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Draw QR in Objective-C The Quartz context

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The Quartz context
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use Quartz to draw to most UIKit objects. To do so, you override the drawRect: method and, inside the object in question, use UIGraphicsGetCurrentContext to retrieve the current context. You may alternatively create a bitmap context in order to create or modify an image that you ll use elsewhere in your program. You do this by using the UIGraphicsBeginImageContext and UIGraphicsEndImageContext functions.
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Warning: inverse coordinate system ahead
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By now, you should be familiar with the standard iPhone OS coordinate system. It has the origin at upper left on the screen, with the main axes running to the right and down. Quartz s default coordinate system is inverted, with the origin at lower left and the main axes running right and up. This isn t usually a problem. The Cocoa Touch methods you use to create and write to graphical contexts usually transform Quartz s default coordinates so that they look like iPhone coordinates to you. Once in a while, though, you ll run into a situation where you ll draw to a UI-derived context and find your content flipped upside down (and in the wrong position). This is a result of accessing Quartz in a way that hasn t been transformed. As of this writing, we re aware of two situations where you ll have to correct Quartz s coordinate system by yourself, even when using one of the UI-derived contexts: if you import images using the native Quartz functions (as opposed to the UIImage methods you saw in the last chapter), and if you write text. We ll talk about each of these when we get to them. Personally, we consider these coordinate inversions bugs, and it s our expectation that they ll eventually be corrected. If you create a context without using Cocoa Touch, expect everything to be inverted. This is something that we don t expect to change in the future.
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You can use a variety of Core Graphics functions to access other sorts of contexts types that you won t usually use on an iPhone or iPad. The functions required to capture a PDF context are one such example. These have two deficits you should be aware of: they depend more heavily on the Core Foundation frameworks, and they use Quartz s inverted coordinate system. One thing to note about graphical contexts is that they re created in a stack: when you create a new context, it s pushed on top of a stack, and when you ve finished with it, it s popped off. This means that if you create a new bitmap context, it s placed on top of any existing context, such as the one associated with your UIView, and stays there until you ve finished with the bitmap. Table 13.1 lists these context-related functions, including both the standard UI context functions and the older Core Graphics function you re most likely to use for PDFs.
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Table 13.1
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Graphics: Quartz, Core Animation, and OpenGL
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Methods for graphical context creation Function Arguments (none) Summary Returns the current context, which is usually the context of the current UIKit object but can also be a context that you create by hand Creates a bitmap context Pops a bitmap context off the stack Returns a bitmap as a UIImage *; used with a bitmap context only Creates a PDF context
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UIGraphicsGetCurrentContext
UIGraphicsBeginImageContext UIGraphicsEndImageContext UIGraphicsGetImageFromCurrentImageContext CGPDFContextCreate
CGSize (none) (none)
CGDataConsumerRef, CGRect, CGDictionaryRef
We won t cover PDFs in this book, but we ll look at how to use each of the UIKit context styles, starting with the UIView.
13.2.1 Drawing to a UIView
In chapter 12, we offered an introductory example of how to write to a UIView graphical context using the drawRect: method. That example was somewhat simplified because the UIKit draw-image commands mostly hide the idea of graphical contexts from you. They automatically write to the current context, which inside drawRect: is the context related to the UIView. For most other functions, you need to do a bit more work: retrieving the graphical context and passing that context along to any drawing commands that you use. Here s how to draw a simple abstract face using this technique:
- (void)drawRect:(CGRect)rect { CGContextRef ctx = UIGraphicsGetCurrentContext(); CGContextBeginPath(ctx); CGContextAddArc(ctx,110,50,30,0,2*M_PI,1); CGContextAddArc(ctx,210,50,30,0,2*M_PI,1); CGContextAddArc(ctx,160,110,15,0,2*M_PI,1); CGContextAddArc(ctx,160,210,25,0,2*M_PI,1); CGContextFillPath(ctx); }
This example is fairly simple. You create a UIView subclass, and then you go to its drawRect: method. Once there, you capture the current context and use it to do whatever Quartz 2D drawing you desire. The function calls won t be familiar to you, but they re calls to draw a bunch of circles; we ll discuss them in the next section. As shown in figure 13.1, the art ends up looking oddly abstract, which shows how Quartz draws continuous
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