Installation • Motivation • Structure • Examples • Routing Algorithm • Improvements • Issues • License

Portus

Portus is a UIKit- and architecture independent routing framework that easily integrates with your iOS apps. Usage enables enhanced features like in-app navigation or deeplinks.

Installation

Installation via Carthage & SwiftPM are both supported.

Carthage

Add the following to your Cartfile:

github "JamitLabs/Portus"

Motivation

Managing screenflow within iOS apps is complex, especially when dealing with advanced requirements like in-app navigation or deeplinks. Even though trivial solutions establish connections among screens, dealing with hard-coded links suffers from insufficient context information and scalabilty issues. Portus mitigates these issues and offers a defined framework for handling screen flow in a standardized way.

Screens within iOS apps are arranged on a stack or presented in succession to provide different level of information. Combined with user interface components, like sliding menus, navigation- or tab bars, paths are established that form the app's navigation tree. Each node in the tree either corresponds to a screen within the application or a branch node that is respondible for managing individual nodes.

Goals and Requirements

Development of this framework was driven by the following requirements:

UIKit Independence

First, the framework is designed to be independent of UIKit. This ensures that Portus does neither need to know about your app's view hierarchy nor about the relationship among view controllers. Rather the framework relies on a tree-based structure that consists of routing nodes, where each node states its routing capabilities by conforming to the Enterable, Leavable or Switchable protocol. Note that being independent of UIKit is crucial to meet the second requirement to be architecturally independent.

Architectural Independence

Second, Portus does not enforce an architectural pattern. Due to limitations of the MVC architecture, the community has come up with several alternatives, like FlowControllers, Coordinators, MVVM, MVVM+RxSwift. To seamlessly integrate with these architectures, we designed Portus to be architectural independent.

Context-Driven

Finally, nodes are entered in different contexts. For instance, the background color of a screen can differ depending on the value that is provided to the node. Hence, even though the requested node might already be open it might be necessary to reenter the node when the color value differs. With the combination of RoutingIdentifier and RoutingContext nodes are uniquely identified.

Structure

Portus uses three protocols, namely Enterable, Leavable and Switchable that inherit from the Routable protocol. Nodes can conform to these protocols and state their routing capabilities as well as whether they manage additional nodes.

Protocols

`Routable`

Conforming to the Routable protocol is required to specify a node's structure. The protocol requires a RoutingEntry that consists of the RoutingIdentifier as well as the RoutingContext and whether additional nodes are managed by the node. The latter requires to also specify an activeEntry that represents the managed node that is currently active. Below you find the definition of the Routable protocol:

protocol Routable: AnyObject {
    var entry: RoutingEntry { get }
}

`Enterable`

In addition to Routable, nodes can state whether they can enter additional nodes using the Enterable protocol:

protocol Enterable: Routable {
    func enterNode(
        with entry: RoutingEntry, 
        animated: Bool, 
        completion: @escaping ((Bool) -> Void)
    )
}

Enterables will receive the entry identifying the node to enter as well as wheter the entrance should be animated or not. Finally the node can indicate whether the entrance succeeded or failed by calling the completion. Note that calling the completion is mandatory.

`Leavable`

Leavable represents an additional routing capability. Routables conforming to this protocol can leave a node for a given entry. Similar to Enterable calling the completion is mandatory and notifies the routing framework whether the operation succeeded.

In addition, leavables can be asked whether they can leave a node for a given entry:

protocol Leavable: Routable {
    func leaveNode(
        with entry: RoutingEntry, 
        animated: Bool, 
        completion: @escaping (Bool) -> Void
    )
    func canLeaveNode(with entry: RoutingEntry) -> Bool
}

`Switchable`

Finally, nodes conforming to the Switchable protocol can determine one of their manage nodes to become active. Hence, nodes conforming to Switchable have to manage additional nodes.

protocol Switchable: Routable {
    func switchToNode(
        with entry: RoutingEntry, 
        animated: Bool, 
        completion: @escaping ((Bool) -> Void)
    )
}

Using above-stated protocols, nodes can be entered, left or become (in)active in a standardised way. Still, routing requires knowledge about the current state and hence requires a tree-based structure to keep track of the current screenflow.

`RoutingTree`

The RoutingTree is used by the RoutingAlgorithm to compute RoutingInstructions to either static or dynamic destinations. Note that the tree consists of individual nodes, where each RoutingNode may contain children. Still only one of a node's children can be active at the same time. This characteristic is crucial to compute an activePath that corresponds to the origin any routing request will start from.

