In my first post I wrote about using NSOperation to grab an image of the current frame of a QuickTime movie while it was playing and save it out to the filesystem. Considering the excitement that is surrounding Core Animation these days, I thought it might be interesting to take that project and change it to instead grab the current frame and animate it across the screen using a Core Animation layer (CALayer). I had to employ a few tricks to get it to work, but the end result, while completely useless, is quite excellent.
A question that I have seen pop up a few times is how to build a Wizard in Cocoa. Having thought about that question a bit I realized that a better answer to it is — how to build a wizard in Cocoa using Core Animation.
The basic concept behind this project is to present a window to the user that will walk them through a list of options. To accomplish this task, I created a number of NSViews that will be presented to the user in order so that they can make the decisions needed. To make this a little easier, I have extended NSView to create MSZLinkedView. The added functionality in this subclass is that the view has a reference to the previous and next views in the wizard. These references are set in interface builder directly so that I do not have to worry about them in code.
This article is inaccurate.
The writer was smoking crack or something when he wrote it and has not been able to duplicate his tests since. This article is left here for historical reasons.
One of the things about Objective-C that I find extremely useful is the ability to resolve a method call at runtime. In addition this same functionality allows us to do some fairly creative things with callbacks, passing messages between threads, etc.
However there is a bit of a trick when it comes to passing primitives though some of these methods. For example, one method that I use quite frequently is performSelectorOnMainThread:withObject:waitUntilDone:. How exactly does one pass a BOOL or an int to this method?
Core Animation is really the buzz among Cocoa developers these days and we are no different from the rest. Learning to do simple fades and frame movement is trivial, but figuring out the more complex effects is quite a challenge. We decided to venture out and try to create some really concise examples of effects you might find used in Leopard. And while these effects may not be done with Core Animation where they are found in the OS, we’ve set out to duplicate them in Core Animation regardless. Our first challenge? Shaking a login window.
Forget Mandelbrot sets (Apple coding headstarts) and Expression Trees. NSOperation is really not that hard.
In his post, Marcus introduced how to use NSOperation to greatly simplify multi-threading tasks in your application. I am now going to provide a step-by-step walk-through sample application that uses NSOperation, NSOperationQueue, and QTKit.
While looking around the Internet, I noticed that the only examples of using NSOperation available were related to scientific applications. I wanted something that I could relate to a little better and since I’ve been working with QTKit a lot lately, I figured it would be a good framework to build from. This application simply grabs images of a movie while it is playing back and saves them out to a file. It’s pretty simple, but it shows how to do something fairly practical.
This post is in response to a few queries that I have received regarding my last post showing an NSOperation example. One of the questions raised that I will focus on is my -(void)dealloc method in the NSOperation subclass. The questions boiled down to:
Why are you using releases at all. Garbage collection is the future!
You should be just doing [self setVar:nil] instead of that [var release], var = nil; crap.
Threading is hard in any language. And what is worse, when it goes wrong, it usually goes wrong in a very bad way. Because of this, programmers either avoid threading completely (and refer to it as the spawn of the devil) or spend a LOT of time making sure that everything is perfect.
Fortunately, Apple has made a lot of progress in OS X 10.5 Leopard. NSThread itself has received a number of very useful new methods that make threading easier. In addition, they have introduced two new objects: NSOperation and NSOperationQueue.