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- (void)viewWillAppear:(BOOL)animated { [super viewWillAppear:animated]; [self.tableView reloadData];
}
There’s one other change from the last time we created a detail view. It’s in the last method, tableView:didSelectRowAtIndexPath:. When we created the Disclosure Button view, we reused the same child controller every time and just changed its values. That’s relatively easy to do when you have a nib with outlets. When you’re using a table view to implement your detail view, the methods that fire the first time and the ones that fire subsequent times are different. Also, the table cells that are used to display and change the data are reused. The combination of these two details means your code can get very complex if you’re trying to make it behave exactly the same way every time and to make sure that you are able to keep track of all the changes. Therefore, it’s well worth the bit of additional overhead from allocating and releasing new controller objects to reduce the complexity of our controller class.
Let’s look at the detail controller, because that’s where the bulk of the new stuff is this time. This new controller is pushed onto the navigation stack when the user taps one of the rows in the BIDPresidentsViewController table to allow data entry for that president. Let’s implement the detail view now.
Creating the Detail View Controller
Please fasten your seatbelts, ladies and gentlemen. We’re expecting a little turbulence ahead. Air sickness bags are located in the seat pocket in front of you.
This next controller is just a little on the gnarly side, but we’ll get through it safely. Please remain seated. Single-click BIDPresidentDetailController.h, and make the following changes:
#import <UIKit/UIKit.h>
@class BIDPresident; |
|
#define kNumberOfEditableRows |
4 |
#define kNameRowIndex |
0 |
#define kFromYearRowIndex |
1 |
#define kToYearRowIndex |
2 |
#define kPartyIndex |
3 |
#define kLabelTag |
4096 |
@interface BIDPresidentDetailController : UITableViewController
<UITextFieldDelegate>
@property (strong, nonatomic) BIDPresident *president; @property (strong, nonatomic) NSArray *fieldLabels;
@property (strong, nonatomic) NSMutableDictionary *tempValues; @property (strong, nonatomic) UITextField *currentTextField;
-(IBAction)cancel:(id)sender;
-(IBAction)save:(id)sender;
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- (IBAction)textFieldDone:(id)sender;
@end
What the heck is going on here? This is new. In all our previous table view examples, each table row corresponded to a single row in an array. The array provided all the data the table needed. For example, our table of Pixar movies was driven by an array of strings, each string containing the title of a single Pixar movie.
Our presidents example features two different tables. One is a list of president names, and it is driven by an array with one president per row. The second table implements a detail view of a selected president. Since this table has a fixed number of fields, instead of using an array to supply data to this table, we define a series of constants we will use in our table data source methods. These constants define the number of editable fields, along with the index value for the row that will hold each of those properties.
There’s also a constant called kLabelTag that we’ll use to retrieve the UILabel from the cell so that we can set the label correctly for the row. Shouldn’t there be another tag for the UITextField? Normally, yes, but we will need to use the tag property of the text field for another purpose. We’ll use another slightly less convenient mechanism to retrieve the text field when we need to set its value. Don’t worry if that seems confusing; everything should become clear when we actually write the code.
You should notice that this class conforms to three protocols this time: the table data source and delegate protocols (which this class inherits because it’s a subclass of
UITableViewController) and a new one, UITextFieldDelegate. By conforming to
UITextFieldDelegate, we’ll be notified when a user makes a change to a text field so that we can save the field’s value. This application doesn’t have enough rows for the table to ever need to scroll, but in many applications, a text field could scroll off the screen, and perhaps be deallocated or reused. If the text field is lost, the value stored in it is lost, so saving the value when the user makes a change is the way to go.
Down a little further, we declare a pointer to a BIDPresident object. This is the object that we will actually be editing using this view, and it’s set in the tableView:didSelectRowAtIndexPath: of our parent controller based on the row selected there. When the user taps the row for Thomas Jefferson, for example, the
BIDPresidentsViewController will create an instance of the BIDPresidentDetailController. The BIDPresidentsViewController will then set the president property of that instance to the object that represents Thomas Jefferson, and push the newly created instance of BIDPresidentDetailController onto the navigation stack.
