Drawing pixels on the screen using CoreGraphics in Swift

Question

The code below is trying to set pixels to an offline bitmap and draw the bitmap to the screen. Unfortunately, it crashes.

import UIKit

class GameView: UIView {

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
    }


    func createBitmapContext(pixelsWide: Int, _ pixelsHigh: Int) -> CGContextRef? {
        let bytesPerPixel = 4
        let bytesPerRow = bytesPerPixel * pixelsWide
        let bitsPerComponent = 8

        let byteCount = (bytesPerRow * pixelsHigh)

        let colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB)

        let pixels = UnsafeMutablePointer<CUnsignedChar>.alloc(byteCount)
        if pixels == nil {
            return nil
        }
        let bitmapInfo = CGImageAlphaInfo.PremultipliedFirst.rawValue | CGBitmapInfo.ByteOrder32Little.rawValue

        let context = CGBitmapContextCreate(pixels, pixelsWide, pixelsHigh, bitsPerComponent, bytesPerRow, colorSpace, bitmapInfo)

        return context
    }

    override func drawRect(rect: CGRect) {
        let width  = 200
        let height = 300
        let boundingBox = CGRectMake(0, 0, CGFloat(width), CGFloat(height))
        let context = createBitmapContext(width, height)

        let data = CGBitmapContextGetData(context)
        var currentPixel: [UInt32] = unsafeBitCast(data, [UInt32].self)

        var n = 0
        for var j = 0; j < height; j++ {
            for var i = 0; i < width; i++ {
                currentPixel[n++] = 0
            }
        }

        let image = CGBitmapContextCreateImage(context!)
        CGContextDrawImage(context!, boundingBox, image)
    }

}

Answer 1

There are couple changes required. 1. Crashing because of there was memory overrun. 2. You are creating image from newly created context and writing to same context instead of current drawing context.

Use this modified drawRect Function:

override func drawRect(rect: CGRect) {
    let width  = 200
    let height = 300
    let boundingBox = CGRectMake(0, 0, CGFloat(width), CGFloat(height))
    let context = createBitmapContext(width, height)

    let data = CGBitmapContextGetData(context)

    let pixels = UnsafeMutablePointer<CUnsignedChar>(data)
    var n = 0
    for var j = 0; j < height; j++ {
        for var i = 0; i < width; i++ {
            pixels[n++] = 0 //B
            pixels[n++] = 255 //G
            pixels[n++] = 0 //R
            pixels[n++] = 255 //A
        }
    }
    let image = CGBitmapContextCreateImage(context!)
    if let currentContext: CGContext! = UIGraphicsGetCurrentContext() {
        UIImagePNGRepresentation(UIImage(CGImage:image!))?.writeToFile("/Users/admin/Desktop/aaaaa.png", atomically: true)
        CGContextDrawImage(currentContext, boundingBox, image)
    }
}