Wednesday, 27 March 2019

Service Locator pattern in SPFx: Using Service Scopes

I have written about this topic in the past but with the recent increase in SharePoint Framework adoption and with more features becoming available (e.g. MSGraphClient, AadHttpClient), I felt it would be a good time to revisit this.

With SPFx solutions getting more complex day by day and with lots of components to manage, passing the web part (or extension) context around to different parts of your code can get difficult to maintain real fast.

The problem:


For example, imagine we have created a custom service which needs the MSGraphClient to make a call to the Microsoft Graph and we are consuming this service in our SPFx webpart. To initialise the service, we need to either pass in the entire web part context to it, or explicitly pass in the MSGraphClient object from the context.

In the first case, we are unnecessarily passing in all other objects in the context to this service as well.

And in the second case, our code becomes tightly coupled i.e. in the future, if our service needs something else from the webpart context, we have to update the service as well as the consuming code (i.e. the webpart) to pass in the new dependency.

The solution:


Using the Service Locator pattern in SPFx through service scopes, we can abstract away the implementation details of our custom services from the calling code (i.e. webparts, extensions).

With service scopes, we can get hold of instances of SPFx classes like MSGraphClient, AadHttpClient, SPHttpClient and PageContext from our services without having to explicitly pass them in from the webparts.



In this post, lets see how to achieve this:

1) Calling the MSGraphClient from a custom service:



Consuming the custom service from an SPFx webpart:


If you observe the code, our calling webpart does not have any knowledge of the implementation details of the custom service, specifically the fact that it used the MSGraphClient internally to retrieve the current user details. In the future, if we wanted to change the implementation of the service, we could do it without changing any web part code.

2) Calling the AadHttpClient from a custom service:


Similarly, we can also use service scopes to get hold of an instance of the AadHttpClient class. In the code below, to keep things simple, I am using the AadHttpClient to make a call to the Microsoft Graph. Eventually, the result is the same as the previous code but it should demo how to use the AadHttpClient through service scopes.


Consuming the custom service from an SPFx webpart:



3) Calling the SPHttpClient from a custom service:


And finally, if we just want to make a call to the SharePoint REST API from our SPFx code, we can also use service scopes to get hold of an instance of the SPHttpClient class. The following code also demonstrates how to get an instance of the PageContext class to get different run time context values like the current web url. Using the current web url, we can make a call to the SharePoint REST API to get the web details:


Consuming the custom service from an SPFx webpart:


And that's it for this post! I am planning more posts on this topic in the future so keep an eye out for those :)

As always, the code from this post is available on GitHub: https://github.com/vman/ServiceScopeTestBench

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