The second instance variable, fieldLabels, is an array that holds a list of labels that correspond to the constants kNameRowIndex, kFromYearRowIndex, kToYearRowIndex, and kPartyIndex. For example, kNameRowIndex is defined as 0. So, the label for the row that shows the president’s name is stored at index 0 in the fieldLabels array. You’ll see this in action when we get to it in code.
Next, we define a mutable dictionary, tempValues, that will hold values from fields the user changes. We don’t want to make the changes directly to the president object because if the user selects the Cancel button, we need the original data so we can go
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back to it. Instead, we will store any value that is changed in our new mutable dictionary, tempValues. For example, if the user edited the Name: field and then tapped the Party: field to start editing that one, the BIDPresidentDetailController would be notified at that time that the Name: field had been edited, because it is the text field’s delegate.
When the BIDPresidentDetailController is notified of the change, it stores the new value in the dictionary using the name of the property it represents as the key. In our example, we would store a change to the Name: field using the key @"name". That way, regardless of whether users save or cancel, we have the data we need to handle it. If the users cancel, we just discard this dictionary; if they save, we copy the changed values over to president.
Next up is a pointer to a UITextField, named currentTextField. The moment the users click in one of the BIDPresidentDetailController text fields, currentTextField is set to point to that text field. Why do we need this text field pointer? We have an interesting timing problem, and currentTextField is the solution.
Users can take one of two basic paths to finish editing a text field. First, they can touch another control or text field that becomes first responder. In this case, the text field that was being edited loses first responder status, and the delegate method textFieldDidEndEditing: is called. textFieldDidEndEditing: takes the new value of the text field and stores it in tempValues.
The second way that users can finish editing a text field is by tapping the Save or Cancel button. When they do this, the save: or cancel: action method is called. In both methods, the BIDPresidentDetailController view must be popped off the stack, since both the save and cancel actions end the editing session. This presents a problem. The save: and cancel: action methods do not have a simple way of finding the just-edited text field to save the data.
The delegate method textFieldDidEndEditing: does have access to the text field, since the text field is passed in as a parameter. That’s where currentTextField comes in. The cancel: action method ignores currentTextField, since the user did not want to save changes, so the changes can be lost without causing any problems. But the save: method does care about those changes and needs a way to save them.
Since currentTextField is maintained as a pointer to the current text field being edited, save: uses that pointer to copy the value in the text field to tempValues. Now, save: can do its job and pop the BIDPresidentDetailController view off the stack, which will bring our list of presidents back to the top of the stack. When the view is popped off the stack, the text field and its value are lost. But since we’ve saved that sucker already, all is cool.
