How to call [UIView drawRect:(CGRect)rect] many times ?

Question

Hi guys.
I made implementation for UIView in order to draw shapes on UIView.
Now I need to call again draw:(CGRect) of my custom UIView.
But I don't know to call it again.
When call again draw:(CGRect), it would be cleared previous drawn shapes.
How can I do it ?
This is my code sample :
//MyView.h
#import <UIKit/UIKit.h>
@interface MyView : UIView {
NSMutableArray *lines;
@property (nonatomic, retain) NSMutableArray *lines;
//MyView.m
#import "MyView.h"
#import "MyPoint.h"
@implementation MyView
@synthesize lines;
- (id)initWithFrame:(CGRect)frame {
if (self = [super initWithFrame:frame]) {
lines = [[NSMutableArray alloc] init];
return self;
- (void)drawRect:(CGRect)rect {
CGFloat red[4] = {1.0f, 0.0f, 0.0f, 1.0f};
CGContextSetStrokeColor(c, red);
CGContextBeginPath(c);
int count = [lines count];
for(int i = 0; i < count - 1; i ++) {
MyPoint *point1 = (MyPoint*)[lines objectAtIndex:i];
MyPoint *point2 = (MyPoint*)[lines objectAtIndex:i + 1];
CGContextMoveToPoint(c, point1.x, point1.y);
CGContextAddLineToPoint(c, point2.x, point2.y);
CGContextStrokePath(c);
I tried to call draw method in NSThread ([self performSelectorOnMainThread:@selector(startDraw) withObject:nil waitUntilDone:NO];) following as :
- (void)startDraw {
MyView *view = (MyView)self.view;
[view drawRect:self.view.frame];
In the console :
Sun Nov 29 10:19:36 apples-imac.local MyApp[11909] <Error>: CGContextSetStrokeColor: invalid context
Sun Nov 29 10:19:36 apples-imac.local MyApp[11909] <Error>: CGContextBeginPath: invalid context
Sun Nov 29 10:19:36 apples-imac.local MyApp[11909] <Error>: CGContextMoveToPoint: invalid context
Sun Nov 29 10:19:36 apples-imac.local MyApp[11909] <Error>: CGContextAddLineToPoint: invalid context
Sun Nov 29 10:19:36 apples-imac.local MyApp[11909] <Error>: CGContextDrawPath: invalid context
What is wrong here ?
Thanks in advance.

Answer 1

Never call drawRect from your code. When you want the entire view to be redrawn, use setNeedsDisplay. E.g.:
// myViewController.m
- (void)updateView:(NSData*)newData {
// compute some new coords based on newData
self.view.newPoint = [self computePoint:newData];
// request redraw
[self.view setNeedsDisplay];
As shown in the example, you normally only need a redraw when something that affects the drawing has changed. You don't need to ask for a redraw when the view is first loaded, since the first drawing is automatic.
Also I'm concerned about your mention of a new thread. Additional threads should be avoided if at all possible, especially if you have less than two years of experience with Cocoa. Although xnav pointed you to some good documentation in reply to your last question, you should do everything you can to get the desired results without NSThread. I promise that extra thread will be nothing but trouble for you. If you explain what you're trying to accomplish, maybe someone here can suggest an implementation that only uses the main thread, ok?
- Ray