图片捕捉 保持高不变 压缩

 UIImage * image = [self captureScreenInRect:viewController.view];
    float ratio = image.size.width/image.size.height;
    
    image = [self resizeImage:image size:CGSizeMake(height*ratio, height)];


- (UIImage *) captureScreenInRect:(UIView *) view {
    CALayer *layer;
    layer = view.layer;
    UIGraphicsBeginImageContext(view.bounds.size);
    
    CGContextClipToRect (UIGraphicsGetCurrentContext(),view.frame);
    [layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *screenImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return screenImage;
}

- (UIImage*) resizeImage:(UIImage*)image size:(CGSize)size {
    if ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)] &&
        ([UIScreen mainScreen].scale == 2.0)) {
        size = CGSizeMake(size.width*2, size.height*2);
    } 
    
    UIGraphicsBeginImageContext(size);
    [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return newImage;
}

如果是手机屏幕图捕捉和显示需要考虑屏幕 所以*2。但是如果相机拍照 然后裁剪上传没必要考虑*2