Question

I am trying to develop transition effect, when one view splits into 2 parts with upper part animated upwards and lower part animated downwards to revel the view behind it.
I am using UIView and UIImageView approach to accomplish that:

// 1. Make a screenshot:
    UIGraphicsBeginImageContext(parentView.frame.size);
    [parentView.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *screenshot = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    // 2. Calculate rectangles for top and bottom part:
    CGRect rectTop, rectBottom;
    CGFloat W = rectBig.size.width;
    CGFloat H = rectBig.size.height;


    rectTop = CGRectMake(0, 0, W, y_cutoff);
    rectBottom = CGRectMake(0, y_cutoff, W, H - y_cutoff);

    // 3. Create top and bottom images:
    CGImageRef imageRefTop      = CGImageCreateWithImageInRect([screenshot CGImage], rectTop);
    CGImageRef imageRefBottom   = CGImageCreateWithImageInRect([screenshot CGImage], rectBottom);


    UIImage *imageTop = [UIImage imageWithCGImage:imageRefTop];
    UIImage *imageBottom = [UIImage imageWithCGImage:imageRefBottom];


// 4. Assign images to image views:
imageViewTop.image = imageTop;
imageViewBottom.image = imageBottom;

// 5. Animate image views:
[UIView beginAnimation:nil context:NULL];
.... animation code here
[UIView commitAnimations];

This code however, is extremely slow on the device and I am sure there is a more efficient way to implement such a transition. Most probably using CALayers, etc..
Can you point me to the right direction?

Answer 1

This is not animation. Your drawing code is slow. If you profile, you will see that renderInContext: (btw. always executed on the main thread) and CGImageCreateWithImageInRect are the limiting factors. What your view (the one you want to split) is about? Is it an option to create two views instead of one from start?