Apple's AppStore a goldmine or ...

You find a lot of blogs on the Internet which speculates how good (or bad) sales are on the AppStore. Usually from people who didn't publish an application themselfs on the AppStore.

Now here I'm gone write my personal experience with the AppStore. About 1 month ago (3th of December) my application was finally approved by Apple.

Let's go over the timeline from concept of the app until today.

1. Concept

The idea came after a discussion with a friend , who's a financial analyst, to build a front end on top of his trading engine.

So the idea was that I should build an iPhone front end and that he would build the web infrastructure so that the iPhone could communicate with the trading engine.

I also did some investigation and I saw that their where only 1000 apps on the AppStore in the finance category , so at least we would be seen.

Of course this was not a game so we wouldn't reach the masses or getting great attention from review sites etc.

(Note: I still find it strange that people buy 600 USdollars phones to play games on it, but oke).

2. Development

And then it was April 2009, we started the development. I bought a development license from Apple for 99 USdollar ( 79 euro), got myself an iPod Touch for development. Got all the contracts signed (that was smooth by the way) and got myself an IRS number.

That was a strange experience for a non-US citizen ;-).

Apparently they don't believe in the Internet because the only to get that number was by phone, mail or fax - no online -.

Anyway we start coding , he was using PHP for the backend and I using Objective-C for the front-end.

And the communication between iPod and backend happened via HTTP using CSV files (nope no XML , that's overkill, the CSV's are generated on the fly during a request).

It took about 2,5 months to develop it, 1 month beta-testing and 1 month for corrections, change requests etc.

So in August 2009 I submitted the app for approval to ItunesConnect. And now the story begins...

3. Approval process

According to Apple it takes about 14 days to get approval (or rejection). So I was thinking , I submit it in August then we have an availability date of September 1th.

Now that was wrong.

After a week I got a mail from Apple to say that it would take longer to approve. But with no further explanation for what reason.

So august went by -> no approval

September went by -> no approval (and no message from Apple)

October went by -> no approval (no message from Apple).

At that time I sent an email to them but I got a standard response back.

And then suddenly in November they started the review of the application and I got an email that the app was rejected because of an usability problem.

I quickly fixed it and resubmitted it, and I was afraid that it would take again 3,5 months for approval.

But luckily for one reason or another they almost immediatly reviewed it and approved it.

So since December 3th the application is on the AppStore for sale !

4. Post Natal depression

So the new born is 1 month old, and how is the sales going ?

Well to be honest, not so good (sold twenty-some). We knew it was a niche market so we didn't expect 1000 downloads per day. But a bit more than 20... would be nice.

Now the AppStore (and ITunes Connect) doesn't give us much help. With more than 100.000 applications available you are just 1 item in a giant catalog (even finance has 1700 apps).

And a catalog with very limited search and browse facilities , after a few days your application is somewhere on page 10 and nobody cares anymore.

ITunes Connect as marketing aid is also nothing, you can enter keywords but you have no idea how many people are landing on your app, which keywords they used etc etc. You only see how many downloads their are.

Now we are spending our time in sending emails to review sites etc. But those guys are also overwhelmed with requests. And most likey they rather like to review games or entertainment apps than boring financial app's.

Conclusion : creating and developing an application for the iPhone is the easy part , its 10 % of the effort , marketing is the other 90%.

For those who think that the AppStore is a goldmine , forget it, a good idea is a prerequisite but some luck and a lot of marketing is needed. And even then you are not sure that you'll win the jackpot. 

 

drawing graphics for the iPhone (not opengl ES)

Don't we all want to make graphics like this : ?

Well , I'll try to explain how I did this (BTW this graphics comes from my iphone application iSignals).

First create a file in your project which is a subclass of UIView (via XCode>File>New File). Call it for example MyGraphView. 

Then you create a XIB with Interface Builder , in the XIB you add a view and you change the class identity to the custom view class MyGraphView.

And now comes the coding part, lets for the sake of clarity say that you have 2 arrays in MyGraphView (usually you'll pass them via a controller class). These arrays are called datapoints[] and ylabels[].

So let's see how the drawRect method is implemented.

