_showImageView.center =self.center;

    _showImageView.size =_showImage.size;

    NSLog(@"第一种: _showImageView.frame ===  %@",NSStringFromCGRect(_showImageView.frame));

    _showImageView.center =self.center;

    _showImageView.bounds = (CGRect){CGPointZero,_showImage.size};

    NSLog(@"第二种: _showImageView.frame ===  %@",NSStringFromCGRect(_showImageView.frame));

2014-09-23 10:41:59.311 NeiHanShe[6205:60b]第一种: _showImageView.frame ===  {{160, 284}, {265, 146.13971}}

2014-09-23 10:41:59.311 NeiHanShe[6205:60b]第二种: _showImageView.frame ===  {{27.5, 210.93015}, {265, 146.13971}}

所有正确设置,应该选择第二种,否则不会居中, 第一种的self.center 会当做_showImageView.frame 的 x y

iOS 同时设置view的 center 跟 size 的注意细节