Question

How to have a Slider in a class view controller that changes the volume of a song playing in the AppDelage, in Objective-C for iOS?
Here is my code in the .h AppDelegate

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>

@interface JARAppDelegate : UIResponder <UIApplicationDelegate>
{
    AVAudioPlayer *musicPlayer;
}

@property (strong, nonatomic) UIWindow *window;

- (void)playMusic;
- (void)setVolume:(float)vol;

@end

And in the .m:

- (void)playMusic
{

    NSString *musicPath = [[NSBundle mainBundle] pathForResource:@"The History of the World" ofType:@"mp3"];
    NSURL *musicURL =  [[NSURL alloc] initFileURLWithPath:musicPath];

    musicPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:musicURL error:nil];
    [musicPlayer setNumberOfLoops:-1];   // Negative number means loop forever
    [musicPlayer setVolume:1.0];

    [musicPlayer prepareToPlay];
    [musicPlayer play];
    NSLog(@"play");
}

- (void)setVolume:(float)vol
{
    [musicPlayer setVolume:vol];
}

and when ‘didFinishLaunchingWithOptions’ I call [self playMusic];
This works and plays the song I want a full volume! Then in a different class called SettingsViewControler:
The .h

#import <UIKit/UIKit.h>
#import "JARAppDelegate.h"

@interface JARSettingsViewController : UIViewController
{

}

@property (strong, nonatomic) IBOutlet UISlider *volumeSliderOutlet;

- (IBAction)volumeSliderActoin:(id)sender;

@end

The .m:

- (IBAction)volumeSliderActoin:(id)sender
{
     NSLog(@"Volume Changed");
     [JARAppDelegate setVolume:sender];
}

Volume Changed is logged everytime you move the slider up and down, and so it should be sending setVolume a value between 0.0 and 1.0. But I get an error that says “No known class method for selector ‘setVolume:’

Answer 1

This is because you are trying to invoke a class method when you call:

[JARAppDelegate setVolume:sender];

but this doesn’t exist. You have created an instance method.

Try creating a singleton for the AVAudioPlayer, then you could do the following:

[[AVAudioPlayer sharedInstance] setVolume:vol];