- (void)drawRect:(CGRect)rect {

    // Drawing code

float width =  rect.size.width ;

float height = rect.size.height ;

CGContextRef currentContext ;

int i = 0 ;

 

 // Get the graphics context that we are currently executing under

currentContext = UIGraphicsGetCurrentContext() ;

   ;

 

In the first lines of the drawRect, I save the width and the height, I'll use this to calculate the scale factors.

To understand this you need to know a bit of coordinatesystems. The iPhone coordinates are simply the pixels that can be drawn. So for example a view can be 200 px wide and 300 px high.

But of course what more important is , is the world coordinatessystem. And these coordinates are the coordinates of your world.

For example if I want to plot 600 points and the range of those points are between 0.5 and 3.5 then my user coordinatessystem is: X-axis =(0,600) Y-axis  = (0.5,3.5).

So to convert user points into iphone points (or view points) I need to apply a scale factor. The scale factor is calculated as follows.

#define OFFSET_X 30.0f

#define OFFSET_Y 10.0f 

scale_factor_x = ( width -  OFFSET_X ) / ( [dataPoints count] - 0 );

scale_factor_y =   (height - OFFSET_Y ) / (max_y - min_y ) ; // max_y = max(dataPoints) ;min_y = min(dataPoints)

Note: OFFSET_X and OFFSET_Y are 2 constants to let some space in the view where I can are  plot the axes and labels of the axes.

So let's draw the axes , please note that in this first version I don't use CGPath's , later I'll refactor the code and use CGPath's.

And also be aware that I use the default orientation and default location of the origin of the view ( point (0,0) is in the upper left corner).

// draw the axes

CGContextSetRGBStrokeColor(currentContext, 0.83, 0.83, 0.83, 0.7); // this is sort of gray

CGContextMoveToPoint( currentContext,   OFFSET_X, height - OFFSET_Y  );

CGContextAddLineToPoint( currentContext,   OFFSET_X, OFFSET_Y );

CGContextMoveToPoint( currentContext,   OFFSET_X, height - OFFSET_Y   );

CGContextAddLineToPoint( currentContext,   width, height - OFFSET_Y  );

CGContextStrokePath(currentContext);

 

And draw some gridlines

        

CGContextSetRGBStrokeColor(currentContext, 0.83, 0.83, 0.83, 1);

CGFloat len[] = {4,2} ;

CGContextSetLineDash( currentContext, 0, len, 2 ) ;

    for ( i = OFFSET_X ; i <= height ; i = i + OFFSET_X )

{

CGContextMoveToPoint( currentContext,   OFFSET_X, height - OFFSET_Y - i );

CGContextAddLineToPoint( currentContext,   width, height - OFFSET_Y - i);

}

CGContextStrokePath(currentContext);

 

And then draw the graph of datapoints

  

CGContextSetRGBStrokeColor(currentContext, 1.0, 0.65, 0, 1);

    UIImage *red = [UIImage imageNamed: @"red.png"] ;

UIImage *green = [UIImage imageNamed: @"green.png"] ;

CGPoint aPoint ;

// draw the history graph

for( i = 0 ; i< [dataPoints count] ; i++ )

{

x =  OFFSET_X +    i * scale_factor_x ;

old_y = [[dataPoints objectAtIndex: i]  floatValue];

y =   (height -OFFSET_Y) - ([[dataPoints objectAtIndex: i]  floatValue] - min_y) * scale_factor_y ;

 

if ( old_x < 0 )

{

CGContextMoveToPoint( currentContext,   x, y  );

old_x = x ;

}

else

{

old_x = x ;

CGContextAddLineToPoint( currentContext,   x, y );

}

 

 

}

CGContextStrokePath(currentContext);

 

And now I show you how to add text in the graph (drawing the Y-labels, I will not give the code for the X-labels).

 

        // this is for the font

CGContextSetRGBStrokeColor(currentContext, 1, 1, 1, 1);

CGContextSetRGBFillColor(currentContext, 1.0, 1.0, 1.0, 0.8);

CGContextSelectFont(

currentContext,

"Helvetica-Bold",

FONT_SIZE,

kCGEncodingMacRoman

);

        // this transformation is to make sure that the text is written in the right direction

CGAffineTransform transform = CGAffineTransformMake(1.0,0, 0.0, -1.0, 0.0, 0.0);

 

        CGContextSetTextMatrix(currentContext, transform);

CGContextSetTextDrawingMode(currentContext, kCGTextFill);

