Every UIView subclass implements the method drawRect:, which contains the drawing code for the view. 

For example, a UIButton's drawRect: method draws a rounded rectangle with a title string in the center.

Each time an instance of UIView needs to be drawn (or redrawn), the system prepares a graphics context specifically for that view. Then the context is activated, and the message drawRect: is sent to the instance of UIView that is being drawn. The graphics context's type is CGContextRef (Core Graphics Context Reference).

A graphics context also stores its drawing state, which inlcudes things like the current drawing color, coordinate system, and the current line width.

- (void)drawRect:(CGRect)rect {// What rectangle am I filling?CGRect bounds = [self bounds];// Where is its center?CGPoint center;center.x = bounds.origin.x + bounds.size.width / 2.0;center.y = bounds.origin.y + bounds.size.height / 2.0;// From the center how far out to a corner?float maxRadius = hypot(bounds.size.width, bounds.size.height) / 2.0;// Get the context being drawn uponCGContextRef context = UIGraphicsGetCurrentContext();// All lines will be drawn 10 points wideCGContextSetLineWidth(context, 10);// Set the stroke color to light gray[[UIColor lightGrayColor] setStroke];// Draw concentric circles from the outside infor (float currentRadius = maxRadius; currentRadius > 0; currentRadius -= 20){CGContextAddArc(context, center.x, center.y,currentRadius, 0.0, M_PI * 2.0, YES);CGContextStrokePath(context);}}

Notice you are passed a CGRect structure. This is the rectangle that needs to be redrawn, sometimes called the dirty rectangle. A CGRect structure contains the members origin and size. The origin is of type CGPoint and contains two float members: x and y. The size if of type CGSize and also has two float members: width and height.

Remember that a structure is not an objective-c object, so you can't send it messages.

Instantiating a UIView

There are two ways to create an instance of your view:

• visually choose and position the view while editing the story board file
• create it programmatically with alloc and initWithFrame: and make it a subview of the window

#import 
     
      //this is a "forward declaration"@class HypnosisView;...HypnosisView *view;
     

This is a forward declaration for the class HypnosisView.When you forward declare a class, you aren't going as far as importing the header file; You are just informing HypnosisterAppDelegate.h of the class HypnosisView so the compiler can validate it. Forward declaring a class saves time when compiling - especially with large projects.

- (BOOL) application: (UIApplication *) applicationdidFinishLaunchingWithOptions:(NSDictionary *) launchOptions{    // Make a CGRect that is the size of the window    CGRect wholeWindow = [[self window] bounds];    // Create an instance of HypnosisView that is the same sizeas the window     view = [[HypnosisView alloc] initWithFrame:wholeWindow];    // Set the background color of that view to "clear"    [view setBackgroundColor:[UIColor clearColor]];    // Add the view to the view hierarchy so that it appears onthe window   [[self window] addSubView:view];   [[self window] makeKeyAndVisible];   return YES;}

Neither HypnosisterAppDelegate nor the window will ever get deallocated because they exist the entire time the application is running.

UIScrollView

UIScrollView *scrollView = [[UIScrollView alloc] initWithFrame:wholeWindow];[[self window] addSubView:scrollView];[scrollView setContentSize:size];//center it in the scroll viewCGPoint offset;offset.x = wholeWindow.size.width*0.5;offset.y = wholeWindow.size.height*0.5;[scrollView setContentOffset:offset];//create the viewview = [[HypnosisView alloc] initWithFrame:reallyBigRect];[scrollView addSubView:view];...}

Zooming

To add zooming, you need to give the scroll view a delegate

@interface HypnosisAppDelegate: NSObject 
     

and in didFinishLaunchingWithOptions...

//enable zooming[scrollView setMinimumZoomScale:0.5];[scrollView setMaximumZoomScale:5];[scrollView setDelegate:self];

in the same file, implement the delegate method viewForZoomingInScrollView:

- (UIView *) viewForZoomingInScrollView: (UIScrollView *)scrollView{    return view;}

You probably don't want to see the time or your remaining battery charge. So, you are going to hide the status bar before you make the window visible.

add a line near the end of application:didFinishLaunchingWithOptions:

[[UIApplication sharedApplication] setStatusBarHidden:YES                                        withAnimation:UIStatusBarAnimationFade];[[self window] makeKeyAndVisible];return YES;

You can also info property list with hidden status bar in project target setting.

Status bar is initially hidden

YES

 Retain Cycles

A view hierarchy is made up of many parent-child relationships. When we talk about view hierarchies, we call parents superviews and their children subviews. 

The solution is simple: children should never retain their parents. When you adhere to this rule, deallocating a parent object appropriately release its child objects. If the parent is the only owner of its children, then these child objects are deallocated.

Redrawing Views

When a UIView instance is sent the message setNeesDisplay, the view is marked for re-display. When a view has marked itself for re-display, it is not immediately redrawn, instead, it is added to a list of views that needs updating. Your application is a giant infinite loop called the run loop, The run loop's job is to check for input(a touch,core location updates, data coming in through a network interface, etc. ) and then find the appropriate handlers for that event.