ios - 在方向更改时维护 UITextView 的 contentOffset

Question

是否可以更改 UITextView 的内容偏移量，以便当用户从纵向移动到横向(或反之亦然)时，他们可见的文本保持不变？

所以如果用户在(纵向模式)

....some text....
....some text2....
....I'm here.... //this is the visible area
....more text....

移到横屏，应该是

....some text....some text2....
....I'm here....more text.... //this is the visible area
.....

UITextView 不可编辑但可滚动。

Answer 1

这里有一个简短的、独立的、可编译的例子:

import UIKit

class ViewController: UIViewController {

    var textView: UITextView!
    var percentOfScroll:CGFloat = 0

    override func viewDidLoad() {

        self.textView = UITextView(frame: CGRectZero)

        self.textView.layer.borderColor = UIColor.blackColor().CGColor
        self.textView.layer.borderWidth = 1
        self.textView.editable = false

        self.textView.text = "Lorem ipsum... some longish text"

        self.view.addSubview(self.textView)

        let leftConstraint = NSLayoutConstraint(item: self.textView, attribute: .Leading, relatedBy: .Equal, toItem: self.view, attribute: .Leading, multiplier: 1, constant: 10)
        let rightConstraint = NSLayoutConstraint(item: self.textView, attribute: .Trailing, relatedBy: .Equal, toItem: self.view, attribute: .Trailing, multiplier: 1, constant: -10)
        let topConstraint = NSLayoutConstraint(item: self.textView, attribute: .Top, relatedBy: .Equal, toItem: self.view, attribute: .Top, multiplier: 1, constant: 44)
        let centerConstraint = NSLayoutConstraint(item: self.textView, attribute: .Bottom, relatedBy: .Equal, toItem: self.view, attribute: .CenterY, multiplier: 1, constant: 0)

        self.textView.translatesAutoresizingMaskIntoConstraints = false
        self.view.addConstraints([leftConstraint, rightConstraint, topConstraint, centerConstraint])
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        let contentSize = self.textView.contentSize
        let newOffset = contentSize.height * percentOfScroll

        self.textView.contentOffset = CGPoint(x: 0, y: newOffset)
    }

    override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {

        super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)

        let contentSize = self.textView.contentSize
        let contentOffset = self.textView.contentOffset

        self.percentOfScroll = contentOffset.y / contentSize.height
    }

}

想法是在旋转之前获取与其高度相关的滚动内容量 (viewWillTransitionToSize) 并使用它计算新的 contentOffset 在 (viewDidLayoutSubviews 大小改变后也会被调用)。

请记住，这并不能给出 100% 精确的结果，但我还没有想出更好的办法，也许这对你来说就足够了。