// set Y-labels

float j1 = min_y ;

float step = (max_y - min_y)/ 7.0f ; // 7 labels

for ( i = 0.0f ; i <=height ; i = i + OFFSET_X )

{

  // set the precision of the label

if ( j1 > 999.0f )

{

    s1 = [NSString stringWithFormat:@"%4.0f", j1] ;

}

else

{

    s1 = [NSString stringWithFormat:@"%4.2f", j1] ;

}

j1 += step ;

CGContextShowTextAtPoint( currentContext,0 , height - OFFSET_Y - i , [s1 UTF8String] , [s1 length] ) ;

}

   

CGContextStrokePath(currentContext);

 

Voila this was it, next time I'll refactor the code and use Paths instead of each time saving and restoring the context

 

Its finally there

After months of hard work and a long review process by Apple, my first iPhone application is finally accepted on the AppStore.

iSignals, for the active investor. This application integrates with an proprietary investment engine , this investment engine holds a computer-generated portfolio with more than 17000 equities on 28 different stockmarkets and with 9 years of historical data. 

On the daily basis the engine generates computer managed portfolios with buy&sells of those equities (portfolio's are organized per market).

The engine is based on a combination of technical analysis models, artificial intelligence (pattern detection) and a neural network.

Via iSignals the user can subscribe to one or more computer managed portfolio's and as such receiving the buy/sells from the engine.

The user can now use those computer generated signals as a guidance in his investment decisions.

The advantage of the application is that the user can consult the engine's portfolio's wherever he wants and whenever he wants. The user can also consult quote data for each instrument (data from finance.yahoo.com) on his iPhone/iPod Touch.

You can find it here

iPhone SMS hack

I 've downloaded the PDF file that explains in full detail the SMS hack.

You can find it here : SMS hack.pdf

Now 4 caveats :

1) They tested it in lab environment, so not by sending real SMS's over a carrier (because that cost money :-) )

2) If some hacker really wants to exploit this he will need a lot of phonenumbers (and cash to send the SMS)

3) The hacker his identity will be quickly revealed as sending of SMS is not anonymous at all.

4) The only thing that the paper described is how to crash an iPhone (and Android GSM) not really taking over the thing. It will be a hard thing to do to really send a program over a carrier via SMS to actually take over an iPhone or an Android GSM 

iPhone user experience

For those developing for the iPhone and struggeling with the UI, I found here a cool article : http://www.smashingmagazine.com/2009/07/21/iphone-apps-design-mistakes-overblown-visuals/

Multitasking for the iPhone: something simple

We all known that the iPhone/iPod has a much slower CPU than a 'standard' Mac or PC. So this means that the user experience can be a bit sluggish.

So when we have for example a time consuming task it comes in handy to use some multi-tasking capabilities built in , in Cocoa Touch.

However ,  I found out for myself a simpler and less difficult method of giving the user a good experience and still doing a lengthy task.

Of course this method only applies to tasks which takes 2-4 seconds , for longer tasks you'll need NSThread.

The trick I do is that I use an UIActivityIndicatorView somewhere on the screen so when a user 'sees' the screen he in fact see the spinning gear.

In the mean time the implementation of the time consuming task I do in the viewDidAppear: method.

So what happens is that the user sees a UI with something moving (the gear) so he knows that the application is actually doing something, and if it only takes 2 to 4 seconds he will never get frustrated.

I found this much easier than going to the route of NSThread with all the difficulties of locking of shared variables etc.

IPhone application, iSignals, goes beta

Oke, it's finally here ! My application goes in beta phase 1.

The purpose of the application is to have a portfolio management system with buy and sell signals generated by a backend.

In the front end you choose the stocks you want to follow and then 3 times a day the backend , a signals engine, will do a technical analysis and send buy/sell signals (if any).

For more information please visit the website.

Screenshots :

Iphone and memory management

Ok, I was busy with some iPhone related developments. Now I realized that the actual memory size of an iPhone is 128 Mb RAM (memory you can use for application, not storage).

So this means that your application, usually can't take more than 50 to 60 Mb of RAM.

