For starters, implement it by adding the code shown in the next fragment:. In this snippet, we have the someValue variable that belongs to the method, and inside the myOperation block we try to change its value. Not even Xcode allows us to proceed until we manage this situation. In our example, our problem can be solved really easy, simply by making this line:. Many developers when they think of blocks, they actually and instantly think of completion handlers and they have a good reason for that, as blocks are used as completion handlers extensively by many framework methods.
But, before we go any further, what a completion handler is? A completion handler is the way technique for implementing callback functionality using blocks. A completion handler is nothing more than a simple block declaration passed as a parameter to a method that needs to make a callback at a later time.
When this time comes, the block is simply called. Besides that, the block definition takes place upon the method invocation, so any actions needed to be taken after a callback has been made can be locally implemented. One important notice before moving forward is that you should never forget that the completion handler should always be the last parameter in a method.
A method can have as many arguments as you want, but always have the completion handler as the last argument in the parameters list. Here is the declaration pattern of a method which contains a completion handler to make callbacks:. When showing a modal view controller, if you want to handle something after the view controller has been presented:. To keep it simple, we will implement a method which adds two integers, and instead of returning the result to the caller, it will send it back using the completion handler.
On the ViewController. As you see, we declare a method with three parameters. The first two are the numbers we want to add, and the last one is the completion handler. Note that you can set any name you want to the completion handler parameter. As you see, the block declaration on the completion handler contains just one parameter, the result we want to send back to the caller of the method.
Anything hard? We just add the two numbers, and we call the completion handler, passing the expected parameter, the result of the addition. When invoking the method, we pass the values 5 and 7 as parameters, and we define the completion handler block here.
Anyway, as this just an example, there is no need to do any special handling, therefore we just log the result of the addition. The last three pieces of code summarize the way you can deal with completion handlers of your own. Xcode makes things even easier to all of us, by suggesting what to type in the editor just when you start writing the method invocation, as follows:.
So, what we are going to do here, is to create a custom class for making a UIActionSheet object to be used with a completion handler, without any delegate methods at all. After having the custom class ready, you can take it as a reusable piece of code for your projects.
Trust me, simply using a completion handler to manage the user selection on a action sheet is much cooler and easy to use than one can imagine. Click Next to move to the next step. In the Subclass of field, make sure to set the NSObject value. Click once again on the Next button, and in the last window that appears click on the Create button to get finished. You should now see on the Project Navigator these two new files:. In our custom class we will make all the usual required action sheet handling.
So, open the CustomActionSheet. Next, declare the following custom init method, so our class can be initialized using the same parameters that an action sheet object gets upon initialization:. Normally, when invoking the showInView: method, the action sheet appears to the view we set as its parameter. In our case, we declare this method using the same name in order to make it look familiar, but the new and great addition is the completion handler at the end.
As you see, we specify two parameters in the block: A NSString and a NSInteger parameter, that represent the title and the index of the tapped button respectively. The first one is an action sheet object that will be used throughout our class. The second one will keep the completion handler that will be provided to the previous method as a parameter, as we will need to call it after the method execution has been finished.
Also, you can observe that this block has been declared like any other property. What we have to do is to initialize our actionSheet object that we declared right before. Nonetheless, take a look at it and then we will discuss it. The action sheet object is initialized, but notice that we do not provide it with the cancel button and the other button titles objects; instead we pass nil values.
Also, the cancel button should be the last button added, so we make it nil as well. All button titles are given in the init method in the form of a nil-terminated string, which is actually a list of strings, and that means that the number of values existing in it may vary. Our job is to get all string values one by one, until we find the nil value, and that exactly happens here. Anyway, the important is that we can add all button titles one by one, using this command:.
After all buttons have been added, we should not forget about the cancel button. Therefore, with these couple of lines:. You may wonder what would happen if we had simply provided the otherButtonTitles parameter to the action sheet initialization method. Well, you may try it out by yourself and see the results, but for those who just want to listen to the answer, the action sheet would simply show only the first button title, ignoring the rest, and considering the otherButtonTitles as a single object, not a list of arguments.
The only noteworthy thing here is that in our class member variable completionHandler we assign the handler block object passed as a parameter to the method. That way, we keep a reference to it even after the method has exited. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service informit.
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Discover some basic block how-tos and pick up a few tricks for your projects. Like this article? We recommend. We recommend Like this article?
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Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog. Does ES6 make JavaScript frameworks obsolete? Blocks make this much easier, however, because you can define the callback behavior at the time you initiate the task, like this:.
This example calls a method to display the progress indicator, then creates the task and tells it to start. The callback block specifies the code to be executed once the task completes; in this case, it simply calls a method to hide the progress indicator. Note that this callback block captures self in order to be able to call the hideProgressIndicator method when invoked. The declaration for the beginTaskWithCallbackBlock: method shown in this example would look like this:. The implementation of the method can invoke the block in the usual way:.
Method parameters that expect a block with one or more arguments are specified in the same way as with a block variable:.
If the method also needs other non-block arguments, the block should come last:. If you need to define more than one block with the same signature, you might like to define your own type for that signature. As an example, you can define a type for a simple block with no arguments or return value, like this:. Custom type definitions are particularly useful when dealing with blocks that return blocks or take other blocks as arguments. Consider the following example:. The complexBlock variable refers to a block that takes another block as an argument aBlock and returns yet another block.
The compiler will warn you for a simple example like this, but a more complex example might involve multiple strong references between objects to create the cycle, making it more difficult to diagnose. If that object is deallocated before the block is called, the weakSelf pointer will simply be set to nil. In addition to general completion handlers, many Cocoa and Cocoa Touch API use blocks to simplify common tasks, such as collection enumeration.
The NSArray class, for example, offers three block-based methods, including:. This method takes a single argument, which is a block to be invoked once for each item in the array:. The block itself takes three arguments, the first two of which refer to the current object and its index in the array.
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