UITableView中的标识符的目的是什么

Question

I'm a little confused to why an identifer (*MyIdentifier) is always required. The code below demonstrates this. I have noticed all tableviews require at least one.

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

if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:MyIdentifier] autorelease];
}

What is the purpose behind having an identifier? Ive have seen a few tutorials where there is more than one. Also, reading the Apple documentation, I was a little confused about why the following is called:

if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:MyIdentifier] autorelease];
}

I would like to know why it takes the input of 'MyIdentifier'? Assuming we had more than one identifier, which one should we choose? To be exact, what if we had a Parent, Child and sub-child etc.

Answer 1

From the docs:

The identifier is a string identifying the cell object to be reused. This parameter must not be nil.

For performance reasons , a table view's data source should generally reuse UITableViewCell objects when it assigns cells to rows in its tableView:cellForRowAtIndexPath: method. A table view maintains a queue or list of UITableViewCell objects that the data source has marked for reuse. Call this method from your data source object when asked to provide a new cell for the table view. This method dequeues an existing cell if one is available or creates a new one using the class or nib file you previously registered. If no cell is available for reuse and you did not register a class or nib file, this method returns nil.

Answer 2

Every time you return a cell you have 2 options - you can create new cell and return it, dequeue cell that already exists and reconfigure it.

So the way this works is when you create cell you give it reuse identifier, so when cell goes off screen it can be used instead of creating new cell. After the cell is dequeued though, you might want to change its properties(like text or image)

You can have different reuse identifiers for different cell types(different content views)

Answer 3

Suppose you have several customized UITableViewCells and each of them have different backgound colors. In your storyboard or xib file, you may name the cell with red background color "redCell" and the one with blue "blueCell". Then you can select what kind of cell to add to a particular row using their identifiers. Let's say, you wanna apply red cells to odd rows and blue ones to even rows then you can use the following code to do this:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 
{
   UITableViewCell *redCell = [tableView dequeueReusableCellWithIdentifier:@"redCell"];

   UITableViewCell *blueCell = [tableView dequeueReusableCellWithIdentifier:@"blueCell"];

   if (indexPath.row % 2 == 0) return redCell;
   else return blueCell;
}

Without specifying an identifier, the system wouldn't know which kind of cell to pick.