Saturday, February 11, 2012

The TreeView in Cocoa : an NSOutlineView tutorial

The MacOS X Finder has a so called ‘List’ viewing mode in which folders have a square triangle to their left side. The triangle can be clicked to expand or collapse the folder, allowing the user to show or hide the contents of the folder. Such an hierarchical tree of items is usually called a TreeView widget. In Cocoa, it is called NSOutlineView.

The NSOutlineView is one of the more difficult Cocoa classes to use. Apple's documentation about it isn't very clear, especially not to beginners, and secondly, it requires a relatively lot of work on your part to get it going. What's also confusing is that there's a NSTreeController that is hard to grasp. In fact, we are not going to bother with NSTreeController. As you will see, it is possible to implement a perfectly good tree view without having to use NSTreeController at all.

The NSOutlineView is a very generic class that handles any kind of hierarchically organized collection of items. So even if you just want to add a simple directory browser to your program, you will have to do the lower level stuff of getting the directory contents and handing them on a plate to the NSOutlineView so that it can display them. On the other hand, if you have a large collection like a zoological family tree of animals, you can also use NSOutlineView to display it and let the end user access this data in a convenient way.

The idea behind NSOutlineView is that it gets its data from a DataSource. Rather than pure data, the DataSource is a class that implements a protocol for providing data items. As said, the DataSource hands the items on a plate to the NSOutlineView.
The data items themselves should be instances of a generic class that is derived from NSObject. So for a directory browser, the data item must have a property for representing the filename. It must also have a pointer to its parent directory, and if it is a directory itself, it must have an array with pointers to data items representing the contents of this directory. As you may suspect by now, implementing the class for the data items is actually harder than implementing the DataSource.

Before starting off, make a new class derived from NSViewController named DirViewController (or so) that has only one property: a pointer to an NSOutlineView. You can bind an NSOutlineView control in IB to this property using Ctrl-dragging. And you can instantiate your new view controller class in IB by dragging an NSViewController to the object area and changing its class name to DirViewController.
We will also bind a DataSource to the NSOutlineView. For this purpose we will create a custom class named DirTreeDataSource. It implements the NSOutlineViewDataSource protocol:
@interface DirTreeDataSource : NSObject <NSOutlineViewDataSource>

From this protocol we need to implement actually only four tiny methods:
- (NSInteger)outlineView:(NSOutlineView *)outlineView numberOfChildrenOfItem:(id)item;
- (BOOL)outlineView:(NSOutlineView *)outlineView isItemExpandable:(id)item;
- (id)outlineView:(NSOutlineView *)outlineView child:(NSInteger)index ofItem:(id)item;
- (id)outlineView:(NSOutlineView *)outlineView objectValueForTableColumn:(NSTableColumn *)tableColumn byItem:(id)item;
Note that you won't call these methods. They will be called when the user clicks in the NSOutlineView control. These methods then tell the NSOutlineView what it can do with this item. The parameter item is of type id, and you can safely typecast it to a pointer of your custom item class. The parameter item may be nil, in which case you should treat it as the root item of the tree.

numberOfChildrenOfItem: must return the number of subdirectories, or zero if item is a file rather than a directory.
isItemExpandable: must return YES if item is a subdirectory, or NO if it is a file.
child:ofItem: must return the item that is at the given index. So for a directory item that holds a number of files in an array, return the array item at the index. Which comes down to return [[item array] objectAtIndex:index];
objectValueForTableColumn:byItem: must return the value of the item. In other words, it returns the filename. I suppose it must return something that the outlineview's cell can draw.

Our custom item class will be called DirTreeItem. Its superclass should be NSObject. As properties, it needs to have a name, a pointer to its parent item, and a mutable array of subitems.
@interface DirTreeItem : NSObject

@property (retain) DirTreeItem *parent;
@property (retain) NSString *name;
@property (retain) NSMutableArray *subdirs;

Of course, DirTreeItem needs some methods. The hardest part is to get the directory contents loaded into the subdirs array. You can do this by using [NSFileManager contentsOfDirectoryAtURL:]. This requires a file:// URL that you can create with [NSURL fileURLAtPath:]. This requires the full path of the current item, which you can get by traversing the item's parent pointers all the way up to the root. Mind not to put filenames into the subdirs array; it must hold DirTreeItems (!) so allocate a new DirTreeItem for each directory entry and add it to subdirs.
Loading the directory contents should be triggered by the data source's numberOfChildrenOfItem: and child:OfItem: methods.

After writing the code for DirTreeItem and DirTreeDataSource, head back to IB. Instantiate the data source by dragging an NSObject to the object area and changing its class name to DirTreeDataSource. Now select the NSOutlineView control and bind its data source to our instantiated DirTreeDataSource object by Ctrl-dragging. We now have a tree view control that is fed by a data source. Our data source will load data items dynamically (as it gets the directory contents) when the user browses the tree.

I have to say, it takes quite some work to get an NSOutlineView going. If you want to add a small image to the items in the outline view, you are going to have to subclass NSCell and do your own drawRect: method. The NSOutlineViewDataSource protocol also supports editing items, drag and drop, and sorting.
It is also possible to bind the outlineview's data source to an NSTreeController. If you go this way, the data items must be key-value-coding compliant. From what I gather, NSTreeController is convenient if you use a Core Data model. Otherwise, you are better off using a data source implementation as shown above.

Although I told you how it works, I did not spell out all the code. If you need more help with this, please study Apple's example.