Implement the takeFloatValueForVolumeFrom: Method

The text field and slider both send a takeFloatValueForVolumeFrom: message to the app delegate. But the method doesn’t update the track volume or the other UI elements yet.

To implement the takeFloatValueForVolumeFrom: method

• In the AppDelegate.m file, change the implementation of the takeFloatValueForVolumeFrom: method as follows:

  - (IBAction)takeFloatValueForVolumeFrom:(id)sender {

  float newValue = [sender floatValue];

  [self.track setVolume:newValue];

  [self updateUserInterface];

  }

There are several pieces to this method:

• float newValue = [sender floatValue];

  This line retrieves the sender’s float value and stores it in a local variable. The sender is the text field or slider that sent this message.

• [self.track setVolume:newValue];

  This line sets the track’s volume to the new value.

• [self updateUserInterface];

  This line invokes a new method to ensure that the user interface is consistent with the value of the track’s volume.

  The method doesn’t exist yet. You could put the code here, but similar functionality will be required in the mute: method; placing the code in its own method means you need write it only once.

Notice that Xcode shows an error icon for the [self updateUserInterface]; statement. It does this because you haven’t declared or implemented the updateUserInterface method yet. You’ll do both next.

Implement the updateUserInterface Method

The information shown in the user interface needs to be kept synchronized with what is stored in the model. In this app, the volume level is shown in two places. It might be tempting to let one of them update the other directly, but doing so tends to be difficult to maintain and debug. The best way to keep the user interface consistent is to always use the value in the model object as the true value and synchronize everything in the user interface against it. In this implementation, you use the text field and slider outlets you created earlier.

To synchronize the user interface

1. In the AppDelegate.h file, declare the updateUserInterface method as follows:

   - (void)updateUserInterface;

2. In the AppDelegate.m file, just before the @end statement, implement the updateUserInterface method as follows:

   - (void)updateUserInterface {

   float volume = [self.track volume];

   [self.textField setFloatValue:volume];

   [self.slider setFloatValue:volume];

   }

   The method first retrieves the track’s volume, then sets that as the float value of both the text field and the slider.

3. Run the app.

   The app should behave as you expect. Moving the slider should update the value shown in the text field, and typing a new value in the text field (and clicking Return) should update the slider.

The next task is to implement the action method for the button.

Implement the mute: Method

The implementation of the mute: method follows the same pattern as the takeFloatValueForVolumeFrom: method, except that the track’s volume is set to zero.

To implement and test the mute: method

1. In the AppDelegate.m file, change the implementation of the mute: method as follows:

   - (IBAction)mute:(id)sender {

   [self.track setVolume:0.0];

   [self updateUserInterface];

   }

2. Run the app.

   Pressing the Mute button should now set the track volume, text field, and slider values to zero.

Although the app appears to be working correctly now, there is one subtle bug. In setting up the nib file, you entered starting values for the text field and slider (both were set to 5). When the app starts, however, the track is created and its volume is set to zero. You need to ensure that the user interface and model values are properly synchronized at launch.

Make the User Interface Consistent at Launch

To ensure that the user interface is consistent with the model values at launch, you can invoke the updateUserInterface method after creating the track object.

To ensure that the app is consistent at launch

1. In the AppDelegate.m file, update the applicationDidFinishLaunching: method to invoke updateUserInterface after setting the track property.

   - (void)applicationDidFinishLaunching:(NSNotification *)aNotification {

   Track *aTrack = [[Track alloc] init];

   [self setTrack:aTrack];

   [self updateUserInterface];

   }

2. Run the app.

   The app should generally behave as it did before, except that when it launches it displays a value of 0 rather than 5.

Recap

You implemented the methods of the app delegate that provide the app’s core functionality—updating multiple views when the model data changes.

The next task is to look more closely at how the app behaves to see whether the user experience might be improved.