How to take a screenshot programmatically on iOS

Question

Considering a check for retina display use the following code snippet:

#import <QuartzCore/QuartzCore.h> 

if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
    UIGraphicsBeginImageContextWithOptions(self.window.bounds.size, NO, [UIScreen mainScreen].scale);
} else {
    UIGraphicsBeginImageContext(self.window.bounds.size);
}

[self.window.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
NSData *imageData = UIImagePNGRepresentation(image);
if (imageData) {
    [imageData writeToFile:@"screenshot.png" atomically:YES];
} else {
    NSLog(@"error while taking screenshot");
}

Below method works for OPENGL objects also

//iOS7 or above
- (UIImage *) screenshot {

    CGSize size = CGSizeMake(your_width, your_height);

    UIGraphicsBeginImageContextWithOptions(size, NO, [UIScreen mainScreen].scale);

    CGRect rec = CGRectMake(0, 0, your_width, your_height);
    [_viewController.view drawViewHierarchyInRect:rec afterScreenUpdates:YES];

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, self.view.opaque, 0.0);
[self.myView.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

NSData *imageData = UIImageJPEGRepresentation(image, 1.0 ); //you can use PNG too
[imageData writeToFile:@"image1.jpeg" atomically:YES];

- (UIImage*) getGLScreenshot {
    NSInteger myDataLength = 320 * 480 * 4;

    // allocate array and read pixels into it.
    GLubyte *buffer = (GLubyte *) malloc(myDataLength);
    glReadPixels(0, 0, 320, 480, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

    // gl renders "upside down" so swap top to bottom into new array.
    // there's gotta be a better way, but this works.
    GLubyte *buffer2 = (GLubyte *) malloc(myDataLength);
    for(int y = 0; y <480; y++)
    {
        for(int x = 0; x <320 * 4; x++)
        {
            buffer2[(479 - y) * 320 * 4 + x] = buffer[y * 4 * 320 + x];
        }
    }

    // make data provider with data.
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer2, myDataLength, NULL);

    // prep the ingredients
    int bitsPerComponent = 8;
    int bitsPerPixel = 32;
    int bytesPerRow = 4 * 320;
    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;

    // make the cgimage
    CGImageRef imageRef = CGImageCreate(320, 480, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);

    // then make the uiimage from that
    UIImage *myImage = [UIImage imageWithCGImage:imageRef];
    return myImage;
}

- (void)saveGLScreenshotToPhotosAlbum {
    UIImageWriteToSavedPhotosAlbum([self getGLScreenshot], nil, nil, nil);  
}

Source.

IN SWIFT

func captureScreen() -> UIImage
{

    UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, false, 0);

    self.view.drawViewHierarchyInRect(view.bounds, afterScreenUpdates: true)

    let image: UIImage = UIGraphicsGetImageFromCurrentImageContext()

    UIGraphicsEndImageContext()

    return image
}

As of iOS10, this gets a bit simpler. UIKit comes with UIGraphicsImageRender that allows you to

... accomplish drawing tasks, without having to handle configuration such as color depth and image scale, or manage Core Graphics contexts

Apple Docs - UIGraphicsImageRenderer

So you can now do something like this:

let renderer = UIGraphicsImageRenderer(size: someView.bounds.size)

let image = renderer.image(actions: { context in
    someView.drawHierarchy(in: someView.bounds, afterScreenUpdates: true)
})

Many of the answers here worked for me in most cases. But when trying to take a snapshot of an ARSCNView, I was only able to do it using the method described above. Although it might be worth noting that at this time, ARKit is still in beta and Xcode is in beta 4

See this post it looks like you can use UIGetScreenImage() for now.