I have a tableview in my viewcontroller. But i have an error which say that [tableview:numberofrowinsection:]:unrecongized selector sent to instance. Can someone please tell me how cna i solve this.
You need to set the UIViewController as the delegate, and also datasource
also you need to implement the necessary methods.
if you look in the apple docs they are described there
but i think the main ones are:
numberOfSectionsInTableView:
tableView:numberOfRows:inSection:
and tableView:cellForRowAtIndexPath:
in the first one you return the number of sections, usually 1, in the second you return the number of rows for the section( usually the count of the array)
and in the final you return the UITableViewCell object with the necessary data put in position.
read the apple docs, and if necessary copy the method names from one of the UITableViewController templates.
hope this helps
Create a new file using the UITableViewController template - it has skeleton code that should help (even if you don't use it to build on, you can use it as an example of implementing the delegate methods and reusing cells).
Related
I'm a little confused as to how I should be using didSelectRowAtIndexPath.
Without using datasources (keeping things simple),if you display a list of names with UITableView and all you want to do is select a name and return it to whatever parent view controller created the UITableView list.. what would you do?
I'm used to Windows MFC programming where you have a dialog class and you call DoModal() to display the dialog and then query member variables of that dialog object to get the results.. With iPhone programming I'm confused with the whole paradigm. I push the UITableView into action, but at what point should I try and retrieve the results? pushViewController is asynchronous in nature isn't it? So I can't expect to retrieve results on the line immediately following the pushViewController method?
I'm using NSMutableArrays of NSStrings to keep things simple.
The patter you are going to use is called delegation, what happens is that you call the new ViewController and let it do its work, the function that called this will return normally,
Now you need to implement didSelectRowAtIndexPath and get the selectedRow, after this you will need to return the selected index or value to the original view controller( this will be recieved in another function) please read more about #protocol and delegateion
I have just been learning iPhone apps development but I have a hard time in understanding what delegate actually means? Can anyone tell me with example what it does and how important it is? Thanks for any helps!
It's a key concept to understand conceptually so getting your head around how to think about it ahead of the technical details is important. Simply put, a delegate is a callback.
Two main scenarios to use delegates:
A class or control wants to abstract out the details on how to do work (like retrieve data).
Allow others to hook code into a pipeline.
Examples:
UITableView - a table view is just a control that knows how to render a list of cells. It handles all the heavy lifting of rendering, scrolling, etc... But, it has no idea how to load your data. So you implement a datasource delegate which has methods to get the cell data for a given row etc... That makes it easy on you. You just use the control and plug in the specifics for your data. The UITableView will do everything for you ... just answer a few specific questions for. A delegate answers those few specific questions.
A text control - you add a text control to your view and voila! you can type in it and alls good. But what if you want to do something when they start typing or when they're done typing? Well, the text control offers a delegate with methods that allow you to hook into the execution pipeline of the text control. It allows the text control to do everything for you and allows you to interject code where you need it. Many times, there's way to interject code to make a decision on whether something is allowed. The control will call back and ask, should I be able to do x? You can interject code and influence the behavior.
If you're creating a control or class, you can create your own protocol, datasource delegates etc... so your control can focus on doing what's advertised. For example, let's say you wanted to create a task control. You could:
First, create a contract. Hey, if you're going to provide data for my control, these are the questions I'm going to ask you. I'll take it from there... In this case, I'm going to ask you the number of tasks and I'm going to have you give me a task given the row number.
#protocol XXTaskBoardDelegate <NSObject>
-(NSInteger*)getTaskCount;
-(XXTask*)getTaskForRow:(NSInteger*)rowNumber;
#end
In the control or class, give the consumer a way to give us the delegate datasource class that will answer the questions the control will ask. At this point, the control is a pure control. It knows nothing about how you get your data. It's asking for an object (id) that implements a contract/protocol. id
#implementation XXTaskBoard
- (void)setDelegate:(id<XXTaskBoardDelegate>)newDelegate
{
// the control stores the delegate so it can callback and ask you questions.
}
Then, for the delegate class, in the header declare you implement that formal protocol
and in the implementation m file you provide the code.
#interface AppController : NSObject<XXTaskBoardDelegate>
{
//...
}
then, implement it in the implementation
#implementation AppController
- (NSInteger*)getTaskCount
{
return [model queryTaskCount];
}
- (XXTask*)getTaskForRow:(NSInteger*)rowNumber
{
return [[model tasks] getItem:(NSInteger*)rowNumber];
}
A delegate is an object that another class can pass messages to. In practice delegate classes have to conform to a delegate protocol.
For instance we will take a subclass of a table view controller. This is a delegate for your table view. First you define that it is a table view delegate by doing this:
MyTableViewController : UITableViewController <UITableViewDelegate>
This says that class MyTableViewController is a subclass of UITableViewController and CONFORMS to the UITableViewDelegate protocol.
Setting [tableView setDelegate:self] (or defining it as such in IB) then passes the self object to the tableview in order for the tableview to send messages to it.
The main message it sends is the didSelectRowAtIndexPath message which tells your class that the user has pressed a table view cell.
So the object that takes the click event (the table view) passes on the message that the cell has been clicked to the delegate object (which in this case is your MyTableViewController object).
Delegate protocols exist so that you can make sure that the delegate object has the necessary methods to deal with your messages. Methods in a delegate protocol can be #optional or enforced. Any methods that are optional don't have to be defined. In your MyTableViewController class the method didSelectRowAtIndexPath is optional - you don't have to have it. If the table view doesn't find the method it just won't call it.
However the cellForRowAtIndexPath is necessary and without it your app won't compile.
I hope this helps and is straightforwards for you. If you need any more info let me know.
Delegates are just way of getting callbacks from something. You pass a delegate (a pointer to an object that conforms to a protocol) to something and when it has new data for you or when an event occurs that something make a method call on the delegate.
For example, when events occur, like your app is put into the background or the app is about to terminate the UIApplication object will call your application delegate to let it know. When a CLLocationManager has a new GPS position is will call your delegate to pass it the new position. UITableViews call their delegates to get UITableViewCells to display in the table. There are many uses of delegates in iOS.
Very new to programming for iOS and Cocoa so please take it easy on me as I try to wrap my brain around the following. I'm trying to display a tableview populated from an XML feed as the opening screen of my app. I've tried to consume the XML from inside my AppDelegate using the ApplicationDidFinishLaunching method (and then making my AppDelegate a delegate for the XML parser which I access using a NSUrlConnection and its delegate methods) but I can't figure out how to take the parsed XML file and pass it to a tableviewcontroller which can then use it as the datasource for a tableview. When I do try, I always get a blank tableview.
I've written the code a few times and nothing seems to work.. I'll post what I have here to show what I've got so far but I'm afraid its mostly vanilla AppDelegate with a few parser methods thrown in.. any pointers in the right direction would be super appreciated.
Thank you in advance!
Hmm, probably a bad idea to do the network call in the AppDelegate. Try to put all that code at the view controller level. Here's a brief structure of what I do (Since it's very similar)
View Controller listens to button events
Use ASIHTTPRequest to talk to your web service. Handles network really well, you can skip the NSURLConnection stuff.
Try to load your data source (an array?) with static values and see if they come up on the table view.
Parse the response from ASIHTTPRequest using NSXMLParser, and load the data you want into the static array you were using. More here.
Call [tableView reloadData] once you're done and the changes will reflect.
Did you specify a UITableViewDataSource for your table view and implement the two required methods?
tableView:cellForRowAtIndexPath:
and
tableView:numberOfRowsInSection:
http://developer.apple.com/library/ios/#documentation/uikit/reference/UITableViewDataSource_Protocol/Reference/Reference.html
When I get blank tables, this is what it is; I've forgotten to specify the data source
I have been using Objective-C for a while and pretty much understand most of its features. However, the concept of delegates eludes me. Can someone please give a succinct and easy to comprehend explanation of what delegates are, how they are used in the iPhone SDK, and how I can best make use of them in my own code?
