Based on that template we can create our own extension. There are two variants of the same data task method, we'll replicate this behavior. The other thing we need is a DownloadTaskPublisher struct, I'll show you the Swift code first, then we'll discuss the implementation details. A Publisher can send an Output or a Failure message to an attached subscriber. You have to create a new typealias for each type, since they both are generic constraints defined on the protocol level. Next, we'll store the session and the request objects for later use.
This method is responsible for attaching a new subscriber through a subscription object. A Publisher and a Subscriber is connected through a Subscription. The Publisher only creates the Subscription and passes it to the subscriber.
The Subscription contains the logic that'll fetch new data for the Subscriber. The Subscriber receives the Subscription, the values and the completion success or failure. Ok, time to create our subscription for our little Combine based downloader, I think that you will understand the relationship between these three objects if we put together the final pieces of the code.
A Subscriber has an Input and a Failure type. A subscriber can only subscribe to a publisher with the same types. This time we can't go with an associatedType, but we have to create a generic value that has a constraint on these requirements by using a where clause. The reason behind this is that we don't know what kind of Subscriber will subscribe to this subscription. It can be either a class A or B , who knows We have to pass a few properties in the init method, store them as instance variables be careful with classes, you should use weak if applicable.
Lastly we implement the value request method, by respecting the demand policy. The demand is just a number. It tells us how many values can we send back to the subscriber at maximum. In our case we'll have max 1 value, so if the demand is greater than zero, we're good to go. You can send messages to the subscriber by calling various receive methods on it.
You have to manually send the completion event with the. Also we have to move the downloaded temporary file before the completion block returns otherwise we'll completely lose it. This time I'm going to simply move the file to the application cache directory.
As a gratis cancellation is a great way to end battery draining operations. You just need to implement a custom cancel method. Let's say that there are 4 kinds of subscriptions. You can use the. If you choose this path you can use the. We then check if there is an error, if there is we call the completion with a.
If there is no error then we call the completion with. This makes it easier for us in the completion handler to handle the failed or successful result. Now that we have both methods created we need to call them.
Let's start with reading the local data. It is super simple to use the readLocalFile method. We pass through the file name and if there is data we will pass that data to our parse method. When the parse method runs and there are no errors, the title and description from our data. We have create a new variable called urlString , this is just to have things look a bit cleaner when we call loadJson. Next we call our loadJson method. Inside the closure we switch on the result.
The two cases that we have are. If the result is successful, we will pass the data that we get from our loadJson method and pass it to our parse method. If loadJson failed, we will print out the error that it passed to this closure. Now that all the code is done you can build and run the app. The content from both the local and the hosted files should be printed in your console.
Before it would have been a bit of an irritation, but codeable makes a world of difference, and with just a few lines we are able to read a JSON file from anywhere. In fact, saving data and saving a string is very similar.
This is how we can save data Note: jsonString is being used from the above section :. If you have read the previous section on how to save a JSON string to a file then this section is very similar, if you only came to see how to save JSON data, then I will go through the above code now. String has a built in method that allows us to convert it to Data. Next we get the document directory. To do this we use FileManager. This will return an array, we only want the first item in the array.
Now that we have the document directory we will create a constant called, pathWithFileName. We will use this to append a file name to the documentDirectory. I have called this file myJsonData.
Lastly we can save the file. Data has a built in method called write to:.
0コメント