UIButton 的imageEdgeInsets 和 titl

通过调整UIButton的 imageEdgeInsets , titleEdgeInsets ，可以任意调换button的标题和图片的位置。
具体原理参看：https://www.jianshu.com/p/034e61768c1f

核心是：

1. 默认内容水平居中、垂直居中情况下，对imageEdgeInsets的设置，会影响整体的安全区域，然后在新的安全区域内重新整体居中摆放，影响了整体的布局（进而影响了image），但不会影响titleLabel的位置；对titleEdgeInsets设置也只会影响整体中心点的布局（进而影响titleLabel），而不会影响image；
2. 重排后整体中心点是保持不变的，在此条件下，分别计算重排后image和titleLabel的中心点偏移量，往后移、下移为正；往左移、上移为负， 然后分别对imageEdgeInsets、titleEdgeInsets赋值；
3. 最后重新计算重排后整体size的变化，如变大， inset值为正，如缩小，inset值为负，对contentEdgeInsets赋值。

代码示例：

- (void)setImagePosition2:(LXMImagePosition)postion spacing:(CGFloat)spacing
{
    CGFloat space = spacing;
    
    CGFloat imageWidth = self.currentImage.size.width;
    CGFloat imageHeight = self.currentImage.size.height;
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
    // Single line, no wrapping. Truncation based on the NSLineBreakMode.
    CGSize size = [self.currentTitle sizeWithFont:self.titleLabel.font];
#pragma clang diagnostic pop
    CGFloat titleWidth = size.width;
    CGFloat titleHeight = size.height;
    
    UIEdgeInsets imageEdgeInsets = UIEdgeInsetsZero;
    UIEdgeInsets titleEdgeInsets = UIEdgeInsetsZero;
    UIEdgeInsets contentEdgeInsets = UIEdgeInsetsZero;
    CGFloat imageCenterOffsetX = 0;
    CGFloat imageCenterOffsetY = 0;
    CGFloat titleCenterOffsetX = 0;
    CGFloat titleCenterOffsetY = 0;
    
    // After reset, the center keeps the same.
    switch (postion) {
        case LXMImagePositionLeft:
            imageCenterOffsetX = -space/2;
            imageCenterOffsetY = 0;
            titleCenterOffsetX = space/2;
            titleCenterOffsetY = 0;
            contentEdgeInsets = UIEdgeInsetsMake(0, space/2, 0, space/2);
            break;
        case LXMImagePositionRight:
            imageCenterOffsetX = ((imageWidth+titleWidth)/2 - imageWidth/2)*2 + space/2;
            imageCenterOffsetY = 0;
            titleCenterOffsetX = -(imageWidth+titleWidth/2 - (imageWidth+titleWidth)/2)*2 - space/2;
            titleCenterOffsetY = 0;
            contentEdgeInsets = UIEdgeInsetsMake(0, space/2, 0, space/2);
            break;
        case LXMImagePositionTop:
        case LXMImagePositionBottom:
        {
            // top
            imageCenterOffsetX = (imageWidth+titleWidth)/2 - imageWidth/2;
            imageCenterOffsetY = -(((imageHeight+titleHeight)/2-imageHeight/2)+space/2);
            titleCenterOffsetX = -((imageWidth+titleWidth/2)- (imageWidth+titleWidth)/2);
            titleCenterOffsetY = (imageHeight+titleHeight - titleHeight/2)-(imageHeight+titleHeight)/2 + space/2;
            CGFloat changeWidth = imageWidth + titleWidth - MAX(imageWidth, titleWidth);
            CGFloat changeHeight = imageHeight+titleHeight - MAX(imageHeight, titleHeight) + space;
            contentEdgeInsets = UIEdgeInsetsMake(changeHeight/2, -changeWidth/2, changeHeight/2, -changeWidth/2);
            
            if (postion == LXMImagePositionBottom) {
                imageCenterOffsetY = -imageCenterOffsetY; // opposite
                titleCenterOffsetY = -titleCenterOffsetY;
            }
        }
            break;
        default:
            break;
    }
    
  //相当于整体平移（imageCenterOffsetX , imageCenterOffsetY,）,但不影响titleLabel
    imageEdgeInsets = UIEdgeInsetsMake(imageCenterOffsetY, imageCenterOffsetX, -imageCenterOffsetY, -imageCenterOffsetX); 
//相当于整体平移（titleCenterOffsetY , titleCenterOffsetX,）,但不影响image
    titleEdgeInsets = UIEdgeInsetsMake(titleCenterOffsetY, titleCenterOffsetX, -titleCenterOffsetY, -titleCenterOffsetX);
    
    self.imageEdgeInsets = imageEdgeInsets;
    self.titleEdgeInsets = titleEdgeInsets;
    self.contentEdgeInsets = contentEdgeInsets;
}

其中imageEdgeInsets = UIEdgeInsetsMake(imageCenterOffsetY, imageCenterOffsetX, -imageCenterOffsetY, -imageCenterOffsetX);的
含义是安全区先顶部减小imageCenterOffsetY，左侧减小imageCenterOffsetX，然后底部增加imageCenterOffsetY，右侧增加imageCenterOffsetX， 相当于整体平移（imageCenterOffsetX , imageCenterOffsetY），自然image的中心也平移了（imageCenterOffsetX , imageCenterOffsetY）。由于这个设置只对image起作用，因此title不受影响。

此代码是对https://github.com/Phelthas/Demo_ButtonImageTitleEdgeInsets/blob/master/Demo_ButtonImageTitleEdgeInsets/LXMButtonImagePosition/UIButton%2BLXMImagePosition.m 的改进，使之更加易懂。