Category Archives: iOS

Suppose that your app has X amount cached objects. And suppose that there is a slight chance that.. since the backend doesn´t actually know what it is exactly that you have cached (and what you don´t have cached), it might send duplicates.

So… what do we do?. Shall we write a for routine checking dupes?. No way. There is a far more performant way to achieve this, and it requires less lines of code.

Steps…

• Implement your own ‘isEquals’ and ‘hash’ methods, in your model object. For instance…

[cc lang=”objc”]
– (BOOL)isEqual:(id)object
{
    SomeClass* secondObject = (SomeClass*)object;
    return ([secondObject isKindOfClass:[self class]] && 
            [[secondObject id] isEqual:[self id]]);
}

– (NSUInteger)hash 
{
   return [_id hash];
}
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• Simply instantiate a NSMutableSet and add the cached objects plus the objects retrieved from the backend. If the set already has an object for any given ID, and you attempt to insert a new one… but with the same id, it´ll just not work.

So… no more filtering things by hand. NextSTEP’s code will do the trick, from now on!

We have seen in a previous post the proper way to write a singleton, with the help of our good friend Grand Central Dispatch.

In this opportunity, i´d like to share a goodie i´ve learnt few days ago. I suppose many of you had problems with the following scenario. You have to perform a task, which is time consuming, and you need to place, onscreen, an Activity Indicator.

Although the Activity Indicator performs the animation on its own thread, you need to “spare some time” in the main thread for it to begin working. I mean… you can execute [indicator startAnimating] right away. But… if you don´t release the main thread, even for a short moment, surprise!. The spinner won´t work.

So… the common solution is to split that method into two parts, and execute ‘performSelector: withObject: afterDelay:’. That´s where GCD comes in.
We can do the exact same thing… but with blocks!!. That´s SO MUCH COOL…!

[cc lang=”objc”]
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW,
0.1f * NSEC_PER_SEC);

dispatch_after(popTime, dispatch_get_main_queue(), ^(void)
{
     // …  Your code
});
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Note: The delay, in this example, is of about 0.1 seconds.

Suppose you wanna insert a new row at the bottom of a table. The first thing you need to do is to calculate the content offset. Suppose you already know the height of the new cell, and it’s stored into the variable ‘newCellsHeight’. Then…

[cc lang=”objc”]
CGPoint contentOffset = CGPointZero;

contentOffset.y = _tableView.contentSize.height
+ newCellsHeight
– _tableView.frame.size.height
+ _tableView.contentInset.bottom;

Once you’ve calculated the contentOffset (considering the current contentInsets)… you should make sure it’s a non-negative value, to prevent quirks:

// Do we need to scroll down?
if(contentOffset.y > 0.0f)
{
[_tableView setContentOffset:contentOffset animated:YES];
}
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The last step would be to actually insert the rows. Please, consider that a delay should be applied, so the insert rows animations won’t break the scroll animation!

I’ve been unable to figure out how to do this, actually, within Xcode. But this can also be accomplished by means of this short script:

[cc lang=”bash”]find . “(” -name “*.m” -or -name “*.mm” -or -name “*.cpp” “)” -print | xargs wc -l[/cc]