On Jan 20, 2010, at 1:40 PM, Steven Degutis wrote:
> Recently I had the same issue you were having, sort of. And I came up with a
> solution I really liked.
>
> When I was playing with Distributed Objects, I fell in love with the abstract
> simplicity. However, it blocks and that's bad. It's even worse when the
> server stops responding, because you could potentially have a 60 second
> timeout before the single method will return. It's a potential disaster.
>
> So, I wrote an elegant compromise. Code is still written inline, no callbacks
> or delegate messages needed. But, it requires Blocks (and thus 10.6) to work.
Steven - this is a really interesting approach. I can see how this basically
achieves the same thing as all the callback methods, but does allow the code to
be written somewhat inline. Unfortunately I need to at least support OS X 10.5
at this point. I definitely need to read-up on blocks b/c I can see how they
can be used to work around some tricky design problems.
>
> Essentially, I wrote some code on top of AsyncSocket (which is a brilliant
> framework by the way) that allows me to wrap up ObjC messages as NSData, send
> it across the server, and unpack it on the other side. The other side then
> responds to the ObjC message as if it was called right inside the
> application. (All this is thanks to NSInvocation's ability to introspect an
> ObjC message, by the way).
>
> The problem came when I had to return values. As long as the return value was
> void, this worked like a charm. But once I wanted to return an array of
> strings or a number, I had to define a method in the sender's protocol to
> receive such information. This is akin to your "thousands of delegate
> messages" you would have to implement, as you stated.
>
> So, using Blocks and NSInvocation and AsyncSocket, I ended up writing code
> that allows me to write code like this:
>
>
> // protocol.h
>
> @protocol ServerProtocol
>
> - (NSNumber*) calculatePiAndKillTime:(NSNumber*)shouldKillTime;
>
> @end
>
>
> // client.m
>
> - (void) someMethod {
> id <ServerProtocol> server;
> NSNumber *sure = [NSNumber numberWithBool:YES];
> [[server calculatePiAndKillTime: sure]
> returnedValue:^(id value) {
> // this will be called later on at some point
> NSLog(@"pi = %@", value);
> }]
> }
>
>
> // server.m
>
> - (NSNumber*) calculatePiAndKillTime:(NSNumber*)shouldKillTime {
> if ([shouldKillTime boolValue])
> // synchronously watch some film
> [self goWatchTheNewStarTrekFilmFrom2009];
>
> return [NSNumber numberWithFloat: 3.14];
> }
>
>
>
> All methods sent to a destination's proxy are sent asynchronously. And, as
> you can see, the return value of the method -calculatePiAndKillTime: is not
> actually an NSNumber, but rather a proxy that waits for a response from the
> destination. When the destination responds to the source with a return value,
> the method -returnedValue: is called with the value.
>
> But that's only half of the coolness.
>
> The other half is that methods can simply return the value they want right
> inside the method, no hacks necessary or anything by the programmer. In this
> case, we just use this line of code: return [NSNumber numberWithFloat: 3.14];
> and then the NSNumber object is packaged up and sent back to the source
> through the proxy, all automagically.
>
> The main downfall of this is that every argument and return value must be an
> ObjC type, no scalars or structs or anything else will work with this system.
> (Mike Ash explains pretty well on this blog why trying to support those
> things can lead to some unfixable trickiness, which I just wanted to avoid
> altogether.)
>
> If you can't support 10.6, then, this won't work. But hopefully you can soon
> ;)
>
> Good luck.
>
> -Steven
>
>
>
> On Wed, Jan 20, 2010 at 11:39 AM, Carter R. Harrison <[email protected]>
> wrote:
> I need some folks experienced with cocoa and socket programming to weigh in
> for me on some design problems I've been having. I'm designing an
> application that acts as a client in a client-server model. The client
> communicates with the server over the network by issuing a request and then
> receiving a response. Requests can only be issued one at a time, meaning
> that a request cannot be sent until a response from any outstanding request
> is first received. My application works in such a way that the it could
> request a handle to an object on the server and then use that handle in
> subsequent requests to retrieve additional information about the object. I
> see two ways of modeling the application - I've tried both and I'm not
> particularly happy with either.
>
> The first is to send a request, and then have the socket block until a
> response is received. This benefit to this model is that it is so much
> easier to write the higher level application code. The issue with this model
> is that over a slow network connection it can take a considerable amount of
> time for the response to come back from the server and while that is
> happening my CPU usage is through the roof b/c the thread is blocking.
>
> The second way is to send a request and then let the NSInputStream call a
> delegate method when the response data is available. The response data is
> then pushed up through my protocol stack and finally up to the higher level
> application code. The benefit to this method is that CPU usage is minimal
> due to the fact that I'm no longer blocking, but the downside is that the
> higher level application code is so much more difficult to write because I
> have to write about a thousand methods to act as a callback for each request
> in a series of requests.
>
> I've provided an example of how I see each working below. My first question
> is, is there other ways to design an application around this client-server
> model that I'm not thinking about? My 2nd question is, if there aren't other
> ways, how can I adapt either method that I have outlined to make it work a
> little bit better?
>
> As an example let's say the server knows about the following objects:
>
> 1. VendingMachine
> - An object that represents a vending machine.
> - A vending machine contains Soft Drink objects.
> 2. SoftDrink
> - Has the following properties: drink name, price, number of calories.
>
> If I use the blocking model, I could write my code like this. The code is
> simple to write but I'm forced to wait for the server to respond with
> information on pretty much every line of code. If the vending machine had
> enough soft drinks it could take a long time to iterate over each one and
> have the server respond with the drink's name of each drink.
>
> -(void)printDrinkNames
> {
> VendingMachine *machine = [server fetchVendingMachine];
> NSArray *softDrinks = [machine getSoftDrinks];
> for (int i = 0 ; i < softDrinks.count ; i++)
> {
> NSString *drinkName = [[softDrinks objectAtIndex:i] name];
> NSLog(@"Found a drink named %@", drinkName);
> }
> }
>
> Likewise if I do the non-blocking approach I would have to have a method that
> gets called for each step in the process (see below). This model drives me
> crazy b/c the higher level application code is long, has tons of methods, and
> is just difficult to read and maintain. The example I have provided is
> simple enough to get the point across, but in reality some of the processes
> I'm trying to drive are much more complex and require numerous callback
> methods to pull off.
>
> -(void)printDrinkNames
> {
> [server fetchVendingMachineWithCallBackObject:self
> selector:@selector(didFetchVendingMachine:)
> }
>
> -(void)didFetchVendingMachine:(VendingMachine *)machine
> {
> [machine fetchSoftDrinksWithCallBackObject:self
> selector:@selector(didFetchSoftDrinks:)];
> }
>
> -(void)didFetchSoftDrinks:(NSArray *)drinks
> {
> for (int i = 0 ; i < drinks.count ; i++)
> {
> SoftDrink *drink = [drinks objectAtIndex:i];
> [drink fetchNameWithCallBackObject:self
> selector:@selector(didFetchDrinkName:)]
> }
> }
>
> -(void)didFetchDrinkName:(NSString *)name
> {
> NSLog(@"Drink name is %@", name);
> }
>
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>
> --
> Steven Degutis
> http://www.thoughtfultree.com/
> http://www.degutis.org/
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