-(UIImage *)takeScreenshot{

    UIGraphicsBeginImageContextWithOptions(CGSizeMake(self.view.frame.size.width, self.view.frame.size.height), YES, 0);
    //设置截屏大小
    [[self.view layer] renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    CGImageRef imageRef = viewImage.CGImage;
    CGRect rect = CGRectMake(0, 0, 300, 300);//这里可以设置想要截图的区域
    CGImageRef imageRefRect =CGImageCreateWithImageInRect(imageRef, rect);
    UIImage *sendImage = [[UIImage alloc] initWithCGImage:imageRefRect];

    //以下为图片保存代码
    UIImageWriteToSavedPhotosAlbum(sendImage, nil, nil, nil);//保存图片到照片库
    NSData *imageViewData = UIImagePNGRepresentation(sendImage);
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSDate *datenow = [NSDate date];
    NSString *pictureName= [NSString stringWithFormat:@"%ld_screenShow.png",(long)[datenow timeIntervalSince1970]];
    NSString *savedImagePath = [documentsDirectory stringByAppendingPathComponent:pictureName];
    [imageViewData writeToFile:savedImagePath atomically:YES];//保存照片到沙盒目录
    CGImageRelease(imageRefRect);
    
    //从手机本地加载图片
    UIImage *bgImage2 = [[UIImage alloc]initWithContentsOfFile:savedImagePath];
    return bgImage2;
}

另一种方法（没读懂如何控制区域）

-(UIImage *)takeScreenshot {
    CGSize imageSize = CGSizeZero;
    CGSize screenSize = [UIScreen mainScreen].bounds.size;
    UIInterfaceOrientation orientation = [UIApplication sharedApplication].statusBarOrientation;
    
    if (UIInterfaceOrientationIsPortrait(orientation)) {
        imageSize = screenSize;
    } else {
        imageSize = CGSizeMake(screenSize.height, screenSize.width);
    }
    
    UIGraphicsBeginImageContextWithOptions(imageSize, false, 0);
    CGContextRef context = UIGraphicsGetCurrentContext();
    if (context) {
        for (UIWindow *window in [UIApplication sharedApplication].windows) {
            CGContextSaveGState(context);
            CGContextTranslateCTM(context, window.center.x, window.center.y);
            CGContextConcatCTM(context, window.transform);
            CGContextTranslateCTM(context, -window.bounds.size.width * window.layer.anchorPoint.x, -window.bounds.size.height * window.layer.anchorPoint.y);
            
            if (orientation == UIInterfaceOrientationLandscapeLeft) {
                CGContextRotateCTM(context, M_PI_4);
                CGContextTranslateCTM(context, 0, -imageSize.width);
            } else if (orientation == UIInterfaceOrientationLandscapeRight) {
                CGContextRotateCTM(context, - M_PI_2);
                CGContextTranslateCTM(context, -imageSize.height, 0);
            } else if (orientation == UIInterfaceOrientationPortraitUpsideDown) {
                CGContextRotateCTM(context, M_PI);
                CGContextTranslateCTM(context, -imageSize.width, -imageSize.height);
            }
            
            if ([window respondsToSelector:@selector(drawViewHierarchyInRect:afterScreenUpdates:)]) {
                [window drawViewHierarchyInRect:window.bounds afterScreenUpdates:YES];
            } else {
                [window.layer renderInContext:context];
            }
            
            CGContextRestoreGState(context);
        }
    }
    
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return image;
}