I like the smell of Swift in the morning…

How to fade in a UITableView section index when the user scrolls

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When you have a UITableView with a lot of sections a section index can be very useful to jump quickly between sections. A good example for this is you iPhone’s address book.

But sometimes you want to hide the section index until the user scrolls the table view. The problem is that you can neither hide or show the section index directly nor officially access the UIView that holds the section index.

However there is a little trick to fade the section index in and out. UITableView has two methods that allow you to set the background and the text color of the section index. When you change the alpha of those two colors according to the UITableView’s contentOffset you can fade the section index when the user scrolls:

To initially hide the section index you have to set those two colors to UIColor.clearColor() in viewDidLoad:

UIScrollView and Auto Layout.

Posted: | Author: | Filed under: iOS, Swift | Tags: , , | 8 Comments »

Some people still seem to struggle when it comes to using Apple’s Auto Layout in a UIScrollView. There are a lot of questions on StackOverflow like “Why is my UIScrollView not scrolling when using AutoLayout?”

So here is a short explanation on how to use Auto Layout with a UIScrollView that should scroll vertically:

There are just a few things you have to take care of:

1. The topmost subview must have a top constraint with the UIScrollView
2. All other subviews must have a top constraint with the bottom constraint of the subview above them
3. The bottommost subview must have a bottom constraint with the UIScrollView

To ensure that the UIScrollView only scrolls vertically you have to make sure that its subviews don’t become wider than the UIScrollView.

Do not rely on left and right constraints to define the width of a subview. If for example you have a UILabel that has a lot of text and should break into several lines, it just won’t, even if you set its numberOfLines property to 0. That’s because the UIScrollView will give it enough space by allowing horizontal scrolling. So if you just set a left and right constraint on the UILabel the UIScrollView will scroll horizontal and the label will be very wide and have only 1 line.

Instead you should define a left and a width constraint. Set the width constraint to the width of the UIScrollView and the UILabel will not become wider than the UIScrollView. It will wrap into multiple lines instead.

If you follow those steps you don’t have to set the UIScrollView’s contentSize property any more to make the UIScrollView scroll. Auto Layout will handle that for you.

To make it more clear, here is an image with the constraints that you have to set:


If you are using Masonry or SnapKit here is a code example on how to set the constraints programmatically:

How to pass an error pointer to a function in Swift

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With the introduction of throwable in Swift 2 more and more functions implement the new error handling. However there are still some functions that expect you to pass an error pointer. This is especially the case when you are using an Objective-C framework in your Swift project.

So for an example let’s look at AFNetworking’s AFHTTPRequestSerializer’s method requestWithMethod:URLString:parameters:error:

If you are trying to implement it like you would in Objective-C you are in for a surprise:

This will cause the following compiler error:

The fix for this is much more simple than the error message suggests. The thing is, that error can be nil if the method is successful. Because of that, error has to be an Optional:

This works fine now

Localizable.strings error: “The data couldn’t be read because it isn’t in the correct format”

Posted: | Author: | Filed under: Objective-C, Swift, Xcode | 4 Comments »

I added some new keys to my Localized.strings files:

Then I got this error from Xcode:

It took me a minute but when I found what caused the error I had to smile. I simply forgot the semicolons at the end of each line:

Those poor semicolons! Coding in Swift really makes you forget they even existed in the first place 😉

Use the Nil Coalescing Operator in Swift to assign default values to properties

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One of the new operators that comes with Swift is the Nil Coalescing Operator. In the documentation it is explained as: The nil coalescing operator (a ?? b) unwraps an optional a if it contains a value, or returns a default value b if a is nil.

This operator already exited in C but for Objective-C developers it is kind of new (more about that at the bottom of this post).

With this operator assigning values to a property with a fallback to a default value becomes really concise:

Instead of doing this:

You can simply do this:

Pretty cool, isn’t it?

Actually something like this was already possible with Objective C:

iOS8 Share extension – completionHandler for loadItemForTypeIdentifier: is never called

Posted: | Author: | Filed under: iOS, Swift | Tags: , | 5 Comments »

Creating a Share Extension in iOS8 is really easy:

  1. In your app choose File > New > Target
  2. Choose Application Extension > Share Extension
  3. Enter a name for your Share Extension and press Finish

That’s all! Xcode creates everything you need for you and puts it in it’s own folder. You’ll find a ShareViewController that is a subclass of SLComposeServiceViewController. That class’ UI looks a lot like the SLComposeViewController that you’ve probably worked with when using iOS’s Social Framework (to share your app’s content to Twitter or Facebook).

In ShareViewController Xcode creates some stubs for you where you have add the code to actually share the content to your service. In my case I want to build an extension for Safari that allows the user to share the page’s URL on my service. So when the user selects my share extension from Safari I need to retrieve the page’s URL to send it to my server.

When the user starts my Share Extension and presses the “Post” button the method didSelectPost is called on the ShareViewController.

Let’s have a look at the stub method that Xcode created:

So far so good. So I added the code to retrieve the page URL. Compared to the easiness of creating the whole extension, the simple task of retrieving the URL takes quite some effort:

That’s a lot of code for just an URL. And the worst thing: It’s not working! The completion handler of the loadItemForTypeIdentifier:options:completionHandler: is never called.

As it turns out calling completeRequestReturningItems:completionHandler: at the bottom of the method causes this problem. In Apple’s documentation it says that you are supposed to call this method to ‘Tell the host app to complete the app extension request with an array of result items’. Calling this methods dismisses the ShareViewController and deallocates it. So the NSItemProvider that contains the URL is also destroyed before it can access the URL.

So the solution is quite simple: The call to completeRequestReturningItems:completionHandler: has to be done AFTER retrieving the URL and sending it to the server. So it has to go into the completionHandler block:

And now the URL is retrieved and can be posted to my server.

In case you need to do this in Swift, here is how that would look like: