Delegates, can't get my head around them

问题:

Hey, I'm looking for useful resources about Delegates. I understand that the delegate sits in the background and receives messages when certain things happen - e.g. a table cell is selected, or data from a connection over the web is retrieved.

What I'd like to know in particular is how to use delegates with multiple objects. As far as I know, specifying the same delegate for an object (e.g. table cell) would cause the same events to be called for both the cells at the same time. Is there anything equivalent to instantiating a delegate for a particular object?

Thanks in advance!

回答1:

In Cocoa, objects almost always identify themselves when calling a delegate method. For example, UITableView passes itself as the first parameter of the delegate message when calling it:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath

If you wanted the same delegate to handle multiple UITableViews, then you just need a some conditional on the tableView object passed to the method:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (tableView == self.myFirstTableView) {
        // do stuff
    } else if (tableView == self.mySecondtableView) {
        // do other stuff
    }
}

}

If you don't want to compare the object pointers directly, you can always use the tag property to uniquely identify your views.

回答2:

Usually, if you have a delegate method that might have to receive messages from many different objects, you simply have the calling object pass itself to the delegate in the message (method call).

For example, if you wanted a delegate method to extract the text from a tableviewcell's label, the method definition would look something like:

-(void) extractTextFromLabelOfTableCell:(UITableViewCell *) theCallingCell{
...
NSString *extractedText=theCallingCell.textLabel.text;
}

You would call the method from a tableviewcell thusly:

[delegate extractTextFromLabelOfTableCell:self];

Each instance of the tableviewcell would send itself to the delegate and the delegate would extract that instance's text. In this way, a single delegate object could handle an arbitrarily large number of cells.

回答3:

A delegate is a way of adding behaviors to a class without subclassing or for attaching a controller to a class.

In the table view example you gave, the delegate is extending or controlling the table, not the cell. The table is designed to have a controller, the cell is not. This design choice is why you can't specify cell-specific delegates.

However, delegate methods will always announce the source object (the one to which the delegate is attached) and relevant parameters (like the cell involved) so you should always be able to handle the action fully.

In your case, if you have a cell and you would like the cell to manage itself, then the delegate method (which will probably be implemented on your UITableViewController) can simply fetch the cell from the source table using its NSIndexPath (passed as a parameter to the delegate method) and invoke a method on the cell subclass to do its work.

回答4:

I always liked Chris Sells' ".NET Delegates: A Bedtime Story"