For myselfs I've put some guidelines to manage that limitation :

1. Never use more memory than needed for the view you are in, so release, release

2. Every viewcontroller should be released as soon it's pushed in a navigation controller

3. Use proxy's (lazy object creation)

4. use SQLite for caching (store your objects in a SQLLite database, this database is not using   RAM)

5. Prepare your application for the worsed and implement the didRecieveMemoryWarning method

Iphone & Cocoa Touch : the final frontier

I'm a bit offline lately because I'm busy with an exciting iPhone app. The goal is to have a business application for the iPhone/iPod which allows the customer to followup a portfolio of stock and getting buy and sell signals.

The generation of the signals is based on a very complicated technical analysis engine (it's in use for 7 years now and it has proven its value).

Now to get an idea how the application will lookalike I post here some screenshots. 

(ps: the website where you can find some more info is www.isignals.biz ).

The iPhone app will be released in a month or so (maybe after holiday season, so I have sometime to do beta testing).

NSDateFormatter in OSX 10.4.11

Today I found some strange behaviour in the NSDateFormatter :
If I execute the following code :

NSDateFormatter *timeFormatter = [[[NSDateFormatter alloc] init] autorelease];
[timeFormatter setDateStyle:NSDateFormatterNoStyle];
[timeFormatter setTimeStyle:NSDateFormatterMediumStyle];
NSDate *stringTime = [NSDate date];
NSString *formattedDateStringTime = [timeFormatter stringFromDate:stringTime];
[theTime setStringValue: formattedDateStringTime] ;

Then , instead of displaying the time, it shows me the date.

If I add the following line on top :

[NSDateFormatter setDefaultFormatterBehavior: NSDateFormatterBehavior10_4] ;

It gives me correctly the time.

is this something specific for the 10.4.11 ? Is this behavior also observed in the 10.5.X ?

Application badge in Dock

In Leopard (OSX v10.5 and higher) you have the capability to set an application badge like you can find on iMail for example :



Now for the poor people who still have a pre 10.5 version, or for the people who want more capabilities than what Leopard offers, I wrote an utility class based on the example code found in CocoaDev.

This utility class does not only gives the capability to draw the badge gives also the possibility to change the position of the badge :



Changing the font size of the badge:



Changing the color of the badge:



The code can be downloaded here.

NSNumberformatter: some examples

I was looking for some examples on the usage of the NSNumberformatter class. During this search I stumbled on the weblog of Sam Lam which provides some good examples.

I copy them here , as I use my own blog as a sort of FAQ or HOW-TO for myself.

-(NSString*) formatCurrencyValue:(double)value
{
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
[numberFormatter setFormatterBehavior:NSNumberFormatterBehavior10_4];
[numberFormatter setCurrencySymbol:@"$"];
[numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];
NSNumber *c = [NSNumber numberWithFloat:value];
return [numberFormatter stringFromNumber:c];
}

-(NSString*) formatPercentValue:(double)value
{
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
[numberFormatter setFormatterBehavior:NSNumberFormatterBehavior10_4];
[numberFormatter setPercentSymbol:@"%"];
[numberFormatter setNumberStyle: NSNumberFormatterPercentStyle];
[numberFormatter setDecimalSeparator:@"."];
[numberFormatter setGeneratesDecimalNumbers:TRUE];
[numberFormatter setMinimumFractionDigits:2];
[numberFormatter setRoundingMode: NSNumberFormatterRoundUp];
[numberFormatter setRoundingIncrement:[[NSNumber alloc]initWithDouble:0.05]];
NSNumber *c = [NSNumber numberWithFloat:value];
return [numberFormatter stringFromNumber:c];
}

-(double) formatDoubleFromCurrency:(NSString*)value
{
double ret ;
if(value)
{
ret = [value doubleValue];
if (ret == 0)
{
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
[numberFormatter setFormatterBehavior:NSNumberFormatterBehavior10_4];
[numberFormatter setCurrencySymbol:@"$"];
[numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];
NSNumber *c = [numberFormatter numberFromString:value];
ret = [c doubleValue];
}
return ret;
}
else
return 0.0;
}

NSButton: setting an image in a button

I always have trouble to know how to set an image in a button. The following code snippet does the job:

NSString* im1 = [[NSBundle mainBundle] pathForResource:@"arrow right" ofType:@"png"];
NSURL* url2 = [NSURL fileURLWithPath:im1];
NSImage *image = [[NSImage alloc] initWithContentsOfURL: url2];


[toggleButton setImage: image] ;

NSString *im2 = [[NSBundle mainBundle] pathForResource:@"arrow left" ofType:@"png"];
url2 = [NSURL fileURLWithPath:im2];
image = [[NSImage alloc] initWithContentsOfURL: url2];
[toggleButton setAlternateImage: image] ;

The code above set's 2 images in a toggle button .

Note that if you want transparant images , you need .png files and not jpeg's (GIF also works)

For telenet users: a telemeter application

Note: the application is updated ! It's now compatible with the new Telemeter service of Telenet (new version since July 2010)

Also available for the iPhone see following blog message .

This little application shows how much data you've downloaded/uploaded from your Telenet account.

(for the impatients : you can find the dmg here ) .

For those with an unlimited download (turbonet, fibernet) the screen will look like follows :

Installation is very easy :

1. Open the dmg file (download here) ;
2. drag and drop the folder 'Telemeter' wherever you want ;
3. Start the Telemeter v2 application.

The very first thing when you start the application a popup will come up to warn you that you need to set your telenet username/password . This can be done clicking the green user icon (on the first line on the right). And if all is oke you should see your usage.

In the screen you see also a button with an arrow (>) , if you click that you'll get the daily usage details.



The application will update the data from Telenet every 30 minutes.



Next enhancements :

1. Make a dashboard widget of it
2. Change the icon colour depending on the used bandwidth


Note: this is a first version , so please report any bugs to me (via blog or email). Comments are also welcome of course.

UPDATE :

I made the following enhancements :

1. you see in the tray bar (system menu bar) the percentage used :



2. In the dock , the icon will also show the percentage used



3. several small bugfixes

UPDATE 2: just found out that I have some memory leaks, will solve them ASAP.

UPDATE 3 : memory leaks solved + some minor improvements

UPDATE 4 : major enhancements

UPDATE 5 : Currently (5 May) telemeter service is down

UPDATE 6: We are 7th of May and the telemeter service is still down. And this in Internet age....

UPDATE 7: And now they've changed the protocol.... pff they really annoy me. I changed the XML processing , so it works again

UPDATE 8: Application rewritten because Telenet has changed completely the webservice

Drawing pie charts

Recently I was looking for a way to draw pie charts (you know, the one with an 'exploded' slice) on my Macbook.
Unfortunealy I didn't find much on the Internet, so I've started to experiment a bit myselfs.
The following code draws a pie chart with one slice exploded from the other :

The trick is that you draw an imaginary circle around the center, on this circle is the center of the 'exploded' slice.

#define PI 3.14159265358979323846

NSBezierPath *greenPath = [NSBezierPath bezierPath] ;

// set some line width

[greenPath setLineWidth: 2 ] ;

// move to the center so that we have a closed slice
// size_x and size_y are the height and width of the view

[greenPath moveToPoint: NSMakePoint( size_x/2, size_y/2 ) ] ;

// draw an arc (perc is a certain percentage ; something between 0 and 1
[greenPath appendBezierPathWithArcWithCenter:NSMakePoint( size_x/2, size_y/2) radius:50 startAngle:0 endAngle: 360 * perc ] ;

// close the slice , by drawing a line to the center
[greenPath lineToPoint: NSMakePoint(size_x/2, size_y/2) ] ;
[greenPath stroke] ;

[[NSColor greenColor] set] ;
// and fill it
[greenPath fill] ;

greenPath = [NSBezierPath bezierPath] ;
[[NSColor blackColor] set] ;
[greenPath setLineWidth: 2 ] ;

// draw the second slice, now exploded from the original center

// so to get it exploded I move (10,7) points from the original center
// but on the imaginary circle (thats why the cos and the sin)
// note mide_angle is the angle halve way from the arc, you can experiment with multiple
// angles, note also that the angle is in degrees
[greenPath moveToPoint: NSMakePoint(size_x/2 - 10 * cos ( PI * mid_angle / 180 ) , size_y/2 - 7 * sin ( PI * mid_angle / 180 )) ] ;

// and now draw the other slice
[greenPath appendBezierPathWithArcWithCenter:NSMakePoint( size_x/2 - 10 * cos ( PI * mid_angle / 180 ) , size_y/2 - 7 * sin ( PI * mid_angle / 180 )) radius:50 startAngle:360 * perc endAngle:360 ] ;

