convert.

func convert(_ point: CGPoint, to view: UIView?) -> CGPoint

// ex.
aView.convert(CGPoint.zero, to: bView)

In the example, the upper left coordinate of aView is converted to the coordinate system of bView.

convert(_: from:) point specifies the coordinate system of view and converts it to the coordinate system of the receiver.

func convert(_ point: CGPoint, from view: UIView?) -> CGPoint

// ex.
aView.convert(CGPoint.zero, from: bView)

In the example, the upper left coordinate of bView is converted to the coordinate system of aView.

Practice

Try converting from the targetView coordinate system to the scrollView and ViewController view coordinate system when the button is pressed!

 @IBAction private func convertPoints(_ sender: Any) {
    //Convert the upper right coordinates of targetView to the coordinate system of scrollView
    let p1 = targetView.convert(CGPoint.zero, to: scrollView)
    print("p1: \(p1)") // p1: (127.5, 900.0)

    //Convert the upper right coordinates of targetView to the view coordinate system
    let p2 = targetView.convert(CGPoint.zero, to: view)
    print("p2: \(p2)") // p2: (127.5, 367.0) *Changes with offset of scrollView

    //Convert the upper right coordinates of targetView to the coordinate system of scrollView
    let p3 = scrollView.convert(CGPoint.zero, from: targetView)
    print("p3: \(p3)") // p3: (127.5, 900.0)

    //Convert the upper right coordinates of targetView to the view coordinate system
    let p4 = view.convert(CGPoint.zero, from: targetView)
    print("p4: \(p4)") // p4: (127.5, 367.0) *Changes with offset of scrollView
}

As you can see from the above, p1 and p3, p2 and p4 have the same result.

Use this to adjust the scroll offset if targetView is off-screen when the button is pressed.

@IBAction private func convertPoints(_ sender: Any) {
    //Convert the lower left coordinates of targetView to the view coordinate system
    let p1 = targetView.convert(CGPoint(x: 0, y: targetView.frame.height), to: view)
    //Check if the lower left of targetView is within the range of view
    if !view.frame.contains(p1) {
        //Scroll so that the targetView fits within the screen
        let p2 = targetView.convert(CGPoint(x: 0, y: targetView.frame.height), to: scrollView)
        scrollView.setContentOffset(.init(x: 0, y: p2.y - scrollView.frame.height), animated: true)
    }
}

It looks like this: tada:

scroll

in conclusion

If you think about the meaning of to and from, you can understand, but which coordinate system should point be attached to? I often get lost. .. ..

Don't get lost with this anymore! !! (Maybe: rolling_eyes :)

By the way, there is also a Rect conversion method.

I wonder if SwiftUI will not use this kind of thing: thinking: