在UITextVIew（iOS7 +）中计算文本高度的4种替代方法，但哪一种方法正确？

Question

我有以下4种方法在UITextView中获取文本高度。我发现价值可能会有所不同。根据我的测试，有时method1和method4是正确的，有时method3是正确的。我无法弄清楚这些是怎么发生的。

//method 1
    float fudgeFactor = [self getTextViewLeftMargin:textView];
        CGSize tallerSize = CGSizeMake(textView.frame.size.width-fudgeFactor*2,999);

        if (SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(@"7.0")) {
            stringSize = [textView.text sizeWithFont:textView.font constrainedToSize:tallerSize lineBreakMode:NSLineBreakByWordWrapping];
        } else {
            stringSize = [textView.text sizeWithFont:textView.font constrainedToSize:tallerSize lineBreakMode:UILineBreakModeWordWrap];
        }

        CGFloat textHeight = stringSize.height;

//method2:     
        CGFloat textHeight2 = [textView sizeThatFits:CGSizeMake(textView.frame.size.width, FLT_MAX)].height;

//method3:    
        CGFloat textHeight3 = textView.contentSize.height;

//method4:
    NSStringDrawingOptions options = NSStringDrawingTruncatesLastVisibleLine |
                NSStringDrawingUsesLineFragmentOrigin;
                NSDictionary *attr = @{NSFontAttributeName: textView.font};
                CGRect labelBounds = [textView.text boundingRectWithSize:tallerSize
                                                                 options:options
                                                              attributes:attr
                                                                 context:nil];

                CGFloat textHeight = labelBounds.size.height;