// close the slice
[greenPath lineToPoint: NSMakePoint( size_x/2 - 10 * cos ( PI * mid_angle / 180 ) , size_y/2 - 7 * sin ( PI * mid_angle / 180 ) ) ] ;
[greenPath stroke] ;
[[NSColor blueColor] set] ;

[greenPath fill] ;

The result of above code is here :



Next time I'll add some code to add text to the slices

SOAP: webservices in Objective-C/Cocoa

Lately I was experimenting a bit with webservices, and this for a good reason : my ISP was trying to bill me extra gigabytes while I was sure I didn't used my full quota.
My ISP (telenet in Belgium) is using a nice web page where you can check your quota but they also offer a webservice which gives full statistics on the bandwidth usage.

So I decided to build a Dashboard widget (in Objective-C, not in Javascript) to see the statistics from my desktop.

Now I didn't do any SOAP/Webservices since a long time , and I never did in Objective-C (my experience goes back to VisualWave, Smalltalk and Java).
So I did some experimentation with the samples from Apple itself :


// SOAP request settings

NSURL *url = [NSURL URLWithString:@"https://telemeter4tools.services.telenet.be/TelemeterService"];

// the name of the webservice
NSString *method = @"getUsage";

// the namespace
NSString *namespace = @"https://telemeter4tools.services.telenet.be/";


// SOAP request params

NSDictionary *params = [NSDictionary dictionaryWithObjectsAndKeys:@"**username**",
@"string",
@"**password**",
@"string0",
nil];

// the parameters itself are in a dictionary, a dictionary is an unordered collection
// so to define the order in which the parameters must be sent
// you need to fill an array with the NAMES of the parameters

NSArray *paramOrder = [NSArray arrayWithObjects:@"string", @"string0"];


// set SOAP request http headers -- some SOAP server impls require even empty SOAPAction headers


NSDictionary *reqHeaders = [NSDictionary dictionaryWithObject:@"" forKey:@"SOAPAction"];


// create SOAP request

WSMethodInvocationRef soapReq = createsoapReq(url, method, namespace, params, paramOrder, reqHeaders);


// invoke SOAP request

NSDictionary *result = (NSDictionary *)WSMethodInvocationInvoke(soapReq);


// get HTTP response from SOAP request so we can see response HTTP status code

CFHTTPMessageRef res = (CFHTTPMessageRef)[result objectForKey:(id)kWSHTTPResponseMessage];

The response of a SOAP requests is a dictionary, and it looks like this :

result: {
"/Result" = { ... <<>> };

"/WSDebugInBody" = "...etc;

"/WSDebugInHeaders" = {
"Accept-Ranges" = none;
Connection = close;
"Content-Length" = 4840;
"Content-Type" = "text/xml; charset=utf-8";
Date = "Wed, 08 Apr 2009 06:44:46 GMT";
"Set-Cookie" = "JSESSIONID=JcHpkYZkhhn2pzp1LY7fvGQ1WXkb24WKDY2LBnf0JpLYyhsk23Q1!-344393110; path=/, st8id=c42908deb564326d86d900fd90459934.01.af0318b08a904d84c5824b76a132ab93; domain=.services.telenet.be; path=/, st8id_wat_%2Eservices%2Etelenet%2Ebe_%2F=SlNFU1NJT05JRA__?1607678332ef30e3c1b9fc43cda54cb6; domain=.services.telenet.be; path=/";
};
"/WSDebugOutBody" = <<>>";
"/WSDebugOutHeaders" = {
"Content-Type" = "text/xml";
Host = "telemeter4tools.services.telenet.be";
Soapaction = "";
"User-Agent" = "Mac OS X; WebServicesCore.framework (1.1.0)";
};
"/kWSHTTPResponseMessage" = <<>>
}

Note that I stripped a bit the contents of the dictionary.

Now to get the result of the webservice you must get the proper data from the dictionary (in this case this is data = [result objectForKey: @"result"].

This will return a string value, so a next step is to create an XMLDocument of it :

NSXMLDocument *document ;
NSError *error ;
NSXMLNode *node ;
document = (NSXMLDocument *) [NSXMLDocument document] ;
[document initWithXMLString: data options: NSXMLDocumentTidyXML error: &error ] ;

And once it is in a XMLDocument , you can use XPath expressions to query the document :

myArray = [document objectsForXQuery: @"/ns1:telemeter[1]/ns1:usage-info[1]/ns1:data[1]/@timestamp" error: &error ] ;

Now this looks to me a very complex process to get something from a webservice.
I'm now looking at WSDL and the utility WSMakeStubs to see if I can simplify this.
The nicest thing would be that we have something like JAXB to 'objectify' webservice responses

Generating mazes

In an attempt to create a video game, I came across the problem of creating a maze (see figure).

Like in most games you need to go from point A to point B via a number of corridors.

Now my problem was to find a good algorithm to create mazes, and after some research on the Internet I found several of them (see wikipedia for an explanantion) .

My first attempt was to use the simpelst algorithm , being the Depth-first search algorithm.

The algorithm goes as follows :

1, Initialize a grid full of walls
2, Start at a particular cell and call it the "exit."
3, Mark the current cell as visited, and get a list of its neighbors. For each neighbor, starting with a randomly selected neighbor:
If that neighbor hasn't been visited, remove the wall between this cell and that neighbor, and then recurse with that neighbor as the current cell.

So to implement this , I'll developed a recursive method (uses more memory and in a later state I can use a stack to avoid recursion).

In objective-C the implementation looks like this (very premature source code of the maze creation program (with animated GUI) can be downloaded here) :

step 1 : Initialize a grid

for ( x = 0 ; x <>
{

for ( y = 0 ; y <>
{
cell = [MazeCell fullCell] ;
[cell x: x ] ;
[cell y: y] ;
[maze atX: x atY: y put: cell]
}
}

A MazeCell is an object that contains four 'wall's' (a wall is here a BOOL that is true or false dending on the fact if the wall is their or not.

Step 2 : the actual generation of the maze

- (void) walkCells: (MazeCell *) aCell
{
int x ;
int y ;

NSMutableSet *neighbors ;
MazeCell *cell1 ;
NSEnumerator *enumerator ;

[aCell visited: YES] ;


x = [aCell x];
y = [aCell y];

neighbors = [NSMutableSet set] ;
if ( x > 0 ) [neighbors addObject: [maze atX: (x-1) atY: y ]] ;

if ( y > 0 ) [neighbors addObject: [maze atX: (x) atY: (y-1) ]] ;

if ( x <>

if ( y <>


enumerator = [ neighbors objectEnumerator] ;

while ( (cell1 = [enumerator nextObject] ) != nil )
{

if ( [cell1 visited] == NO )
{
if ( [cell1 x] == (x-1) )
{
[cell1 rightWall: NO] ;
[aCell leftWall: NO] ;
}

if ( [cell1 x] == (x+1) )
{
[aCell rightWall: NO] ;
[cell1 leftWall: NO] ;
}
if ( [cell1 y] == (y-1) )
{
[cell1 upWall: NO] ;
[aCell downWall: NO] ;
}

if ( [cell1 y] == (y+1) )
{
[aCell upWall: NO] ;
[cell1 downWall: NO] ;
}


[self walkCells: cell1] ;

}
}


}


A little explanation is needed here , in the beginning of the method you see that I build a list of the neighbours of cell.
Now it is important that the list is build in a random order, so therefor I use a NSMutableSet as datastructure. The position in the set depends on the hash that is generated for the object that is inserted.
In my case these objects are of type MazeCell.
So the trick to get them in random order was to implement the hash method on MazeCell and this hash method just returns a random number.

Drawing in views : background in custom views

Sometimes it could be usefull to have a background image in view instead of a color. For example if we want to have a texture in a view.

In the following example I draw a 'wall' as the background of a view :


The texture I want to apply to my view is the following :



Now its very easy to define this as a texture, you just create a new color , and not just a color but a colorpattern :

Step 1 : find the texture in my resources

NSString* fileName = [[NSBundle mainBundle] pathForResource: @"wall" ofType:@"jpg"];

NSURL* url = [NSURL fileURLWithPath: fileName];

Step 2 : upload the image from the URL
background = [[NSImage alloc] initWithContentsOfURL: url];

Step 3 : define a new color

color = [NSColor colorWithPatternImage:background ] ;
[color set] ;

Step 4 : fill the background
NSRectFill( [self bounds] ) ;

Note that you need to implement this code also in the drawRect: method, to ensure that each time your view get's redrawn that the background is drawn also.

