Databases
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PO
 

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    <p>Yes — in the first case you create a block and post that off to the main queue. <code>dispatch_async</code> will <code>copy</code> the block. Copying a block causes it to retain every object it references. So <code>self</code> will be retained, as would any other object you mention in the block. The mechanism that keeps hold of the block is Grand Central Dispatch.</p> <p>In the second case you simply schedule a call to <code>doSomething</code> on the main thread. The mechanism that makes the call will be a run loop. <code>self</code> and at most one argument object will be retained.</p> <p>A difference between GCD and run loops is that run loops have modes. You queue things up to be executed only when the run loop is in a compatible mode. In practice you'll most commonly use that by queueing something up to occur in <code>NSDefaultRunLoopMode</code>. When the user is interacting with the user interface it switches into a mode other than the default. Therefore your work will not occur until the user stops interacting. E.g. on iOS the user is considered to be interacting while their finger is down. So you can arrange for work that you know is too heavy to allow, say, smooth scrolling to continue so that it doesn't happen until immediately after the user has finished the interaction.</p> <p>The selector you nominate will use <code>NSRunLoopCommonModes</code> which includes tracking (user interaction) mode but compare and contrast with <code>performSelectorOnMainThread:...modes:</code>.</p>
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    1. POIs there any different between these two method to call main thread in obj-c?
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    1. COAre you certain? I'm pretty sure `performSelectorInMainThread:…` does retain the receiver as well as the argument.
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    2. CO@Chuck I couldn't find a definitive answer in the documentation but simple testing suggests you're right, so it's a pretty safe guess the call goes through `NSRunLoop` `- performSelector:target:argument:order:modes:`
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Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

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