如何在MonoTouch中限制UITextView字符限制？

Question

我希望用户能够在UITextView中输入不多于一些数字字符。这也需要使用复制粘贴。我找到了this solution，但它是Objective C，它有一个缺点，即在达到限制后重置光标位置。

我想要的是继承自UITextView的类，揭示MaxCharacters属性并完成所有工作。

Answer 1

此代码基于manicaesar's answer，但在达到限制后不重置插入符号的位置。

[Register ("LimitedTextView")] 
public class LimitedTextView : UITextView 
{ 
    public int MaxCharacters { get; set; } 

    void Initialize() 
    { 
     MaxCharacters = 140; 
     ShouldChangeText = ShouldLimit; 
    } 

    static bool ShouldLimit (UITextView view, NSRange range, string text) 
    { 
     var textView = (LimitedTextView)view; 
     var limit = textView.MaxCharacters; 

     int newLength = (view.Text.Length - range.Length) + text.Length; 
     if (newLength <= limit) 
      return true; 

     // This will clip pasted text to include as many characters as possible 
     // See https://stackoverflow.com/a/5897912/458193 

     var emptySpace = Math.Max (0, limit - (view.Text.Length - range.Length)); 
     var beforeCaret = view.Text.Substring (0, range.Location) + text.Substring (0, emptySpace); 
     var afterCaret = view.Text.Substring (range.Location + range.Length); 

     view.Text = beforeCaret + afterCaret; 
     view.SelectedRange = new NSRange (beforeCaret.Length, 0); 

     return false; 
    } 

    public LimitedTextView (IntPtr handle) : base (handle) { Initialize(); } 
    public LimitedTextView (RectangleF frame) : base (frame) { Initialize(); } 
}