Saving data to files

In some circumstances you want to save data to disk, in some cases you will use a database for that. But if you need , for example, to save the highscores of a game then a database will be a bit overkill.

In that case you can use core data, so lets look at a small example of how this works.
(Consider you already have an application in which you want to include a mechanism to save data).

Step 1 : add a data model

In XCode , you open the file assistant (via File>New File) and select 'data model'



Once you defined the data model , you'll get something like this :


I already added an Entity called HighScores , you add the entity via the menuitem (Design>Data Model>Add Entity).
I a same fashion you can add attributes to the entity (Desgin>Data Model>Add Attribute).
In my case I just defined 3 attributes : name,rank and score.

So this is all their is to define a datamodel (of course this is a very simple datamodel).
Now we need some coding to save information to the datamodel (and to file).

Step 2 : datastore identification

First we need to define in which file we will store the information :

NSError *error ; // this is for error reporting ; see later

NSURL *url = [NSURL fileURLWithPath:@"/Users/Herman/file.xml"];

Step 3 : define the datamodel we will use (= the datamodel in the project)

NSArray *bundles = [NSArray arrayWithObject:[NSBundle mainBundle]];

NSManagedObjectModel *model = [NSManagedObjectModel mergedModelFromBundles:bundles];

Step 4 : associate the datamodel with the datastore (= the file) via a coordinator

NSPersistentStoreCoordinator *coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model];
[coordinator addPersistentStoreWithType:NSXMLStoreType configuration:nil URL:url options: nil error:&error];

Step 5 : Get the context

context = [[NSManagedObjectContext alloc] init];
[context setPersistentStoreCoordinator: coordinator];

Step 6 : Save some data to the file

NSManagedObject *score = [NSEntityDescription insertNewObjectForEntityForName:@"HighScores" inManagedObjectContext:context];
[score setValue:@"bla bla" forKey:@"name"] ;
[score setValue:[NSNumber numberWithInt: 100] forKey:@"score"] ;
[score setValue:[NSNumber numberWithInt: 1] forKey:@"rank"] ;
[context save: &error ] ;

Et voila, we have a simple way of saving data to a file.
In a next message I'll discuss how to retrieve data and do some more complex queries on files.

Drawing in views: rectangles

When you want to draw a rectangle (or fill one), you basically have 2 choices :

1. You want absolute performance and less accuracy then you can use the functions defined in NSGraphics.h (NSFrameRect, NSRectFill etc).

2. You need absolute accuracy and don't care so much about performance then you can use the methods defined in NSBezierPath.

(the reason why the NSBezierPath is less performant is explained in this article ).

The following code extract is from a drawRect: method of a custom view :
It shows the different methods of drawing and filling a rectangle.

NSBezierPath *aPath ;

NSBezierPath *myPath ;

// reset the bounds
[self setBounds: NSMakeRect(0,0,1000,1000)] ;

//---------------------------------
//1. filling rectangles
//---------------------------------
[[NSColor grayColor] set] ;

// convenience method for filling a rectangle
// less precise , but good performance
// C functions defined in NSGraphics.h
NSRectFill( rect ) ;

// filling a rectangle using a class method of NSBezierPath
[NSBezierPath fillRect: NSMakeRect( 0,500,75,75) ] ;


// filling a rectangle using an instance method of NSBezierPath
// this usefull if you want to do additional drawing in that path
myPath = [NSBezierPath new] ;
[myPath appendBezierPathWithRect: NSMakeRect ( 600,0,80,80) ] ;
[myPath fill] ;


// -----------------------------
//2. drawing a rectangle
//------------------------------

[[NSColor redColor] set ] ;

// drawing a rectangle using a class method of NSBezierPath
[NSBezierPath strokeRect: NSMakeRect( 500,500,80,80 ) ] ;

// drawing a rectangle using an instance method of NSBezierPath
// this usefull if you want to do additional drawing in that path
[[NSColor greenColor] set ] ;
aPath = [NSBezierPath bezierPathWithRect: NSMakeRect( 450,600,100,100) ] ;
[aPath stroke] ;


// absolute performance, less precise
// C functions defined in NSGraphics.h
NSFrameRectWithWidth( NSMakeRect( 700,700,70,70), 10 ) ;

NSFrameRect( NSMakeRect( 0,700,80,80 ) );