What is kvo and kvc

The difference is that the first method expects a key which is a string value , while the second expects a keypath also a string value. So, what is a key and what a keypath? In our example, we had the firstname property, and that was exactly the key we used when we set its value using the KVC. For example, the someObject. On Xcode, begin by creating a new class. Next, go to the second step and set the NSObject value to the Subclass of field. In the Class textfield, specify the Children value as the name of the new class.

Get finished with the guide, and let Xcode to create the new class and add the couple new files to the project. Now, open the Children. It is time to see KVC in action now. Open the ViewController.

If we assign values to the name and age properties of the child1 object, and then show them on the debugger as follows…. If you want, either comment out or delete the previous lines in the viewDidLoad , just make sure to leave the child1 object initialisation intact. In the first couple of rows we set the desired values to both properties using the setValue:forKey: method. Pay attention to the fact that the age is a number, therefore it cannot be passed directly as an argument to that method.

Instead, we must convert it to a NSNumber object first. Next, we perform the exact opposite task. We extract the values out of the properties using the valueForKey: method, and we assign them to two local variables. Focusing on the age again, notice that we want to get an unsigned integer value, so we convert the returned data type from NSNumber to NSInteger. Finally, we display everything to the debugger.

The output in this case is similar to the previous one:. If you run the app once again then… Boom! The app crashes! This crash message is quite explanatory. Okay, no big deal as we can go back to our code and fix this error. With all the above, we have managed to see how to write KVC-styled code, and how to set and get values using keys. Also, we have seen and underlined what happens if a key name is mistyped. For starters, go to the Children. With this, we can represent in code the child of a child and so on.

Return to the ViewController. Now, add the next lines that initialize the related objects and assign their initial values:.

Nothing new here, we just initialize the child2 object of our custom class, we set the values of the name and the age properties, and we then initialize its child object. The point now is how we can set values to the properties of the child object using the KVC style. According to what we said in the introduction of this tutorial, we must use the setValue:forKeyPath: method to do so.

The key that we will provide is going to be a string with the dot syntax to point out the property of the object we want to change. In code:. Now, what if the child of the child has a child too? So, now you know how to deal with both keys and keypaths, and how to write KVC compliant code.

If you want, feel free to write more examples and see what results you get back. Here we will see what actions should be taken in order to be able to track down changes on properties. First of all, let me introduce you as a list the steps needed to implement KVO:. The most important thing when we want to observe for changes of a property, is to make our class observe for these changes.

This is done more or less with as with the casual notifications NSNotifications , but using another method. This method is the addObserver:forKeyPath:options:context.

In order to become more specific, I will use our previous example where we used the Children class. In this class we had three properties, the name , the age and the child which was of type Children as well. Now that we have made our class able to observe for any changes in the above two properties, we must implement the observeValueForKeyPath:ofObject:change:context: method. Its implementation is mandatory, and it has one great disadvantage.

That is the fact that is called for every KVO change, and if you observe many properties then you must write a lot of if statements so as to take the proper actions for each property. However, this can be easily overlooked as the benefits of the KVO are greater than this limitation.

As you see, we do nothing special here for the sake of the demonstration. Active 5 months ago. Viewed 41k times. Improve this question. Sanoj 4, 4 4 gold badges 45 45 silver badges 70 70 bronze badges. Krishnan Krishnan This is The best for it.. Add a comment.

Active Oldest Votes. Improve this answer. Darren Darren Is this the same "darren" as above? LOL — Jacob Relkin. No, but it looks like we're both from the same town, answering the same questions, at the same time What are the odds? That is so weird. And with roughly the same amount of rep and badges.

Darren, what does this "addObserver" do?. Can you explain it? Show 4 more comments. Hi, could you please flesh this out that one final step--I do not understand the "power" of KVC yet. What is KVO in infusion pump?

An order indicating that the patency of an intubated vessel be maintained so that subsequent intravenous solutions or medicines can be administered. This is done using the lowest possible infusion rate with a microdrop set or a volume controller.

Joao Tafula Explainer. What is didSet in Swift? Property observers observe and respond to changes in a property's value. Property observers are called every time a property's value is set, even if the new value is the same as the property's current value. Dios Dorster Explainer. What is a value coding? If you fail to do this and then allow the observer to be deallocated, then future notifications to the observer may cause your application to crash.

For 1 , just send removeObserver:forKeyPath message to the sender in the -dealloc function of the observer. In -dealloc , just free non-object resources, or clean up tasks like removing observers. In -dealloc under ARC mode, you can not call [super dealloc] , as the compiller did it for you and this why there is an error if you call this manually.