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Month: September 2017

DispatchGroup with Swift

Sometimes you have a need to wait on several asynchronous tasks before doing something in your code. DispatchGroup is a powerful API that lets you group together these tasks into one task.

DispatchGroup allows for aggregate synchronization of work. You can use them to submit multiple different work items and track when they all complete, even though they might run on different queues. This behavior can be helpful when progress can’t be made until all of the specified tasks are complete.

For example you have a ViewController that shows data from three different APIs. You need data from all 3 API’s before you can render the page. One way to do this would be to chain network calls and render the page after the 3rd API completed. But this leads to ugly nest code and requires that the APIs be called synchronously, rather than asynchronously.

Let’s looks look at how to solve this problem by chaining first.

I’ll be using httpbin.org for these examples. It’s a great tool for testing HTTP requests and responses.

This URL allows you to make an HTTP request that will wait a duration in seconds. For example, this will take 10 seconds to load:
https://httpbin.org/range/1024?duration=10

I’m using this API so you can easily see the time difference.

I wrote a simple method called makeNetworkRequest(), which take a duration in seconds and uses NSURLSession to call httpbin.org:

    func makeNetworkRequest(duration:Int, completion: @escaping (_ result: String?) -> Void) {
        let config = URLSessionConfiguration.default
        let session = URLSession(configuration: config)
        let url = URL(string: "https://httpbin.org/range/1024?duration=\(Int(duration))")!
        NSLog("makeNetworkRequest() \(url)")
        let task = session.dataTask(with: url) { (data, response, error) in
            if error != nil {
                NSLog(error!.localizedDescription)
                completion(nil)
            } else {
                if let str = String(data: data!, encoding: String.Encoding.utf8) {
                    completion(str)
                } else {
                    completion(nil)
                }

            }
        }
        task.resume()
    }

 

Here is an example using nesting:

makeNetworkRequest(duration: 2) { (str) in
	NSLog("Request #1 \(str ?? "nil"))\n")

	makeNetworkRequest(duration: 3) { (str) in
		NSLog("Request #2 \(str ?? "nil"))\n")

		makeNetworkRequest(duration: 10) { (str) in
			NSLog("Request #3 \(str ?? "nil"))\n")
			
			NSLog("Done!")
		}
	}
}

As you can see this accomplishes a solution to our problem, but there is a lot of nested code and called the API’s synchronously. It took 15 seconds to wait for all 3 responses.

Now let’s try this with DispatchGroup. With DistpatchGroup you first create a DistpachGroup:

let group = DispatchGroup()

For each block of code you want to execute you call:

group.enter()

Before the API and then

Group.leave()

When it finishes executing.

Finially group.notifiy(…) will be called when all group.enter() calls have a successful group.leave() response:

group.notify(queue: DispatchQueue.global(qos: .background)) {
NSLog(“All 3 network reqeusts completed”)
completion(strings)
}

Here’s a complete example:

func makeNetworkRequests(completion: @escaping (_ result: String?) -> Void) {
        let group = DispatchGroup()
        var strings = "start"
        
        group.enter()
        makeNetworkRequest(duration: 2) { (str) in
            NSLog("Request #1 \(str ?? "nil"))\n")
            if let str = str {
                strings = strings + str
            }
            group.leave()
        }
        
        group.enter()
        makeNetworkRequest(duration: 3) { (str) in
            NSLog("Request #2 \(str ?? "nil"))\n")
            if let str = str {
                strings = strings + str
            }
            group.leave()
        }
        
        group.enter()
        makeNetworkRequest(duration: 10) { (str) in
            NSLog("Request #3 \(str ?? "nil"))\n")
            if let str = str {
                strings = strings + str
            }
            group.leave()
        }
        
        group.notify(queue: DispatchQueue.global(qos: .background)) {
            NSLog("All 3 network reqeusts completed")
            completion(strings)
        }
    }

Notice how much easier this code is to read. No more nested blocks. It’s also asynchronous. It only takes 10 seconds for all 3 to return, because we’re only waiting on the longest API to respond.

