Observe changes of instance variable in Objective-c - objective-c

Question

I have a UITableView with a property(strong, nonatomic)NSMutableArray *currentContent, I also have a property(strong, nonatomic)NSMutableArray *cellHeights to keep track of the cell heights cos the user could expand or collapse each cell. The self.currentContent is set by another controller which load data from a web service, so it will change as the data load, I want to keep both of these variables in sync. As soon as currentContent is updated, I want to update cellHeights. How do I do that?
I tried:
- (void)setCurrentContent:(NSMutableArray *)currentContent{
_currentContent = currentContent;
self.cellHeights = [NSMutableArray arrayWithDefaultHeightsForCellCount:[currentContent count]];
}
But it's not working, cos it will only be set at the first time when I set currentContent, when it's empty. So self.cellHeights currently will stay empty. When there is finally value in self.currentContent, self.cellHeights was not updated.

Answer 1

I've done a similar thing before, with variable cell heights depending on the content from your web-service, and I'd advise that keeping an array of cell heights might not be the best idea.
What I did was to create a 'fake' cell in the viewDidLoad: method, that I use just to calculate cell heights.
Then I use the 'heightForRowAtIndexPath' method to specify how tall cell should be by populating the 'fake' cell with the data for the index path, then finding out how tall that cell is. For example:
#interface MyTableViewController()
#property (nonatomic, strong ) MyCustomTableViewCell *cellForTestingHeight;
#end
#implementation MyTableViewController
- (void)viewDidLoad {
[super viewDidLoad]
self.cellForTestingHeight = [[MyCustomTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault];
}
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
NSDictionary *myData = [self.data objectAtIndex:indexPath.row];
self.cellForTestingHeight.viewData = myData;
return self.cellForTestingHeight.height;
}
#end
This code assumes that you've created a class called MyCustomTableViewCell which has a method to set the viewData property on it, and that after setting that property you'll be able to tell how tall that cell will be by accessing a height property.

Answer 2

What you need to do is observe changes to the mutable array itself, not your property which references the array. This should be straightforward but unfortunately there is a twist...
OK, let's assume there is no twist and rather than an NSMutableArray your property is of type MyObservableClass. Then to setup the observing you would do something like this (all code is pseudo - i.e. typed into the answer):
- (void)setCurrentContent:(MyObservableClass *)currentContent
{
if(_currentContent)
[_currentContent removeObserver:self forKeyPath:#"myObservableProperty"];
_currentContent = currentContent;
[_currentContent addObserver:self forKeyPath:#"myObservableProperty" ...];
}
Now whenever the currentContent is changed your code stops observing the properties of its old value and starts observing properties of its new value.
Easy, but its not quite that simple for arrays...
The twist, though KVO will inform an observer of additions, deletions and changes to a collection the NSMutableArray class doesn't issue those notifications. You need to use a proxy for the array, and such a proxy is provided by the frameworks - see for example NSArrayController and mutableArrayValueForKey:. However the second part of the twist is that the changes to the array must be done through this proxy - you can't have your client call setCurrentContent and pass any NSMutableArray if you are going to observe it, your client needs to pass in the proxy... Whether you can use this easily in your design you'll have to figure out. You'll need to read Automatic Change Notification in this document and other Apple docs to sort this out.
This sounds more complicated that it needs to be, maybe somebody else will provide a more succinct way of doing this.