Dependency Injection in React Native modules

Question

We're going through a gradual conversion of my application to React Native. And I keep getting into problems with Dependency Injection in React Native on iOS.

I have some services in my app I'd like to use in a native module. Currently, they are injected through Typhoon and everything works just fine.

However, react native itself initializes and maintains any native module as a singleton. That prevents me from letting Typhoon initialize them, and so I can't inject dependencies into them.

What can be done? Creating the RCTBridge myself is an option, but feels very low-level, and still need to figure out how to inject it into the UIView in the first place.

Answer 1

I'm not exactly sure why your question didn't receive more up-votes; I myself was struggling to answer the same question and thought I should hop on and answer my first StackOverflow question!

Digging around the RCTBridge class provided the answer. What you need to do is initialise a RCTBridge manually with an instance of a class that implements the RCTBridgeProtocol (and importantly the method 'extraModulesForBridge'; you could even implement this protocol in your View Controller if you wanted to.

// Initialise a class that implements RCTBridgeDelegate
// Be warned, RCTBridge won't store a strong reference to your delegate.
// You should there store this delegate as a property of your initialising class, or implement the protocol in the View Controller itself. 
id<RCTBridgeDelegate> moduleInitialiser = [[classThatImplementsRCTBridgeDelegate alloc] init];

// Create a RCTBridge instance 
// (you may store this in a static context if you wish to use with other RCTViews you might initialise.  
RCTBridge *bridge = [[RCTBridge alloc] initWithDelegate:moduleInitialiser launchOptions:nil];

// Initialise an RCTRootView
RCTRootView *rootView = [[RCTRootView alloc]
                     initWithBridge:bridge
                         moduleName:kModuleName
                  initialProperties:nil];

// Note that your class that implements RCTBridgeDelegate SHOULD implement the following methods and it might look something like this.

// This will return the location of our Bundle
- (NSURL *)sourceURLForBridge:(RCTBridge *)bridge
{
    return [NSURL URLWithString:@"http://localhost:8081/index.ios.bundle?platform=ios"];
}

// Importantly, this is the method we need to implement to answer this question
// We must return an array of initialised Modules
// Be warned, I am writing this off of the top of my head, so excuse any syntax errors there might be!
- (NSArray *)extraModulesForBridge:(RCTBridge *)bridge
{
   return @[[OurNativeModule alloc] initWithCustomInitialiser:customDependency]];
}

Edit: I added this to the React-native documentation. https://facebook.github.io/react-native/docs/native-modules-ios.html#dependency-injection

Answer 2

The code above works just fine. Here's the Swift 4 version of the code.

@objc(RNModuleInitialiser)
final class RNModuleInitialiser: NSObject {

    //Inject your dependencies here
    init() {

    }

}

extension RNModuleInitialiser: RCTBridgeDelegate {

    func sourceURL(for bridge: RCTBridge!) -> URL! {
        return URL(string: "http://localhost:8081/index.ios.bundle?platform=ios")!
    }

    func extraModules(for bridge: RCTBridge!) -> [RCTBridgeModule]! {

        var extraModules = [RCTBridgeModule]()

        //Initialise the modules here using the dependencies injected above

        return extraModules
    }

}

When initializing the bridge, pass the moduleInitialiser:

//Make sure to hold the strong reference to the moduleInitaliser

self.moduleInitialiser = RNModuleInitialiser()
self.bridge = RCTBridge(delegate: self.moduleInitialiser, launchOptions: nil)