为什么在制作自定义表格视图单元格时两次使可重用单元格出队

Question

I am following a tutorial of making custom table view cell with storyboard. I drag a UILabel as subview of the cell and set its tag to 1. I have two questions regarding the data source code.

1. What's the purpose of the second dequeue statement? I know it's an init method while not using storyboard to make the custom cell.

2. What's the difference between tableview and self.tableview?

    -(UITableViewCell*)tableView:(UITableView*)tableView cellForRowAtIndexPath:(NSIndexPath*)indexPath { static NSString *CellIdentifier = @"Cell"; UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier]; if (cell == nil) { cell = [self.tableView dequeueReusableCellWithIdentifier:CellIdentifier]; } NSDictionary *dToAccess = (self.tableView==tableView)?[self.arForTable objectAtIndex:indexPath.row] : [self.arForSearch objectAtIndex:indexPath.row]; [(UILabel*)[cell viewWithTag:1] setText:[dToAccess valueForKey:@"name"]]; [(UILabel*)[cell viewWithTag:2] setText:[dToAccess valueForKey:@"value"]]; return cell; } 

Answer 1

For your first question, the second dequeueReusableCellWithIdentifier: looks like a mistake.

Here is how a UITableView works:

You might have 50 rows in your table, but if only 10 rows are visible at a time, you only need to make 10 cells, and then when the user scrolls, you can reuse cells that have gone offscreen instead of always releasing them and init'ing new cells that come onscreen. A UITableView keeps a list of cells that have gone offscreen and when you call dequeueReusableCellWithIdentifier:, it removes it from the list of offscreen cells and returns it to you. From here you can customize the cell for re-use (change its text, color, etc) and return it. Again, this is not an "init" method, this is returning a pre-existing cell.

So, let's look at what happens when this UITableView is first displayed -- in this example there are 10 visible cells, so the tableView will call tableView:cellForRowAtIndexPath: 10 times to get cells to display in these 10 slots. Every time this is called, you will need to initialize and return a new UITableViewCell to display. (At this point dequeueReusableCellWithIdentifier: will return nil, because you don't have any offscreen cells to re-use yet)

When a user scrolls your list, cells will begin to go offscreen, and new cells will need to appear. You don't need to make new cells, because you have already created as many as will need to be onscreen at a time. You should call dequeueReusableCellWithIdentifier: to get a reference to a cell that has gone offscreen, which you can then re-use.

I would alter your code like this:

static NSString *CellIdentifier = @"Cell";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier: CellIdentifier];

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

Now you are checking for reusable cells before creating new ones.

For your second question,

In your example, tableView refers to the tableView that was passed in (see the "tableView" in your method signature). Separately, if your class has defined a property called tableView, then self.tableView will call the getter for this property.

Answer 2

When apple developed the UITableView for the first iphone they had a problem in performance when scrolling through it. Then one clever engineer discovered that the cause of this was that allocation of objects comes with a price, so he came up with a way to reuse cells.

dequeueReusableCellWithIdentifier method is used to returns a cell if it has been marked as ready for reuse.

So Whenever there are many number of rows in a table view and you are going to scroll it, then the cells which are just passed away from your previous screen before scrolling are get reused instead of creating new one.

And to know the ans of your second que. I think you should refer this link :

http://www.iphonedevsdk.com/forum/iphone-sdk-development/17669-when-use-self-objectname-just-objectname.html

Answer 3

不必两次出队，此代码块将被破坏。