How to use drawRect to draw in a existing view? - objective-c

Question

I'm doing a GPS tracking app. Every time it receives a Latitude/Longitude it converts it to (x,y) coordinates and calls drawRect to draw a line between two (x,y) pairs.
However, the drawRect method just clear all the old contents before it draw new thing. How I can make the drawRect to draw new thing on the existing canvas? Thanks in advance
here is my viewController.h
#import <UIKit/UIKit.h>
#import <CoreLocation/CoreLocation.h>
#class routetrack;
#interface simpleDrawViewController : UIViewController <CLLocationManagerDelegate> {
CLLocationManager *locationManager;
IBOutlet routetrack *routeroute;
float latOld;
float longOld;
float latNew;
float longNew;
}
- (IBAction) gpsValueChanged;
#property (nonatomic,retain) CLLocationManager *locationManager;
#property (retain,nonatomic) routetrack *routeroute;
ViewController.m
#import "simpleDrawViewController.h"
#import "routetrack.h"
#implementation simpleDrawViewController
#synthesize locationManager,btn,routeroute;
- (IBAction) gpsValueChanged{
[routeroute setLocationX:longNew setLocationY:latNew
oldLocation:longOld oldLocationY:latOld ];
[routeroute setNeedsDisplay];
}
- (void)viewDidLoad {
[super viewDidLoad];
if (nil == locationManager)
locationManager = [[CLLocationManager alloc] init];
locationManager.delegate = self;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
[locationManager startUpdatingLocation];
}
- (void)didReceiveMemoryWarning {
// Releases the view if it doesn't have a superview.
[super didReceiveMemoryWarning];
// Release any cached data, images, etc that aren't in use.
}
- (void)viewDidUnload {
[super viewDidUnload];
// Release any retained subviews of the main view.
// e.g. self.myOutlet = nil;
}
- (void) locationManager:(CLLocationManager *)manager
didUpdateToLocation:(CLLocation *)newLocation
fromLocation:(CLLocation *)oldLocation
{
if (oldLocation) {
latOld=oldLocation.coordinate.latitude;
longOld=oldLocation.coordinate.longitude;
latNew=newLocation.coordinate.latitude;
longNew=newLocation.coordinate.longitude;
}
else {
latOld=53.3834;
longOld=-6.5876;
latNew=53.3835;
longNew=-6.5877;
}
[self gpsValueChanged];
}
- (void) locationManager:(CLLocationManager *)manager didFailwithError:
(NSError *)error
{
if([error code] == kCLErrorDenied)
[locationManager stopUpdatingLocation];
NSLog(#"location manger failed");
}
- (void)dealloc {
[locationManager release];
[super dealloc];
}
#end
my drawing class, a subclass of UIView
#import "routetrack.h"
#implementation routetrack
- (void) setLocationX:(float) loX setLocationY:(float) loY
oldLocation:(float) oldX oldLocationY:(float) oldY {
self->locationX = loX;
self->locationY = loY;
self->oldLoX = oldX;
self->oldLoY = oldY;
scaleX= 36365.484375;
scaleY= 99988.593750;
maxLat= 53.3834;
maxLog= -6.5976;
drawX = locationX;
drawY = locationY;
tempX=(maxLog - drawX) * scaleX;
tempY=(maxLat - drawY) * scaleY;
lastX = (int) tempX;
lastY = (int) tempY;
drawX = oldLoX;
drawY = oldLoY;
tempX=(maxLog - drawX) * scaleX;
tempY=(maxLat - drawY) * scaleY;
startX = (int) tempX;
startY = (int) tempY;
}
- (id)initWithFrame:(CGRect)frame {
self = [super initWithFrame:frame];
if (self) {
}
return self;
}
- (void)drawRect:(CGRect)rect {
int firstPointX = self->startX;
int firstPointY = self->startY;
int lastPointX = self->lastX;
int lastPointY = self->lastY;
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBStrokeColor(context, 1.0, 0.0, 0.0, 1.0);
CGContextSetLineWidth(context, 2.0);
CGContextMoveToPoint(context, firstPointX, firstPointY);
CGContextAddLineToPoint(context, lastPointX , lastPointY);
CGContextStrokePath(context);
}

Answer 1

UIView does not have a canvas model. If you want to keep a canvas, you should create a CGLayer or a CGBitmapContext and draw onto that. Then draw that in your view.
I would create an ivar for a CGLayerRef, and then drawRect: would look something like this (untested):
- (void)drawRect:(CGRect)rect {
if (self.cgLayer == NULL) {
CGLayerRef layer = CGLayerCreateWithContext(UIGraphicsContextGetCurrent(), self.bounds, NULL);
self.cgLayer = layer;
CGLayerRelease(layer);
}
... various Core Graphics calls with self.cgLayer as the context ...
CGContextDrawLayerInRect(UIGraphicsContextGetCurrent(), self.bounds, self.cgLayer);
}
Whenever you wanted to clear your canvas, just self.cgLayer = NULL.
setCgLayer: would be something like this:
- (void)setCgLayer:(CGLayerRef)aLayer {
CGLayerRetain(aLayer);
CGLayerRelease(cgLayer_);
cgLayer_ = aLayer;
}

Answer 2

What exactly to you mean by "old contents"? If you want to draw a line from your GPS data, you have to draw all points every time in the implementation of drawRect.