The UIKit Graphics System IPHONE APPS

In iPhone OS, all drawing—regardless of whether it involves OpenGL, Quartz, UIKit, or Core Animation—occurs within the confines of a UIView object. Views define the portion of the screen in which drawing occurs. If you use system-provided views, this drawing is handled for you automatically. If you define custom views, however, you must provide the drawing code yourself. For applications that draw using OpenGL, once you set up your rendering surface, you use the drawing model specified by OpenGL.

For Quartz, Core Animation, and UIKit, you use the drawing concepts described in the following sections.

The View Drawing Cycle

The basic drawing model for UIView objects involves updating content on demand. The UIView class makes the update process easier and more efficient, however, by gathering the update requests you make and delivering them to your drawing code at the most appropriate time.

Whenever a portion of your view needs to be redrawn, the UIView object’s built-in drawing code calls its drawRect: method. It passes this method a rectangle indicating the portion of your view that needs to be redrawn. You override this method in your custom view subclasses and use it to draw the contents of your view. The first time your view is drawn, the rectangle passed to the drawRect: method contains your view’s entire visible area. during subsequent calls, however, this rectangle represents only the portion of the view that actually needs to be redrawn. There are several actions that can trigger a view update:

  • Moving or removing another view that was partially obscuring your view
  • Making a previously hidden view visible again by setting its hidden property to NO
  • Scrolling a view off the screen and then back on
  • Explicitly calling the setNeedsDisplay or setNeedsDisplayInRect: method of your view

After calling your drawRect: method, the view marks itself as updated and waits for new actions to arrive and trigger another update cycle. If your view displays static content, then all you need to do is respond to changes in your view’s visibility caused by scrolling and the presence of other views. If you update your view’s content periodically, however, you must determine when to call the setNeedsDisplay or set Needs Display InRect: method to trigger an update. For example, if you were updating content several times a second, you might want to set up a timer to update your view. You might also update your view in response to user interactions or the creation of new content in your view.

Coordinates and Coordinate Transforms

As described in “View Coordinate Systems”, the origin of a window or view is located in its top-left corner, and positive coordinate values extend down and to the right of this origin.When you write your drawing code, you use this coordinate system to specify the location of individual points for the content you draw.

If you need to make changes to the default coordinate system, you do so by modifying the current transformation matrix. The current transformation matrix (CTM) is a mathematical matrix that maps points in your view’s coordinate system to points on the device’s screen. When your view’s drawRect: method is first called, the CTM is configured so that the origin of the coordinate system matches the your view’s origin and its positive axes extend down and to the right. However, you can change the CTM by adding scaling, rotation, and translation factors to it and thereby change the size, orientation, and position of the default coordinate system relative to the underlying view or window.

Modifying the CTM is the standard technique used to draw content in your view because it involves much less work. If you want to draw a 10 x 10 square starting at the point (20,20) in the current drawing system, you could create a path that moves to (20, 20) and then draws the needed set of lines to complete the square. If you decide later that you want to move that square to the point (10, 10), however, you would have to recreate the path with the new starting point. In fact, you would have to recreate the path every time you changed the origin. Creating paths is a relatively expensive operation, but creating a square whose origin is at (0, 0) and modifying the CTM to match the desired drawing origin is cheap by comparison.

In the Core Graphics framework, there are two ways to modify the CTM. You can modify the CTM directly using the CTM manipulation functions defined in CGContext Reference. You can also create a CG Affine Transform structure, apply any transformations you want, and then concatenate that transform onto the CTM. Using an affine transform lets you group transformations and then apply them to the CTM all at once. You can also evaluate and invert affine transforms and use them to modify point, size, and rectangle values in your code.

Graphics Contexts

Before calling your custom drawRect: method, the view object automatically configures its drawing environment so that your code can start drawing immediately. As part of this configuration, the UIView object creates a graphics context (a CG Context Ref opaque type) for the current drawing environment. This graphics context contains the information the drawing system needs to perform any subsequent drawing commands. It defines basic drawing attributes such as the colors to use when drawing, the clipping area, line width and style information, font information, compositing options, and several others.

You can create custom graphics context objects in situations where you want to draw somewhere other than your view. In Quartz, you primarily do this when you want to capture a series of drawing commands and use them to create an image or a PDF file. To create the context, you use the CGBitmapContextCreate or CGPDFContextCreate function. Once you have the context, you can pass it to the drawing functions needed to create your content.

When creating custom contexts, the coordinate system for those contexts is different than the native coordinate system used by iPhone OS. Instead of the origin being in the upper-left corner of the drawing surface, it is in the lower-left corner and the axes point up and to the right. The coordinates you specify in your drawing commands must take this into consideration or the resulting image or PDF file may appear wrong when rendered.

Points Versus Pixels

The Quartz drawing system uses a vector-based drawing model. Compared to a raster- based drawing model, in which drawing commands operate on individual pixels, drawing commands in Quartz are specified using a fixed-scale drawing space, known as the user coordinate space. iPhone OS then maps the coordinates in this drawing space onto the actual pixels of the device. The advantage of this model is that graphics drawn using vector commands continue to look good when scaled up or down using an affine transform.

In order to maintain the precision inherent in a vector-based drawing system, drawing coordinates are specified using floating-point values instead of integers. The use of floating- point values for coordinates makes it possible for you to specify the location of your program's content very precisely. For the most part, you do not have to worry about how those values are eventually mapped to the device’s screen.

The user coordinate space is the environment that you use for all of your drawing commands. The units of this space are measured in points. The device coordinate space refers to the native coordinate space of the device, which is measured in pixels. By default, one point in user coordinate space is equal to one pixel in device space, which results in 1 point equaling 1/160th of an inch. You should not assume that this 1-to-1 ratio will always be the case, however.

Color and Color Spaces

iPhone OS supports the full range of color spaces available in Quartz; however, most applications should need only the RGB color space. Because iPhone OS is designed to run on embedded hardware and display graphics on a screen, the RGB color space is the most appropriate one to use.

The UIColor object provides convenience methods for specifying color values using RGB, HSB, and grayscale values. When creating colors in this way, you never need to specify the color space. It is determined for you automatically by the UIColor object. You can also use the CG Context Set RGB Stroke Color and CG Context Set RGB Fill Color functions in the Core Graphics framework to create and set colors. Although the Core Graphics framework includes support for creating colors using other color spaces, and for creating custom color spaces, using those colors in your drawing code is not recommended. Your drawing code should always use RGB colors.

Supported Image Formats

Table lists the image formats supported directly by iPhone OS. Of these formats, the PNG format is the one most recommended for use in your applications.

Supported image formats

Supported image formats

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