Single-click BIDPresidentDetailController.m, and make the following changes:
#import "BIDPresidentDetailController.h"
#import "BIDPresident.h"
@implementation BIDPresidentDetailController
@synthesize president; @synthesize fieldLabels;
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@synthesize tempValues; @synthesize currentTextField;
- (IBAction)cancel:(id)sender {
[self.navigationController popViewControllerAnimated:YES];
}
- (IBAction)save:(id)sender {
if (currentTextField != nil) {
NSNumber *tagAsNum = [NSNumber numberWithInt:currentTextField.tag]; [tempValues setObject:currentTextField.text forKey:tagAsNum];
}
for (NSNumber *key in [tempValues allKeys]) { switch ([key intValue]) {
case kNameRowIndex:
president.name = [tempValues objectForKey:key]; break;
case kFromYearRowIndex:
president.fromYear = [tempValues objectForKey:key]; break;
case kToYearRowIndex:
president.toYear = [tempValues objectForKey:key]; break;
case kPartyIndex:
president.party = [tempValues objectForKey:key]; default:
break;
}
}
[self.navigationController popViewControllerAnimated:YES];
NSArray *allControllers = self.navigationController.viewControllers; UITableViewController *parent = [allControllers lastObject]; [parent.tableView reloadData];
}
- (IBAction)textFieldDone:(id)sender { [sender resignFirstResponder];
}
#pragma mark -
- (void)viewDidLoad { [super viewDidLoad];
NSArray *array = [[NSArray alloc] initWithObjects:@"Name:", @"From:", @"To:", @"Party:", nil];
self.fieldLabels = array;
UIBarButtonItem *cancelButton = [[UIBarButtonItem alloc] initWithTitle:@"Cancel" style:UIBarButtonItemStylePlain target:self action:@selector(cancel:)];
self.navigationItem.leftBarButtonItem = cancelButton;
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UIBarButtonItem *saveButton = [[UIBarButtonItem alloc] initWithTitle:@"Save" style:UIBarButtonItemStyleDone target:self action:@selector(save:)];
self.navigationItem.rightBarButtonItem = saveButton;
NSMutableDictionary *dict = [[NSMutableDictionary alloc] init]; self.tempValues = dict;
}
#pragma mark -
#pragma mark Table Data Source Methods
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
return kNumberOfEditableRows;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
static NSString *PresidentCellIdentifier = @"PresidentCellIdentifier";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier: PresidentCellIdentifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:PresidentCellIdentifier];
UILabel *label = [[UILabel alloc] initWithFrame: CGRectMake(10, 10, 75, 25)];
label.textAlignment = UITextAlignmentRight; label.tag = kLabelTag;
label.font = [UIFont boldSystemFontOfSize:14]; [cell.contentView addSubview:label];
UITextField *textField = [[UITextField alloc] initWithFrame: CGRectMake(90, 12, 200, 25)];
textField.clearsOnBeginEditing = NO; [textField setDelegate:self]; textField.returnKeyType = UIReturnKeyDone; [textField addTarget:self
action:@selector(textFieldDone:)
forControlEvents:UIControlEventEditingDidEndOnExit]; [cell.contentView addSubview:textField];
}
NSUInteger row = [indexPath row];
UILabel *label = (UILabel *)[cell viewWithTag:kLabelTag]; UITextField *textField = nil;
for (UIView *oneView in cell.contentView.subviews) { if ([oneView isMemberOfClass:[UITextField class]])
textField = (UITextField *)oneView;
}
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label.text = [fieldLabels objectAtIndex:row]; NSNumber *rowAsNum = [NSNumber numberWithInt:row]; switch (row) {
case kNameRowIndex:
if ([[tempValues allKeys] containsObject:rowAsNum]) textField.text = [tempValues objectForKey:rowAsNum];
else
textField.text = president.name; break;
case kFromYearRowIndex:
if ([[tempValues allKeys] containsObject:rowAsNum]) textField.text = [tempValues objectForKey:rowAsNum];
else
textField.text = president.fromYear; break;
case kToYearRowIndex:
if ([[tempValues allKeys] containsObject:rowAsNum]) textField.text = [tempValues objectForKey:rowAsNum];
else
textField.text = president.toYear; break;
case kPartyIndex:
if ([[tempValues allKeys] containsObject:rowAsNum]) textField.text = [tempValues objectForKey:rowAsNum];
else
textField.text = president.party; default:
break;
}
if (currentTextField == textField) { currentTextField = nil;
}
textField.tag = row; return cell;
}
#pragma mark -
#pragma mark Table Delegate Methods
- (NSIndexPath *)tableView:(UITableView *)tableView willSelectRowAtIndexPath:(NSIndexPath *)indexPath {
return nil;
}
#pragma mark Text Field Delegate Methods
- (void)textFieldDidBeginEditing:(UITextField *)textField { self.currentTextField = textField;
}
- (void)textFieldDidEndEditing:(UITextField *)textField { NSNumber *tagAsNum = [NSNumber numberWithInt:textField.tag]; [tempValues setObject:textField.text forKey:tagAsNum];
}
@end
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