The following operations can be performed on the RoutingTree and allow applications to provide information about when they enter, leave or switch to one of their managed nodes. This information is required by the RoutingAlgorithm to compute appropriate instructions to predefined destinations.

public func didEnterNode(withEntry entry: RoutingEntry)
public func didLeaveNode(with entry: RoutingEntry)
public func switchNode(
    withEntry entry: RoutingEntry, 
    didSwitchToNodeWithEntry targetEntry: RoutingEntry
)

`Router`

The Router represents the main component of Portus and is used by applications to either request a route to a StaticRoutingDestination, i.e., a destination that is predefined in the RoutingTable, or to enter a DynamicRoutingDestination, i.e., a destination that does not depend on the current context and can be entered from everywhere. While dynamic destinations only consist of a single node, static destinations are represented by a list of nodes, where each list starts with the application's root node. Below you can find the signature of the operations as offered by the Router:

public func routeTo(
    staticDestination: StaticRoutingDestination,
    animated: Bool = true,
    executedInstructions: RoutingInstructions = [],
    completion: ((Result<RoutingInstructions, RoutingError>) -> Void)? = nil
) {
    ...
}

public func enter(
    dynamicDestination entry: RoutingEntry, 
    animated: Bool = true, 
    completion: ((Bool) -> Void)? = nil
) {
    ...
}

To request a route, static- and dynamic destinations need to be predefined in the RoutingTable. The RoutingTable includes knowledge about all reachable destinations using the routing framework. For instance, when requiring a deeplink to a screen within your app, you add a static route, describing the screen's location relative to your app's routing tree. However, having to state static destinations in the RoutingTable at compile time does not mean that they can not adapt to context information known at runtime. Below examples includes both a static route to a colorList not requiring any parameters, as well as a route to the color details screen depending on a hexadecimal value representing the color that is shown in the details screen. This way, you can adapt static routes at runtime as soon as the required context information is known.

extension RoutingTable.StaticEntries {
    static let colorList = [.root, .colorList].entries
    static func colorDetail(withHexString hexString: String) -> [RoutingEntry] {
        return [
            RoutingEntry(identifier: .root),
            RoutingEntry(identifier: .colorList),
            RoutingEntry(identifier: .colorDetail, context: ["hex": "\(hexString)"])
        ]
    }

    ...
}

In comparison, dynamic routes represent a single destination that is reachable from everywhere within your app:

extension RoutingTable.DynamicEntries {
    static let a = RoutingEntry(identifier: .a)

    ...
}

Examples

We have included two demo applications, namely Portus-iOS-Demo and Portus-iOS-Demo2 to demonstrate the framework's ability to deal with different architectures. The first demo application uses an architectural pattern called Imperio, while the second sticks to the classic MVC approach. You can find visualizations of the example app's routing trees below:

Illustration, visualizing the routing tree of the first demo application, namely Portus-iOS-Demo:

Illustration, visualizing the routing tree of the second demo application, namely Portus-iOS-Demo2:

To include a non-trival structure, we included a PageViewController in the third tab of the second example app. The difficulty consists of keeping track of open paths when switching the active child of a switchable node. You can click through the sample app and open multiple screens each modifying the routing tree. Then, routes to static- as well as dynamic destinations are requested using the RoutingMenu as accessed using the device's shake-gesture.

Routing Algorithm

If you are interested in the underlying algorithm that is used by Portus to compute RountingInstructions, keep on reading. Otherwise, you may consider to skip this section.

The key component of the RoutingAlgorithm is computeNextRoutingInstructions(...). Using this method the Router computes the next chunk of routing instructions in an iterative process and executes them until the desired destination is reached. The signature of computeNextRoutingInstructions(...) is given as follows:

internal static func computeNextRoutingInstructions(
    to destination: StaticRoutingDestination,
    completion: @escaping (Result<RoutingInstructions, RoutingError>) -> Void
) {
    ...
}

Note that RoutingInstructions is a typealias for [RoutingInstruction], where a single instruction corresponds to one of the following cases: - enter(entry: RoutingEntry): represents a node to be entered - leave(entry: RoutingEntry): represents a node to be left - switchTo(entry: RoutingEntry, switchNodeEntry: RoutingEntry): represents a node to switch to

