如何为 UITextField 实现类似 NSLineBreakByTruncatingHead 的功能？

我需要实现类似的目标NSLineBreakByTruncatingHead for UITextField如此处所示。我们假设原文是：

这是无法在 UITextField 内显示的长文本

我需要它像：

...无法显示在 UITextField 内

但目前我得到类似的东西：

这篇文章很长，不能...

只是开头的截断。这lineBreakMode未给出财产UITextField。我怎样才能实现它？

我采取了解决方案here http://iosdevelopertips.com/cocoa/truncate-an-nsstring-and-append-an-ellipsis-respecting-the-font-size.html并将其修改为截断字符串的头部而不是尾部。请注意，它仅在未编辑字段时显示省略号。

注意：此解决方案仅适用于 iOS 7+。要在 iOS 6 中使用，请使用sizeWithFont:代替sizeWithAttributes: in the NSString+TruncateToWidth.m文件。
EDIT:添加了对 iOS 6 的支持

NSString+TruncateToWidth.h

@interface NSString (TruncateToWidth)
- (NSString*)stringByTruncatingToWidth:(CGFloat)width withFont:(UIFont *)font;
@end

NSString+TruncateToWidth.m

#import "NSString+TruncateToWidth.h"

#define ellipsis @"…"

@implementation NSString (TruncateToWidth)

- (NSString*)stringByTruncatingToWidth:(CGFloat)width withFont:(UIFont *)font
{
    // Create copy that will be the returned result
    NSMutableString *truncatedString = [self mutableCopy];

    // Make sure string is longer than requested width
    if ([self widthWithFont:font] > width)
    {
        // Accommodate for ellipsis we'll tack on the beginning
        width -= [ellipsis widthWithFont:font];

        // Get range for first character in string
        NSRange range = {0, 1};

        // Loop, deleting characters until string fits within width
        while ([truncatedString widthWithFont:font] > width)
        {
            // Delete character at beginning
            [truncatedString deleteCharactersInRange:range];
        }

        // Append ellipsis
        [truncatedString replaceCharactersInRange:NSMakeRange(0, 0) withString:ellipsis];
    }

    return truncatedString;
}

- (CGFloat)widthWithFont:(UIFont *)font
{
    if([self respondsToSelector:@selector(sizeWithAttributes:)])
        return [self sizeWithAttributes:@{NSFontAttributeName:font}].width;
    return [self sizeWithFont:font].width;
}

使用它：

...
// Make sure to import the header file where you want to use it
// assumes instance variable holds your string that populates the field
fieldString = @"abcdefghijklmnopqrstuvwxyz1234567890";
// Size will need to be less than text field's width to account for padding
_myTextField.text = [fieldString stringByTruncatingToWidth:(_myTextField.frame.size.width - 15) withFont:_myTextField.font];
...

// use textFieldShouldBeginEditing to make it animate from the start of the field to the end of the string if you prefer that. I found it a little distracting
- (void)textFieldDidBeginEditing:(UITextField *)textField
{
    textField.text = fieldString;
}

- (BOOL)textFieldShouldEndEditing:(UITextField *)textField
{
    fieldString = textField.text;
    textField.text = [textField.text stringByTruncatingToWidth:(textField.frame.size.width - 15) withFont:textField.font];

    return YES;
}