How to open other applications like phone dialer, SMS, Safari, Google Maps, iTunes or AppStore or any other iPhone application?

“openURL” is the API to use to achieve any of the above and many more.
If you need to let your application users quickly dial APPLE from within your app, just include the following line

[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"tel:8006927753"]];

Similarly, you could initiate SMS application by using openURL with “sms: “, but unfortunately you cannot fill in the SMS content as Apple does not yet support it.
There are many more things you could do with this API like launching Google Maps, Safari with any website like you want or even the AppStore links to your apps which you want the users to buy.

Checking support for URLs of other applications
Until the release of iPhone OS 3.0 it was not possible to find if there are any applications installed on the device which handle the URL scheme you are about to use. But now it is possible through the following API

- (BOOL)canOpenURL:(NSURL*)url

Call the above API before using “openURL” to make sure your call will “certainly” be handled by a registered application. If you were developing an iphone app which lets a phone call be made from within it, do the following

// this will fail if the device is an iPod
if ([[UIApplication sharedApplication] canOpenURL:[NSURL URLWithString:@"tel:111"]])
{
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"tel:111"]];
}
else
{
// show an Alert here indicating that the call can’t be completed
}

Apart from using “canOpenURL”, do check the return value from “openURL” too and take appropriate action.

You should also use “canOpenURL” to find out if any features (like calling a number and SMS) that are not accessible on iPod should be disabled or hidden. Showing “phone” icons and letting them be clicked in your application on an iPod is a sure shot way to get your application rejected for uploading to AppStore.

Registering and handling custom URL schemes
If you are developing a great tool and can be used by other applications, you can register custom URL schemes and implement them. To create custom URL schemes just add them to your application’s “info.plist” file.

• To the root element of your application’s “info.plist” file, and a row with key named “URL types”.
• Expand it to find “item1″ in it, expand it and edit the value for key “URL identifier” to look something like “com.yourcompany.yoururlscheme”
• Add another row to “item1″ with key “URL Schemes”. Type in the first few characters of your URL scheme.
  For an example, if you wish to use url formats like abc://part1/part2, just enter “abc” for URL Schemes

Once you have created and registered your own URL formats/schemes, you can handle incoming URL requests by implementing the following UIApplicationDelegate method

- (BOOL)application:(UIApplication*)application handleOpenURL:(NSURL*)url

Before parsing and/or handling the URL, make sure it is in the right format and follows all rules of your URL scheme.

The above method was the only place where you can handle incoming URLs until iPhone OS 3.0 arrived. For iPhone OS 3.0 and above, the preferred place to handle incoming URLs is

- (BOOL)application:(UIApplication*)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions

This method should return whatever “handleOpenURL” API would have returned if it were implemented. If you implement the above delegate method of UIApplication, IPhone OS will not invoke the following methods

- (void)applicationDidFinishLaunching:(UIApplication*)application
- (BOOL)application:(UIApplication*)application handleOpenURL:(NSURL*)url