在iOS7中，我需要逻辑来为tableview返回正确的高度:highightforrowatindexpath方法，但sizeWithAttributes总是返回相同的高度，而不管字符串长度，因为它不知道它将被放在一个固定宽度的表格单元格。我发现这对我来说很有用，并计算出正确的高度，考虑到表格单元格的宽度!这是基于上面T先生的回答。

NSString *text = @"The text that I want to wrap in a table cell."

CGFloat width = tableView.frame.size.width - 15 - 30 - 15;  //tableView width - left border width - accessory indicator - right border width
UIFont *font = [UIFont systemFontOfSize:17];
NSAttributedString *attributedText = [[NSAttributedString alloc] initWithString:text attributes:@{NSFontAttributeName: font}];
CGRect rect = [attributedText boundingRectWithSize:(CGSize){width, CGFLOAT_MAX}
                                           options:NSStringDrawingUsesLineFragmentOrigin
                                           context:nil];
CGSize size = rect.size;
size.height = ceilf(size.height);
size.width  = ceilf(size.width);
return size.height + 15;  //Add a little more padding for big thumbs and the detailText label

正如你在苹果开发者网站上看到的sizeWithFont，它已被弃用，所以我们需要使用sizeWithAttributes。

#define SYSTEM_VERSION_LESS_THAN(v) ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] == NSOrderedAscending)

NSString *text = @"Hello iOS 7.0";
if (SYSTEM_VERSION_LESS_THAN(@"7.0")) {
    // code here for iOS 5.0,6.0 and so on
    CGSize fontSize = [text sizeWithFont:[UIFont fontWithName:@"Helvetica" 
                                                         size:12]];
} else {
    // code here for iOS 7.0
   CGSize fontSize = [text sizeWithAttributes: 
                            @{NSFontAttributeName: 
                              [UIFont fontWithName:@"Helvetica" size:12]}];
}

基于@bitsand，这是一个新方法，我刚刚添加到我的NSString+Extras类别:

- (CGRect) boundingRectWithFont:(UIFont *) font constrainedToSize:(CGSize) constraintSize lineBreakMode:(NSLineBreakMode) lineBreakMode;
{
    // set paragraph style
    NSMutableParagraphStyle *style = [[NSParagraphStyle defaultParagraphStyle] mutableCopy];
    [style setLineBreakMode:lineBreakMode];

    // make dictionary of attributes with paragraph style
    NSDictionary *sizeAttributes = @{NSFontAttributeName:font, NSParagraphStyleAttributeName: style};

    CGRect frame = [self boundingRectWithSize:constraintSize options:NSStringDrawingUsesLineFragmentOrigin attributes:sizeAttributes context:nil];

    /*
    // OLD
    CGSize stringSize = [self sizeWithFont:font
                              constrainedToSize:constraintSize
                                  lineBreakMode:lineBreakMode];
    // OLD
    */

    return frame;
}

我只是使用结果帧的大小。

创建一个接受UILabel实例的函数。并返回CGSize

CGSize constraint = CGSizeMake(label.frame.size.width , 2000.0);
// Adjust according to requirement

CGSize size;
if([[[UIDevice currentDevice] systemVersion] floatValue] >= 7.0){

    NSRange range = NSMakeRange(0, [label.attributedText length]);

    NSDictionary *attributes = [label.attributedText attributesAtIndex:0 effectiveRange:&range];
    CGSize boundingBox = [label.text boundingRectWithSize:constraint options: NSStringDrawingUsesLineFragmentOrigin attributes:attributes context:nil].size;

    size = CGSizeMake(ceil(boundingBox.width), ceil(boundingBox.height));
}
else{
    size = [label.text sizeWithFont:label.font constrainedToSize:constraint lineBreakMode:label.lineBreakMode];
}

return size;

正如@Ayush的回答:

正如你在苹果开发者网站上看到的sizeWithFont，它已被弃用，所以我们需要使用sizeWithAttributes。

好吧，假设在2019+你可能会使用Swift和String而不是Objective-c和NSString，下面是使用预定义字体获取字符串大小的正确方法:

let stringSize = NSString(string: label.text!).size(withAttributes: [.font : UIFont(name: "OpenSans-Regular", size: 15)!])

在Xamarin接受的答案将是(使用sizeWithAttributes和UITextAttributeFont):

        UIStringAttributes attributes = new UIStringAttributes
        { 
            Font = UIFont.SystemFontOfSize(17) 
        }; 
        var size = text.GetSizeUsingAttributes(attributes);