Computing the next chunk of routing instructions involves completing the following steps:

First, we need to check whether we are already at the destination. If this is the case, we are done and can return an empty list of instructions:
```
guard origin != destination else {
    completion(.success([]))
    return
}
```

Otherwise, i.e., when the destination is not yet reached, we determine the first mismatch index, i.e., the first index from which on the origin and destination path differ. Using this index, we can handle Leavables, Switchables and Enterables respectively.

 let firstMismatchIndex: Int? = {
    for index in 0 ... maxIndex {
        guard
            let originEntry = origin[try: index],
            let destinationEntry = destination[try: index],
            originEntry == destinationEntry
        else {
            return index
        }
    }

    return nil
}()

We need to ensure that all leavable nodes along the path starting from the node corresponding to the first mismatch index up to the active leaf that are not managed by their parent (as indicated by isManagedByParent) can be left. This way we can gurantee that the desired destination is reached from the current context. For each of these nodes we introduce a new routingInstruction using .leave(entry: ...):

var leavingRoutingInstructions: RoutingInstructions = []
let canLeaveCurrentContext: Bool = {
    for node in activePath.suffix(from: index).reversed() {
        guard let leavable = node.entry.routable as? Leavable, !node.isManagedByParent else { continue }

        guard leavable.canLeaveNode(with: node.entry) else { return false }

        leavingRoutingInstructions += [.leave(entry: node.entry)]
    }

    return true
}()

guard canLeaveCurrentContext else { return [] }
guard leavingRoutingInstructions.isEmpty else { return leavingRoutingInstructions }

Next, when all nodes could be left that do not match the destination path, we need to check whether the predecessor of the active leaf is Switchable and contains the target node as one of its children. In this case, we can set the active child to the target node, by introducing a new routing instruction of the form switchTo(...):

if
    let switchNode = (RoutingTree.default.root?.activePath() ?? [])[try: index - 1],
    switchNode.children.count > 1
{
    guard switchNode.children.map({ $0.entry.identifier }).contains(destination[index].identifier) else { return [] }

    return [.switchTo(entry: destination[index], switchNodeEntry: switchNode.entry)]
}

Otherwise, if there is no Switchable we simply enter the part of the destination path by introducing .enter(entry: ...):
```
if let enterEntry = destination[try: index] {
    return [.enter(entry: enterEntry)]
}
```
Finally, if the algorithm returns the next chunk of instructions as computed above, or indicates that the destination is not reachable using the following Error: destinationNotReachable

To illustrate this procedure, consider the following example:

Example:
- Origin: .switchNode1, .node2, .switchNode3, .node12
- Destination: .switchNode1, .node1, .node5, .node8
- Computed Instructions: .leave(.switchNode3) .switchTo(.node1, switchNode: .switchNode1) .enter(.node5) .enter(.node8)
Example:
- Origin: .switchNode1, .node1, .node5, .node8
- Destination: .switchNode1, .switchNode2, .node4
- Computed Instructions: .leave(.node8) .leave(.node5) .switchTo(.switchNode2, switchNode: .switchNode1) .switchTo(.node4, switchNode: .switchNode2)
Example:
- Origin: .switchNode1, .switchNode2, .node4
- Destination: .switchNode1, .node1, .node6, .node11
- Computed Instructions: .switchTo(.node1, switchNode: .switchNode1) .enter(.node6) .enter(.node11)
Example:
- Origin: .switchNode1, .node1, .node6, .node11
- Destination: .switchNode1, .node2, .switchNode3, .node13
- Computed Instructions: .leave(.node11) .leave(.node6) .switchTo(.node2, switchNode: .switchNode1) .enter(.switchNode3)

Note that we have included Unit Tests based on the presented example to ensure that RoutingInstructions are properly computed by the RoutingAlgorithm. You can find them in Portus/Tests/PortusTests.

Improvements

The following issues/enhancements can be addressed in future revisions:

Enhance the demo applications by including some fancy UI
Enhance the framework by proving a visualization of the current routing tree that can be used for debugging purposes.
Introduce different routing algorithms, other then the minimum route to destination.
Route to a given destination within a single animation, i.e. without animating each step in between
Simulate routing instructions before executing them. This way, developers using the framework can be informed that a destination is not reachable before execution.

License

This library is released under the MIT License. See LICENSE for details.