Building Cocoa Applications: A Step by Step Guide

Adding Mouse Tracking to GraphPaper

Now we know enough to add mouse tracking to GraphPaper. The modifications will involve the following four parts:

• Modifying the initWithFrame: method to set up the tracking rectangle around the on-screen ColorGraphView instance

• Adding a mouseEntered: method that will tell the NSWindow to start sending mouse-moved events

• Adding a mouseMoved: method to process mouse-moved events

• Adding a mouseExited: method that will reset the window’s event-handling status

Rather than adding this new functionality to the GraphView or ColorGraphView classes, we’ll subclass ColorGraphView to make a TrackingGraphView class. Tracking functionality is separate from graphing functionality, and it makes sense to separate them in the code.

Let’s get on with it!

Changes to the GraphPaper Interface

1. Back in IB, make sure that MainMenu.nib is open and is the selected window (again, we recommend that you minimize the other nibs that are open in IB).

2. Subclass the ColorGraphView class and rename the new subclass “TrackingGraphView”.

3. Add outlets called xCell and yCell to the TrackingGraphView class.

4. Change the class of the ColorGraphView instance in the GraphPaper window to TrackingGraphView in the Class Info dialog.

5. Verify that the Graph button is still connected to the TrackingGraphView instance. Make sure it sends the graph: action.

6. Add two NSTextField objects inside the GraphPaper window and label them “x:” and “y:”, as shown in Figure 18-1. It may be necessary to resize the ...