iOS + Swift, How to Redirect to Itunes Purchase Page

Open AppStore through button

Here. But I highly suggest you learn the basics of Swift!

UIApplication.sharedApplication().openURL(NSURL(string: "itms://itunes.apple.com/de/app/x-gift/id839686104?mt=8&uo=4")!)

If you wanna open the AppStore in Swift 5:

if let url = URL(string: "itms-apps://itunes.apple.com/app/id1629135515") {
    UIApplication.shared.open(url)
}

Is it allowed in iOS to redirect user to website for payment?

External payment is required for physical goods, you can't pay physical goods using your iTunes account. The only allowed way to pay for physical goods using the Apple infrastructure is using Apple Pay if it's available in your country:

(Excerpt taken from App Store Review Guidelines)

3.1.5(a) Goods and Services Outside of the App: If your app enables people to purchase goods or services that will be consumed outside of the app, you must use purchase methods other than in-app purchase to collect those payments, such as Apple Pay or traditional credit card entry.

Anyway Apple is quite picky about privacy, security and user experience, so try not to take the user out of the application and make the paying process as secure as you can. I would try to integrate whatever payment gateway you are using inside your app, or at least open a web view inside the app instead of sending the user to Safari with an external URL.

Is there a URL forwarding to the App Store review creator in iOS 11?

You need to add a name after the app parameter of your url (here I use 'itunes-u') :

https://itunes.apple.com/us/app/itunes-u/id\(appID)?ls=1&mt=8&action=write-review

Also note that the scheme seems to be working properly so you could instead call :

itms-apps://itunes.apple.com/us/app/itunes-u/id\(appID)?ls=1&mt=8&action=write-review

Redirect to application if installed, otherwise to App Store

There is no way to check for this.
However, there is a nice workaround.

The idea is basically this:

1. The first time you open your app, you open up mobile safari from within your app to a predefined URL on your server
2. On that URL you set up a cookie, like appInstalled to the users mobile safari
3. You then kick the user back to your app with your registered scheme (same as FB does with SSO)
4. All your email links point to your website, but on the website you check if the browser is mobile Safari and if the appInstalled cookie exists
5. If either the browser is not mobile Safari or the cookie is not found, you redirect to the AppStore, or stay in your webpage.
6. If the conditions of #4 are true, you redirect the user to your app with the registered scheme
7. If the app has been deleted by the user, so the custom url scheme fails, you have a fail-safe redirect to the appstore

The 2 last steps are explained on this SO post

How to link to apps on the app store

Edited on 2016-02-02

Starting from iOS 6 SKStoreProductViewController class was introduced. You can link an app without leaving your app. Code snippet in Swift 3.x/2.x and Objective-C is here.

A SKStoreProductViewController object presents a store that allows the
user to purchase other media from the App Store. For example, your app
might display the store to allow the user to purchase another app.

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From News and Announcement For Apple Developers.

Drive Customers Directly to Your App
on the App Store with iTunes Links
With iTunes links you can provide your
customers with an easy way to access
your apps on the App Store directly
from your website or marketing
campaigns. Creating an iTunes link is
simple and can be made to direct
customers to either a single app, all
your apps, or to a specific app with
your company name specified.

To send customers to a specific
application:
http://itunes.com/apps/appname

To send
customers to a list of apps you have
on the App Store:
http://itunes.com/apps/developername

To send customers to a specific app
with your company name included in the
URL:
http://itunes.com/apps/developername/appname

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Additional notes:

You can replace http:// with itms:// or itms-apps:// to avoid redirects.

Please note that itms:// will send the user to the iTunes store and itms-apps:// with send them to the App Store!

For info on naming, see Apple QA1633:

https://developer.apple.com/library/content/qa/qa1633/_index.html.

Edit (as of January 2015):

itunes.com/apps links should be updated to appstore.com/apps. See QA1633 above, which has been updated. A new QA1629 suggests these steps and code for launching the store from an app:

1. Launch iTunes on your computer.
2. Search for the item you want to link to.
3. Right-click or control-click on the item's name in iTunes, then choose "Copy iTunes Store URL" from the pop-up menu.
4. In your application, create an NSURL object with the copied iTunes URL, then pass this object to UIApplication' s openURL: method to open your item in the App Store.

Sample code:

NSString *iTunesLink = @"itms://itunes.apple.com/app/apple-store/id375380948?mt=8";
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:iTunesLink]];

iOS10+:

 [[UIApplication sharedApplication] openURL:[NSURL URLWithString:iTunesLink] options:@{} completionHandler:nil];

Swift 4.2

   let urlStr = "itms-apps://itunes.apple.com/app/apple-store/id375380948?mt=8"
    if #available(iOS 10.0, *) {
        UIApplication.shared.open(URL(string: urlStr)!, options: [:], completionHandler: nil)
        
    } else {
        UIApplication.shared.openURL(URL(string: urlStr)!)
    }

Launching App Store from App in Swift

You have too many protocols in your URL. Get rid of https: so the URL reads

itms-apps://itunes.apple.com/app/bars/id706081574

How to open app store in ios app?

Your URL can simply be:

https://itunes.apple.com/app/id<App ID Here>?mt=8

Better yet, use SKStoreProductViewController so you can show the app in an App Store page without the need to leave your app.

And as of iOS 10.3 you can SKStoreReviewController specifically to allow a user to post a review or rate an app.

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