Cross Platform C#
Similar to the UITableView in iOS and ListView in Android, the XF adaptation can be acclimated to accumulation images for use in added locations of an application.
I’ve consistently been anxious aback alive with images. I’m consistently accurate with how my applications use them, as images can be almost ample and apps will about download them via a cellular or apathetic Wi-Fi connection. So you ability not be so anxious with angel admeasurement aback it takes alone a few hundred milliseconds to download, but those milliseconds can add up aback your app is aggravating to download a ample cardinal of them.
In a cardinal of antecedent columns, I’ve focused on how to accomplish a Xamarin Forms (XF) appliance attending and act added like a built-in application. I’ll abide forth and this time I’ll attending at how you can use the Xamarin.Forms ListView to accumulation images so that you can use the images in added locations in your appliance with appliance achievement in mind.
Xamarin.Forms ListView is the agnate of the UITableView in iOS and the ListView in Android. It allows you to present a simple filigree of abstracts to the user. The user is able to calmly annal through the data, and bounden textual abstracts to it is additionally a snap.
One of the problems that I see with XF ListView is that there has alone been one way to calmly bind the angel in an ImageCell in a ListView, and that’s by casual the URL of the angel to the cell. ListView is afresh amenable for downloading the capacity of the URL, administration the capacity of the download, formatting the angel capacity and announcement the image.
The botheration is that I would like to download the bytes of an angel once, to a cache, and afresh accept the buried bytes accessible whenever the appliance needs them. For example, the golf appliance I’ve been developing forth with this commodity alternation displays a account of aggregation pictures. I would like it to be able to blow the cell, accept addition awning accessible and affectation the angel after activity to the server to get it. To do this, I charge to be able to accumulation the images. Because the appliance will accumulation the angel agreeable as a byte array, the appliance will charge to catechumen from a byte arrangement to an image, so there will charge to be a Xamarin.Forms angel advocate to handle the binding.
So why is this an issue? Here’s what I’ve appear up with:
There are abundant means to handle break the problems here. I will present one way and add some thoughts on how to advance the code. In this scenario, you do this by blockage the accumulation to see if you accept the angel already, downloading the all-important images, converting them to a byte array, autumn the images for approaching use, and appliance the byte array. Already the affairs has the byte array, the appliance will charge to catechumen the byte arrangement into an image. To do this, the cipher break bottomward into two areas:
Caching CodeThe caching cipher is adequately simple. The cipher food its capacity in an in-memory concordance in a concordance commodity that’s stored in the accepted App class. By autumn it and accessing it via the App class, the capacity are accessible all through the application. The afterward cipher holds the angel concordance in the Appliance class:
The aing footfall is to get the cipher to affectation the pictures of the teams. Teams and pictures are a simple accord in a one-to-many relationship. The aforementioned is accurate with teams and tournaments. A distinct clash will accept assorted teams. On top of that, a distinct aggregation can accept assorted pictures associated with it. The cipher in Listing 1 displays a bureaucracy of tournaments, teams and their pictures, by activity through these steps:
The ConverterThe aing footfall of the appliance is to catechumen the byte arrangement into an angel that can be displayed. This is the job of the converter, as apparent in Listing 2. In this example, it takes a alternation of ambit and afresh converts it into an ImageSource, and the ImageSource is afresh returned. The ImageSource is created by converting the byte arrangement by creating an in-memory stream, pointing the beck aback to the beginning, and afresh appliance the .FromSource changeless adjustment to catechumen the beck into an ImageSource.
The final allotment of the addle is the binding. The ListView, in this example, uses an ImageCell and angel that is actuality apprenticed in a ListView’s cell. With this as the basis, the afterward cipher will set the binding:
Please agenda that the appliance in Listing 3 does not accurately use an ImageCell. However, the corpuscle that’s acclimated inherits from the ImageCell and accordingly will act like it. The bounden advice becomes absorbing in that it has the corpuscle annul the angel on the server and afresh the app will re-query the Web account to get adapted images.
What Could This Cipher Do Better?This is some archetype code, and it works appropriately in the book that it’s acclimated in, which is an appliance with a almost baby cardinal of images that accept already been compressed. If there were a ample cardinal of images or beyond images, or this cipher ran on some earlier phones with a abate bulk of memory, it would run into anamnesis problems. If the appliance crashed, images would charge to be reloaded from the server. This cipher needs a abiding caching area to abundance images. Thankfully, there is a band-aid to this. iOS and Android accept the SQLite database anchored in anniversary OS. By appliance the database, you get that abiding apparatus to accumulation images amid restarts of the application. SQLite has several datatypes that are acceptable for this.
A added catechism is how to acknowledge to low-memory issues. XF by itself doesn’t handle low-memory issues — that’s the albatross of the basal OS. On iOS and Android, it would be accessible to apparatus the all-important methods to t low-memory letters in the basal OS, use the MessagingCenter to accelerate a bulletin up to Xamarin.Forms, and afresh to handle accepting rid of the all-important altar that are in memory.
SummaryIn this article, it has looked at how to accumulation images, bind the buried byte arrangement of images to the ListView, and attending at some added improvements to the application.
Thank you for your time.
Wallace (Wally) B. McClure has authored books on iPhone programming with Mono/Monotouch, Android programming with Mono for Android, appliance architecture, ADO.NET, SQL Server and AJAX. He’s a Microsoft MVP, an ASPInsider and a accomplice at Scalable Development Inc. He maintains a blog, and can be followed on Twitter.
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