You can find a sample application using this API here:

https://github.com/dougdiego/DispatchGroupDemo

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Cocoapods – Unable to find a specification for

When trying to install CodableAlamofire, I kept getting this error

[!] Unable to find a specification for `CodableAlamofire` depended upon by…

I check and the pod existed in the CocoaPods Specs.  I also noticed it said it was updated (in this case added) 2 days ago:

The problem was that my CocoaPods was out of date.  I ran this command to update it:

pod repo update master

After updating and running pod install again, I was able to install it.

Installing CodableAlamofire (1.0.0)
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D.L. Bliss Camping Tips

One of our favorite campsites is D.L. Bliss State Park. After camping there many times, here are a few tips we have learned over the years.

Map not to Scale. When you arrive and check in, the ranger will give you a map of the campsite. There are 3 main loops at Bliss and the beach. The map shows this. Make sure you note the text that says “Not to scale”. The loops are not close to each other. Our first time at Bliss we stayed at the top loop. We looked at the map and saw the beach was just below the 2 lower loops. We decided it would be a quick walk. We packed out gear and started walking down. Needless to say, it was not a quick walk. It’s walkable, just be aware.

Bear Boxes. All food and any scented item must go in the bear box (not your car). This includes soaps, lotions, gum, etc. There are bears. We’ve seen them. They are not as aggressive as in Yosemite, but do exist. Make sure all your stuff will fit in the bear box. The inside dimensions of the food lockers 36″ deep, 43″ wide, and 22″ high. Make sure your cooler(s) fits. And if you’re sharing, make sure whoever is sharing stuff will all fit together in 1 bear box.  Note: if you have a Yeti cooler or other bear-proof cooler, you can leave it outside the bear box as long as you have locks on it.

Ice. Tahoe gets warm during the summer. The metal bear boxes heat up in the sun and get really hot. The ice in your cooler melts quickly. Luckily the campsite host sells ice. When it’s hot, we refresh it daily.  Sometime the camp host will drive around in their ATV’s and sell ice and firewood. Fresh ice is also good if you want to plan on making cocktails.

Water, Bathroom and Showers. Each loop has a bathroom and showers. The showers take coins instead of quarters. You must buy these coins from the ranger kiosk when you enter the park. There are water spigots at about every couple of campsites. We drink the water from here. It’s drinkable and tastes good.

Beach. One of our favorite parts of D.L. Bliss is the beach. There is a small beach with a parking lot. One side of the beach is houses and the other side is rocks. On a busy day the beach will fill up fast. People come early and setup camp to reserve a spot on the beach. It’s best to go down around 7am and setup a spot on the beach. It’s best to setup a sunshade and some towels. If you’re early enough get one of the handful of picnic benches. There is a parking lot at the beach too. This also fills up quickly. On a busy day the lot is full by 8 or 9am. Another idea is to take your breakfast to the beach. You can setup on the beach and then make breakfast there or at the picnic tables by the parking lot.

Paddle Boards and Kayaks. Bliss is a great spot for Paddle Boards and Kayaks. There is a rocky cove just to the right of the beach. It’s fun spot to paddle around. The rocks are a popular spot for jumping into the water. It’s also popular with boats, so be careful.

Yellow Jackets. Towards the end of the camping season, the yellow jackets can be bad. One year (2016) was so bad we almost went home. The ranger suggested yellow jacket traps. We tried that and it didn’t do much. We found it best to just try and ignore them. They didn’t bite, but were just annoying.

Hiking. There are several good hikes around Bliss. The closest is the Rubicon Trail. It starts by the beach parking lot and is a 8.3 mile round trip to Emerald Bay. The gift shop at Emerald Bay sells popsicles and other packaged ice cream, which is a nice treat for the kids. Eagle Lake is another favorite hiking spot. The trailhead starts at the top of Emerald Bay. It’s a 1.8 mile round trip hike. There is a parking lot and you can park on the street, but during a busy weekend it fill early. I suggest being there by at least 8am. You start off by hiking next to a waterfall, then you cross it and then take a trail to Eagle Lake.  If you want to go further, there are lots of trails into Desolation Wilderness.

Cell Phone Reception.  There is some Cell Phone Reception at the lower campsites and beach.

South Lake Tahoe. It’s only 13 miles to South Lake Tahoe if you need to run out and get supplies.  With no traffic, this only takes about 25 minutes.  But on a busy weekend this could take over an hour.  The traffic gets bad because of people going to the beaches and Camp Richardson.  If you need to get out, I suggest doing it before 10am.

 

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