在Objective-C中代表

Question

I am new at Objective-C, and I'm trying to understand delegates. I have searched and read a lot, but nothing is really helping me understand. I think that best way to understand this might be asking a question with a sample app.

I'm trying to create a grade calculator application to test my skills. Here are my files:

mainTableViewController.h

#import <UIKit/UIKit.h>

@interface mainTableViewController : UITableViewController

@end

mainTableViewController.m

#import "mainTableViewController.h"
#import "addLectureViewController.h"


@interface mainTableViewController ()

@end

@implementation anaTableViewController

- (id)initWithStyle:(UITableViewStyle)style
{
    self = [super initWithStyle:style];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    [lectureName count];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];

    return cell;
}

@end

addLectureViewController.h

#import <UIKit/UIKit.h>

@interface addLectureViewController : UIViewController 
@property (weak, nonatomic) IBOutlet UITextField *lectureNameTextField;
- (IBAction)addButtonPressed:(id)sender;

@property NSMutableArray *lectureName; 
@property NSObject *lecture;

@end

addLectureViewController.m

#import "addLectureViewController.h"

@interface addLectureViewController ()

@end

@implementation addLectureViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    _lectureName = [[NSMutableArray alloc] init];
    _lecture = [[NSObject alloc] init];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];

}

- (IBAction)addButtonPressed:(id)sender {

    _lecture = _lectureNameTextField.text;
    [_lectureName addObject:_lecture];
    NSLog(@"%@",_lectureName);


}
@end

Everything is okay so far. But when i try to use the _lectureName NSMutableArray at mainTableViewController.m, I can't see the array.

I know the codes for printing the array in tableView. I know they are not at there right now. I just don't understand implement delegate codes to my code.

Answer 1

If You want to display something on the rows of the table, You can take an NSArray and you have to return the count of the array in the delegate method:

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
 return array.count;
}

Otherwise, table will not display any elements. And delegate method cellForRowAtIndexPath will only be called if you are returning a particular array count of number count in your numberOfRowsInSection method.

You can take reference from these links to understand delegates: http://www.tutorialspoint.com/ios/ios_delegates.htm

How do I set up a simple delegate to communicate between two view controllers?

http://code.tutsplus.com/tutorials/ios-sdk-custom-delegates--mobile-10848

But in the case of tableView, the delegate methods are internally defined and triggered internally. We just need to set those delegates to the controller which acts as a listener.

Answer 2

The below code might have syntax errors but they can provide a summary of delegates for this code.

Make the following changes :-

mainTableViewController.h

#import <UIKit/UIKit.h>

@interface mainTableViewController : UITableViewController
@property(strong, nonatomic) NSMutableArray *lectureName
@end

Synthesize the property lectureName in mainTableViewController.m

Then make the following changes in addLectureViewController.h

#import <UIKit/UIKit.h>
#import "mainTableViewController.h"

@interface addLectureViewController : UIViewController 
@property (weak, nonatomic) IBOutlet UITextField *lectureNameTextField;
- (IBAction)addButtonPressed:(id)sender;
@property(weak, nonatomic) id<mainTableViewController> delegate;
@property NSMutableArray *lectureName; 
@property NSObject *lecture;

@end

Synthesize the property delegate in addLectureViewController.m

Then make the following change :-

- (IBAction)addButtonPressed:(id)sender {

    _lecture = _lectureNameTextField.text;
    [_lectureName addObject:_lecture];
    NSLog(@"%@",_lectureName);
    delegate.lectureName=_lectureName;
}

Assuming that you are pushing addLectureViewController from mainTableViewController, also include the following code in prepareForSegue of mainTableViewController (or whatever method in which you are pushing addLectureViewController). :-

addLectureViewController *vc=[[addLectureViewController alloc] init];
vc.delegate=self;
// Push vc

The above code actually creates a weak property of type id<mainTableViewController> called delegate (weak in order to prevent reference cycles). This way, addLectureViewController can update mainTableViewController's property.

Answer 3

Points:

1. Why is the class in mainTableViewController.m named anaTableViewController . It should be mainTableViewController (almost always, until you get more advanced).

2. mainTableViewController and addLectureViewController should start with capital letters.

3. The only way for mainTableViewController to access lecture is through a addLectureViewController object, eg

    addLectureViewController *alvc = // *something* here NSArray *localLecture = [alvc lecture]; 

   
   Figure out how you have access to an addLectureViewController and you will have solved your problem.