iOS tutorial 1: Link against GStreamer

This page last changed on May 06, 2013 by xartigas.

Goal

The first iOS tutorial is simple. The objective is to get the GStreamer version and display it on screen. It exemplifies how to link against the GStreamer library from Xcode using objective-C.

Hello GStreamer!

The code for this project can be found in the tutorials folder of your GStreamer SDK iOS installation. It was created using the GStreamer Single View Application template. The view contains only a UILabel that will be used to display the GStreamer's version to the user.

The User Interface

The UI uses storyboards and contains a single View with a centered UILabel. The ViewController for the View links its label variable to this UILabel as an IBOutlet.

ViewController.h

#import <UIKit/UIKit.h>
 
@interface ViewController : UIViewController {
    IBOutlet UILabel *label;
}

@property (retain,nonatomic) UILabel *label;

@end

The GStreamer backend

All GStreamer-handling code is kept in a single Objective-C class called GStreamerBackend. In successive tutorials it will get expanded, but, for now, it only contains a method to retrieve the GStreamer version.

The GStreamerBackend is made in Objective-C so it can take care of the few C-to-Objective-C conversions that might be necessary (like char * to NSString *, for example). This eases the usage of this class by the UI code, which is typically made in pure Objective-C. GStreamerBackend serves exactly the same purpose as the JNI code in the Android tutorials.

GStreamerBackend.m

#import "GStreamerBackend.h"

#include <gst/gst.h>

@implementation GStreamerBackend

-(NSString*) getGStreamerVersion
{
    char *version_utf8 = gst_version_string();
    NSString *version_string = [NSString stringWithUTF8String:version_utf8];
    g_free(version_utf8);
    return version_string;
}

@end

The getGStreamerVersion() method simply calls gst_version_string() to obtain a string describing this version of GStreamer. This Modified UTF8 string is then converted to a NSString * by NSString:stringWithUTF8Stringand returned. Objective-C will take care of freeing the memory used by the new NSString *, but we need to free the char * returned by gst_version_string().

The View Controller

The view controller instantiates the GStremerBackend and asks it for the GStreamer version to display at the label. That's it!

ViewController.m

#import "ViewController.h"
#import "GStreamerBackend.h"

@interface ViewController () {
    GStreamerBackend *gst_backend;
}

@end

@implementation ViewController

@synthesize label;

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    gst_backend = [[GStreamerBackend alloc] init];

    label.text = [NSString stringWithFormat:@"Welcome to %@!", [gst_backend getGStreamerVersion]];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

Conclusion

This ends the first iOS tutorial. It has shown that, due to the compatibility of C and Objective-C, adding GStreamer support to an iOS app is as easy as it is on a Desktop application. An extra Objective-C wrapper has been added (the GStreamerBackend class) for clarity, but calls to the GStreamer framework are valid from any part of the application code.

The following tutorials detail the few places in which care has to be taken when developing specifically for the iOS platform.

It has been a pleasure having you here, and see you soon!

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