Thank you!
There are a couple main reasons to use delegates in Objective-C, which are subtly different:
Enhancing the base functionality of a framework class. For example, a UITableView is pretty boring on its own, so you can give it a delegate to handle the interesting bits (creating table cells, adding text to section headers, what have you). This way, UITableView never changes, but different table views can look and act very differently.
Communicating to parent objects in your dependency hierarchy. For example, you may have a view with a button that the user may push to do something that affects other views. The view will have to send a message to its parent view, or perhaps the view controller, so that it can create or destroy or modify other views. To do this you'd pass the parent object into your view, most likely through a protocol, as a weak reference (in Objective-C, an assign property). The view could then send any message declared in the protocol to the parent, or delegate, object.
This approach need not involve views. For example NSURLConnection passes event back to its delegate, which may be the object that created it, using this mechanism.
Essentially, all a delegate is, is an object that accepts feedback from another object. Put simply, when stuff happens to an object, it tells its delegate (assuming it has one).
For instance, lets say I have a UIViewController with a UITextView placed in the middle of the view. I set up my UIViewController to be the delegate of the UITextView. Then, when certain actions are performed on the text view (begin editing, text changes, end editing, etc), it tells it's delegate so it can do whatever logic it needs to do, like spell checking every time characters change, or dismissing the keyboard when it receives a return key press.
Delegate methods perform a similar function to callback functions in C.
Hope that makes sense :)
Best and simple concept I got from a Lynda.com Tutorial was: When you set a Delegate it means you have been given work to do. So, if you want to use methods that are written in a protocol method, you must implement them by searching in the Delegate Class Reference and using them. I hope it helped.
By the way, Delegates are excellents. They are your friends. They have been made to make your life as a programmer much easier.
for example we use this method in the tableview
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
return 16;
}
i want to learn we don't call this method in anywhere but application reads this value how is it being? there are a lot of methods like this we did not call.
Your object has been set as the data source of the UITableView somewhere. Most likely, by making a connection in InterfaceBuilder, though it is straightforward to do so in code by setting the dataSource property of the UITableView:
- (void) setUpMyJunkMan
{
myTableView.dataSource = self;
}
Once you have set your object as the data source, the table view will invoke the method as needed to determine what it needs to draw or how it needs to respond to events.
Your object is required to implement the UITableViewDataSource protocol (though, if you connected the data source via InterfaceBuilder, there may not be a complaint if you don't -- it is more of a compile time validation than a runtime one).
If you look at the declaration of UITableViewDataSource, you'll see that a number of methods are #optional. The rest are #required; you must implement them to fulfill the contract of the protocol.
This is the key difference between data sources and delegates. Delegates can optionally implement any of the declared methods. Data sources create a much more formal relationship between the two objects wherein some of the methods must be implemented.
An easy way see why a method is being called - set a breakpoint, run in debug mode, and then look at the stack trace.
For this particular case - It's being called automatically by the framework when it renders the table view.
I think you really need to take a look at The Table View Programming Guide so that you have a good understanding of what methods you need to override (and not override) when using Table Views. If you are extending the TableViewController class the framework does a lot of the heavy lifting and you barely have to write any code.
numberOfSectionsInTableView: is being called by the table view.
You implement numberOfSectionsInTableView: as part of the UITableViewDataSource protocol. Each UITableView is given a dataSource. Normally, UITableView will be constructed by a UITableViewController which will set itself as the view's dataSource.
When the view is shown, it calls numberOfSectionsInTableView: on its dataSource.
This is explained in the Table View Programming Guide for iPhone OS.
This is part of a Delegate Interface.
At some point in your application (possibly in your UIBuilder) you have specified that the object that contains the method is actually the delegated object. This means that when you want to adjust the behaviour (in this case of a UITableView) you can without actually extending UITableView but mearly changing the delegate methods.
basically the UITableView class will look somehting like this.
- (void) AMethodinUiTableView
{
int colums =[self.delegate numberOfSectionsInTableView:self];
}
for more info i would check out delgate programing and selectors.