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KVO, which stands for Key-Value Observing, is one of the techniques for observing the program state changes available in Objective-C and Swift. The concept is simple: when we have an object with some instance variables, KVO allows other objects to establish surveillance on changes for any of those instance variables.
Key-Value Observing, KVO for short, is an important concept of the Cocoa API. It allows objects to be notified when the state of another object changes. That sounds very useful.
According to Apple: Key-value coding is a mechanism for accessing an object's properties indirectly, using strings to identify properties, rather than through invocation of an accessor method or accessing them directly through instance variables.
I'm having the worst time getting key value observing working in with a UITextView's text property. I can successfully add the observer, I can even remove that same observer. I have a tableview with several cells - some have UITextFields, some have UISegmentSelectors and one has a UITextView. ALL the rest of the fields are successfully observed by my core data object (subclass of NSMangedObject) EXCEPT for the UITextView. I can post code if needed.
Posted Code:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *UserProfileCellIdentifier = @"UserProfileCellIdentifier";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:
UserProfileCellIdentifier];
if (cell == nil)
{
[_tableCellsNib instantiateWithOwner:self options:nil];
switch (indexPath.row)
{
// UserName Row
case UserNameRowIndex:
_textFieldCell.cellLabel.text = NSLocalizedString(@"UserNameLabel", @"User Profile TableView");
_textFieldCell.cellType = CellTypeNormal;
_textFieldCell.boundProperty = @"UserName";
_textFieldCell.tag = indexPath.row;
_textFieldCell.errorHandler = self;
_textFieldCell.boundControl = _textFieldCell.cellTextField;
[_textFieldCell addObserver:[MBUtilities getAppDelegate].userProfile forKeyPath:@"cellTextField.text" options:NSKeyValueObservingOptionNew context:NULL];
cell = _textFieldCell;
self.textFieldCell = nil;
break;
case PasswordRowIndex:
_textFieldCell.cellLabel.text = NSLocalizedString(@"PasswordLabel", @"User Profile TableView");
_textFieldCell.cellType = CellTypeNormal;
_textFieldCell.cellTextField.secureTextEntry = YES;
_textFieldCell.boundProperty = @"Password";
_textFieldCell.tag = indexPath.row;
_textFieldCell.errorHandler = self;
_textFieldCell.boundControl = _textFieldCell.cellTextField;
[_textFieldCell addObserver:[MBUtilities getAppDelegate].userProfile forKeyPath:@"cellTextField.text" options:NSKeyValueObservingOptionNew context:NULL];
cell = _textFieldCell;
self.textFieldCell = nil;
break;
case PasswordConfirmRowIndex:
_textFieldCell.cellLabel.text = NSLocalizedString(@"PasswordConfirmLabel", @"User Profile TableView");
_textFieldCell.cellType = CellTypeNormal;
_textFieldCell.cellTextField.secureTextEntry = YES;
_textFieldCell.tag = indexPath.row;
cell = _textFieldCell;
self.textFieldCell = nil;
break;
case FirstNameRowIndex:
_textFieldCell.cellLabel.text = NSLocalizedString(@"FirstNameLabel", @"User Profile TableView");
_textFieldCell.cellType = CellTypeNormal;
_textFieldCell.boundProperty = @"FirstName";
_textFieldCell.tag = indexPath.row;
_textFieldCell.errorHandler = self;
_textFieldCell.boundControl = _textFieldCell.cellTextField;
[_textFieldCell addObserver:[MBUtilities getAppDelegate].userProfile forKeyPath:@"cellTextField.text" options:NSKeyValueObservingOptionNew context:NULL];
cell = _textFieldCell;
self.textFieldCell = nil;
break;
case LastNameRowIndex:
_textFieldCell.cellLabel.text = NSLocalizedString(@"LastNameLabel", @"User Profile TableView");
_textFieldCell.cellType = CellTypeNormal;
_textFieldCell.boundProperty = @"LastName";
_textFieldCell.tag = indexPath.row;
_textFieldCell.errorHandler = self;
_textFieldCell.boundControl = _textFieldCell.cellTextField;
[_textFieldCell addObserver:[MBUtilities getAppDelegate].userProfile forKeyPath:@"cellTextField.text" options:NSKeyValueObservingOptionNew context:NULL];
cell = _textFieldCell;
self.textFieldCell = nil;
break;
case DateOfBirthRowIndex:
_textFieldCell.cellLabel.text = NSLocalizedString(@"BirthDateLabel", @"User Profile TableView");
_textFieldCell.cellType = CellTypeDatePicker;
_textFieldCell.boundProperty = @"DateOfBirth";
_textFieldCell.tag = indexPath.row;
_textFieldCell.errorHandler = self;
_textFieldCell.boundControl = _textFieldCell.cellTextField;
[_textFieldCell addObserver:[MBUtilities getAppDelegate].userProfile forKeyPath:@"cellTextField.text" options:NSKeyValueObservingOptionNew context:NULL];
cell = _textFieldCell;
self.textFieldCell = nil;
break;
case GenderSelfRowIndex:
_genderSelectCell.cellLabel.text = NSLocalizedString(@"UserSexLabel", @"User Profile TableView");
_genderSelectCell.boundProperty = @"GenderSelf";
_genderSelectCell.errorHandler = self;
_genderSelectCell.boundControl = _genderSelectCell.cellGenderSegment;
_genderSelectCell.tag = indexPath.row;
[_genderSelectCell addObserver:[MBUtilities getAppDelegate].userProfile forKeyPath:@"cellGenderSegment.selectedSegmentIndex" options:NSKeyValueObservingOptionNew context:NULL];
cell = _genderSelectCell;
self.genderSelectCell = nil;
break;
case GenderInterestedInRowIndex:
_genderSelectCell.cellLabel.text = NSLocalizedString(@"UserInterestedInLabel", @"User Profile TableView");
_genderSelectCell.boundProperty = @"GenderInterestedIn";
_genderSelectCell.errorHandler = self;
_genderSelectCell.boundControl = _genderSelectCell.cellGenderSegment;
_genderSelectCell.tag = indexPath.row;
[_genderSelectCell addObserver:[MBUtilities getAppDelegate].userProfile forKeyPath:@"cellGenderSegment.selectedSegmentIndex" options:NSKeyValueObservingOptionNew context:NULL];
cell = _genderSelectCell;
self.genderSelectCell = nil;
break;
case IntroductionRowIndex:
_textViewCell.cellLabel.text = NSLocalizedString(@"IntroductionLabel", @"User Profile TableView");
_textViewCell.boundControl = _textViewCell.cellTextView;
_textViewCell.errorHandler = self;
_textViewCell.boundProperty = @"Introduction";
_textViewCell.tag = indexPath.row;
[_textViewCell addObserver:[MBUtilities getAppDelegate].userProfile forKeyPath:@"cellTextView.text" options:NSKeyValueObservingOptionNew context:NULL];
cell = _textViewCell;
self.textViewCell = nil;
break;
};
}
return cell;
}
Discussion: Each cell is loaded from an external NIB file then initialized with different properties. All of the cells work with key value observing except for the very last one with the UITextView. Let me know if any additional info is needed.
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