UITableView中的可见单元格数

Question

I have A UITableView that is slightly smaller than its parent view. The height of each cell is the same as the height of the table (only one cell is fully visible at a time).

The UITableView has clipsToBounds set to NO to be able to see the top of the next cell.

But the next cell is not loaded in visibleCells as it doesn't fit in the UITableView . When I start to scroll it appears (or if I set the cell height to be 1 less pixel). How can I make the UITableView to always have 2 cells loaded instead of only the one that fits in the table?

On this illustration, the second UITableViewCell is not loaded by default and loads only when I start to scroll.

PS: I'm also using pagingEnabled on the table view.

Answer 1

You could make the tableview bigger than a single cell, then set the tableview content insets so that the top and bottom cells appear in the correct places at max/min scroll. You could also put an invisible view over the portion of the tableview that is not supposed to be there (the bigger part that you don't currently have there) so that this part of the tableview is still not selectable.

It could be a bit of work, but the only way to ensure that the cell is drawn is to make it on screen.

In further research, this guy says the same thing, but for a different reason.

2. UITableView buffering

This may be one of the simplest changes you can make to improve perceived performance in any iOS application. Simply enough, if you want to preload data of you table view beyond the visible bounds of the table view, you need only extend the table view itself to achieve the desired buffer. For example, you have an iPhone app that has a full screen table view (320x480 or 320x568). If you want the table view to buffer an extra table view height of data, you just make your table view twice as tall (320x960 or 320x1136) and then counter the height extension using UIScrollView's contentInset and scrollIndicatorInsets properties. It really is that easy. With this simple change to your table views, you now have the ability to avoid showing table view cells with blank content as the content is downloaded and then have the content jarringly show up.

Answer 2

in viewDidAppear or viewWillAppear

for (int i=0 ; i <10 ; i ++) {
    [self tableView: self.myTableView cellForRowAtIndexPath: indexPath];
     // give your indexpath for section=0 and row=i; 
}

this line will Allocate all UITableViewCell but UITableView will discard them if they are not in Visiblecells.

To Do it add all your cells in NSMutableArray and in cellForRowAtIndexPath: load them from NSMutableArray.

- (UITableViewCell *)tableView:(UITableView *)tableView 
        cellForRowAtIndexPath:(NSIndexPath *)indexPath  {
    return [preCompiledCellArray objectAtIndex:indexPath.row]; 
}

Answer 3

Unless there are a lot of rows or you need some table-like feature you have not mentioned, you might be a lot happier not using UITableView at all here. What you are describing sounds much more like an ordinary UIScrollView.

Suppose, for example, there are just four "cells". Then this is just an ordinary UIScrollView and you can populate its entire content (the four "cells") beforehand.

On the other hand, if there are 100 "cells", you could run out of memory that way. But you can still remove subviews when they are no longer visible and create them as needed, as the user scrolls; see the WWDC 2010 video on scroll views for how to manage that.