UIImageView

停靠模式

带有scale单词的：图片有可能会拉伸
没有scale单词的：图片绝对不会被拉伸，保持图片的原尺寸
将图片拉伸至填充整个imageView
UIViewContentModeScaleToFill 

带有aspect单词的：保持图片原来的宽高比
UIViewContentModeScaleAspectFit
保证刚好能看到图片的全部

UIViewContentModeScaleAspectFill
拉伸至图片的宽度或者高度跟imageView一样

//下面的就是字面意思
UIViewContentModeCenter 

UIViewContentModeTop

UIViewContentModeBottom

UIViewContentModeLeft

UIViewContentModeRight

UIViewContentModeTopLeft

UIViewContentModeTopRight

UIViewContentModeBottomLeft

UIViewContentModeBottomRight

修改imageView的size等于图片的size

//修改imageView的size等于图片的size
UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(50, 200, 300, 300)];
imageView.image = [UIImage imageNamed:@"header"];

//image.size = 144*144
CGRect rect = imageView.frame;
rect.size = imageView.image.size;
imageView.frame = rect;

//实际显示的imgView的size就是144*144
[self.view addSubview:imageView];