1、使用文字换行符“ \n”

UILabel *label = [[UILabel alloc]initWithFrame:CGRectMake(20, 450, 320 - 20, 45)]；
    [label setTextColor:[UIColorblackColor]];
    [label setText:@"图片不会被拉伸。\n设置按钮上的图片"];
    [label setFont:[UIFont boldSystemFontOfSize:15]];
    label.textAlignment = NSTextAlignmentLeft;
    label.numberOfLines = 0;
    [self.view addSubview:label];

2、让label上的文字多行显示

//文字居中显示
    label.textAlignment = UITextAlignmentCenter;
    //自动折行设置
    label.lineBreakMode = UILineBreakModeWordWrap;
    label.numberOfLines=0;

3、求出label中文字的行数和每一行的内容

原文链接

函数返回的数组的count 即为label中文字行数 ；数组中元素即为某行内容。

- (NSArray *)getLinesArrayOfStringInLabel:(UILabel *)label{
    NSString *text = [label text];
    UIFont *font = [label font];
    CGRect rect = [label frame];
    
    CTFontRef myFont = CTFontCreateWithName(( CFStringRef)([font fontName]), [font pointSize], NULL);
    NSMutableAttributedString *attStr = [[NSMutableAttributedString alloc] initWithString:text];
    [attStr addAttribute:(NSString *)kCTFontAttributeName value:(__bridge  id)myFont range:NSMakeRange(0, attStr.length)];
    CFRelease(myFont);
    CTFramesetterRef frameSetter = CTFramesetterCreateWithAttributedString(( CFAttributedStringRef)attStr);
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathAddRect(path, NULL, CGRectMake(0,0,rect.size.width,100000));
    CTFrameRef frame = CTFramesetterCreateFrame(frameSetter, CFRangeMake(0, 0), path, NULL);
    NSArray *lines = ( NSArray *)CTFrameGetLines(frame);
    NSMutableArray *linesArray = [[NSMutableArray alloc]init];
    for (id line in lines) {
        CTLineRef lineRef = (__bridge  CTLineRef )line;
        CFRange lineRange = CTLineGetStringRange(lineRef);
        NSRange range = NSMakeRange(lineRange.location, lineRange.length);
        NSString *lineString = [text substringWithRange:range];
        CFAttributedStringSetAttribute((CFMutableAttributedStringRef)attStr, lineRange, kCTKernAttributeName, (CFTypeRef)([NSNumber numberWithFloat:0.0]));
        CFAttributedStringSetAttribute((CFMutableAttributedStringRef)attStr, lineRange, kCTKernAttributeName, (CFTypeRef)([NSNumber numberWithInt:0.0]));
        //NSLog(@"''''''''''''''''''%@",lineString);
        [linesArray addObject:lineString];
    }
    
    CGPathRelease(path);
    CFRelease( frame );
    CFRelease(frameSetter);
    return (NSArray *)linesArray;
}