截取全屏

- (UIImage *)makeImageWithView:(UIView *)view withSize:(CGSize)size
{
    /**
     1.第一个参数表示区域大小
     2.第二个参数表示是否是非透明的。如果需要显示半透明效果，需要传NO，否则传YES
     3.第三个参数就是屏幕密度了，关键就是第三个参数 [UIScreen mainScreen].scale
     */
    UIGraphicsBeginImageContextWithOptions(size, YES, [UIScreen mainScreen].scale);
    //渲染view.layer
    [view.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    //保存图片到照片库
    UIImageWriteToSavedPhotosAlbum(image, nil, nil, nil);
    return image;
}

//方法解释：
void UIGraphicsBeginImageContextWithOptions(CGSize size, BOOL opaque, CGFloat scale);
size——同UIGraphicsBeginImageContext
opaque—透明开关，如果图形完全不用透明，设置为YES以优化位图的存储。
scale—–缩放因子 iPhone 4是2.0，其他是1.0。虽然这里可以用[UIScreen mainScreen].scale来获取，但实际上设为0后，系统就会自动设置正确的比例了。

根据自定义尺寸截屏

CGRect snapshotFrame = CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height);
 UIEdgeInsets snapshotEdgeInsets = UIEdgeInsetsZero;
    /**
     snapshotFrame 想要截取屏幕的frame
     */
- (UIImage *)snapshotViewFromRect:(CGRect)rect withCapInsets:(UIEdgeInsets)capInsets {
    UIGraphicsBeginImageContextWithOptions(rect.size, NO, [UIScreen mainScreen].scale);
    CGContextRef currentContext = UIGraphicsGetCurrentContext();
    CGContextTranslateCTM(currentContext, - CGRectGetMinX(rect), - CGRectGetMinY(rect));
    [self.view.layer renderInContext:currentContext];
    UIImage *snapshotImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    UIImageView *snapshotView = [[UIImageView alloc] initWithFrame:rect];
    snapshotView.image = [snapshotImage resizableImageWithCapInsets:capInsets];
    //保存图片到照片库
    UIImageWriteToSavedPhotosAlbum(snapshotView.image, nil, nil, nil);
    return snapshotView.image;
}