PXListView

An optimized list view control for Mac OS X 10.5 and greater. It was created after I wrote this post on the subject.

PXListView is licensed under the New BSD license.

PXListView uses similar optimizations as UITableView for the iPhone, by enqueuing and dequeuing NSViews which are used to display rows, in order to keep a low memory footprint when there are a large number of rows in the list, yet still allowing each row to be represented by an NSView, which is easier than dealing with cells.

The architecture of the control is based on the list view controls which are present in both Tweetie (Mac) and Echofon (Mac).

The project is still very much a work in progress, and as such no documentation exists at current.

How the control works

Each row in the list view is displayed using an instance of PXListViewCell (which is a subclass of NSView). The delegate of PXListView responds to three messages in order for the control to function:

1. numberOfCellsInListView:
2. -listView:cellForRow:
3. -listView:heightOfRow:

###Setting up the List View in Interface Builder### Setting up PXListView in Interface Builder can be accomplished in a few simple steps:

1. Drag an NSScrollView to where you want to lay out the List View.
2. Set the class of the NSScrollView to PXListView in the Identity Inspector.
3. Set the class of the document view of the NSScrollView to PXListViewDocumentView.
4. With the scroll view's document view selected, alter the resizing mask so that only the bottom and the left anchors are selected.

###Using PXListViewCell### PXListViewCell is an abstract superclass, implementing the bare minimum for such features as cell selection and declaring methods relied on by the list view.

You should create a concrete subclass of PXListViewCell when using it in the list view, where drawRect: can be overridden to do custom drawing, and properties for cell UI outlets or data can be declared on this subclass. The example project (as part of the repository) shows this. Since PXListViewCells are views, it is easy to use a NIB to design your cell template, and makes adding text fields, buttons, images etc a much simpler process.

###Returning cells### When responding to -listView:cellForRow:, the delegate should first call -dequeueCellWithReusableIdentifier: on the list view, passing in the reusable cell identifier, to see if there are any reusable cells available. If this returns nil then a new cell can be created using the initializer initWithReusableIdentifier: (declared on PXListViewCell). this keeps the memory footprint of the control as low as possible by reusing cells that have been scrolled offscreen, removed from the view hierarchy and cached.

You can also load cells from NIBs easily, by using PXListViewCell's class method +cellLoadedFromNibNamed:reusableIdentifier:. This loads the NIB whose name is passed in, and returns the first list view cell it finds. To create a NIB which is compatible with this feature, just create a blank NIB and add a view. Make sure you set its class to your PXListViewCell subclass name, layout your cell as you see fit, and save. When you call +cellLoadedFromNibNamed:reusableIdentifier: with the name of your NIB, your new cell will be returned autoreleased, which can then be returned from -listView:cellForRow:. There is no need to set a File's Owner for your new NIB.

###Live Resize### PXListView has a property, usesLiveResize which determines whether the control should be updated continuously during a resize or not. By default, the cells will be updated continuously as the control is resized. Although visually preferable, especially when dealing with large data sets, this can cause the UI to become slow, so this can be turned off by setting the property to NO.

###Optimizations### PXListView only keeps the bare minimum of list view cells in the view hierarchy to be performant, and when rows are scrolled onscreen new cells are added to the view hierarchy to display the rows, and when the rows are scrolled offscreen the associated cells are removed from the view hierarchy.

Attributions

Thanks to Mike Abdullah for optimizations related to cell dequeuing.

Thanks to Uli Kusterer for additions and fixes to PXListView including momentum scrolling, keyboard navigation, changes to variable row heights (using CGFloats), accessibility as well as drag and drop support.

Thanks to Tom for fixing a memory issue with reloading data.