barcode maker vb.net Figure 9-7. Pointing Shark to a symbolicated binary in Objective-C

Generate Denso QR Bar Code in Objective-C Figure 9-7. Pointing Shark to a symbolicated binary

Figure 9-7. Pointing Shark to a symbolicated binary
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CHAPTER 9: Fake It Til You Make It: Tips and Tricks for Improving Interface Responsiveness
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Figure 9-8. Shark profile showing the data parsing and plist writing bottlenecks
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Even if you ship with 20 plists (which I probably would for this application), they ll almost definitely be stale once the application gets into users hands. You don t want your application to be this unresponsive the first time it is run. What can you do about this You have several options. At the moment, you re downloading and parsing all three months worth of data from Yahoo, because that s the easiest thing to do. You could figure out how much data you already have on disk and download only the missing data. You are also spending a fair amount of time converting NSNumbers to the NSDecimalNumbers you need for Core Plot. You could change Core Plot to accept NSNumbers, or you could change your storage to CoreData, which retrieves NSDecimalNumbers without conversion. The problem with these optimizations, some of which you may choose to do before shipping, is that they will all incur unpredictable amounts of overhead on the main thread. Thus, you would have to test a lot of use cases. It may also prove difficult to predict just how much data you ll need. If your user uses your application often enough to pull down small chunks (in this case, fewer days) of data, which is not guaranteed, you might do well to avoid downloading duplicate data. You might also want to allow the user to add stocks to plot, which would definitely require a lot of parsing the first time the stock data is downloaded; it also adds yet another stock to the queue on application launch. Perhaps you would do well to try to pull the processing off of the main thread so you can unblock the user interface once and for all, thus freeing yourself from all of these problems at once.
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Concurrency
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Cause it s gonna be the future soon. And I won't always be this way. When the things that make me weak and strange get engineered away. Lyrics for The Future Soon by Jonathan Coulton
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Wait a tick. Did I just suggest multithreading OK, threading is hard, but the engineers at Apple and elsewhere keep making it easier for us. We have all of these cores on our desktops because the hardware engineers keep slicing silicon, so concurrency keeps getting more and more important. Who
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CHAPTER 9: Fake It Til You Make It: Tips and Tricks for Improving Interface Responsiveness
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knows Perhaps one day we ll all be walking around with multicore processors in our phones. If so, you ll be ready to write software for them.
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NSOperation, NSOperationQueue, and Blocks
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NSOperationQueue and NSOperation remove much of the pain of multithreading. NSOperationQueue is a sort of thread pool that maintains an array of pending NSOperation objects that it schedules to run in a background thread based on a number of factors from system hardware to system state to the relative priority of a given NSOperation. You can even declare one NSOperation dependent on the completion of another. You normally subclass NSOperation to override one method, main, which is where you put the work you want on a background thread. It s called when the operation is run. The only things we as programmers have to be wary of in this situation are the usual data access caveats. Try not to mutate data at the same time you re reading it. There are tools for this, too. We can use the various permutations of performSelectorOnMainThread:, and @synchronized() directives are useful, too. Before you dig in, I highly recommend reading Apple s concurrency document at http://developer.apple.com/Cocoa/managingconcurrency.html. There is a helpful tool in other languages for this kind of problem called blocks. Blocks are another name for closures, with which you may have familiarity with from using Ruby, LISP, Python, Smalltalk, and others. They re like function pointers that take a (usually const) snapshot of their local stack variables so you can run them later with the information you shove in them now. They re little portable units of computation that carry their state around and are extraordinarily useful with concurrent operations. Because they have a snapshot of their state, they re easier to deal with in a concurrent environment. Useful though they may be, they don t officially exist yet. They re being added to Objective-C by the folks who are bringing us the open source Clang and LLVM projects. (See http://lists.cs.uiuc.edu/pipermail/cfe-dev/2008-August/002670.html and http://www.macresearch.org/cocoa-scientists-part-xxvii-getting-closureobjective-c.) There is no guarantee, though it seems likely, that Apple will bring them to the iPhone. These additions to the Objective-C language and runtime are free and open source, and they ve been implemented in GCC 4.2, so it is actually quite possible to backport them to the iPhone, so of course they have been. Plausible Blocks from Plausible Labs is available at http://code.google.com/p/plblocks/ and is, as of this writing, shipping its second beta of a gingerly patched version of the standard, stable GCC 4.2 compiler that ships with the OS X Leopard (10.5) and iPhone software development kits. I have found it to be very stable, and it works with both iPhone OS 3.0 and 2.2.1 targets. There is some example code for their use on the primary author s GitHub repository available at http://github.com/landonf/block_samples/tree/master. Next you ll install the Plausible Blocks compiler and add its static framework to your project so you can easily place your downloading, parsing, and saving code in a block to be executed by an NSOperation to be scheduled by an NSOperationQueue (in the house that Jack built). If or when Apple does add blocks support to the iPhone, switching from Plausible Blocks will be simple. You ll revert to Apple s compiler and remove the Plausible blocks framework
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