如何在Xamarin.Forms中获取以前的数据

Question

I am working on an application which is going to show updated dollar and euro rates for Turkey. I want to print green and red arrows depending on if rates went up or down since the last time user opened the app. So my question is how can I get previous data and how can I compare them with the current data?

CODE-BEHIND;

namespace Subasi.A.M.D
{
    public partial class MainPage : ContentPage
    {
        float banknoteSellingUSD = 0;
        float banknoteBuyingUSD = 0;

                public MainPage()
        {
            InitializeComponent();

            if (Device.OS == TargetPlatform.iOS)
                Padding = new Thickness(10, 50, 0, 0);
            else if (Device.OS == TargetPlatform.Android)
                Padding = new Thickness(10, 20, 0, 0);
            else if (Device.OS == TargetPlatform.WinPhone)
                Padding = new Thickness(30, 20, 0, 0);
        }






            private void Button_Clicked(object sender, EventArgs e)
            {


                XmlDocument doc1 = new XmlDocument();
                doc1.Load("http://www.tcmb.gov.tr/kurlar/today.xml");
                XmlElement root = doc1.DocumentElement;
                XmlNodeList nodes = root.SelectNodes("Currency");

                foreach (XmlNode node in nodes)
                {

                    var attributeKod = node.Attributes["Kod"].Value;
                    if (attributeKod.Equals("USD"))
                    {

                        var GETbanknoteSellingUSD = node.SelectNodes("BanknoteSelling")[0].InnerText;
                        var GETbanknoteBuyingUSD = node.SelectNodes("BanknoteBuying")[0].InnerText;
                     //if (banknoteSellingUSD > float.Parse(GETbanknoteSellingUSD)) isusdup = false;
                    //else isusdup = true;

                     banknoteSellingUSD = float.Parse(GETbanknoteSellingUSD);
                     banknoteBuyingUSD = float.Parse(GETbanknoteBuyingUSD);
                        labelUSDBuying.Text = banknoteSellingUSD.ToString("0.00");
                        labelUSDSelling.Text = banknoteBuyingUSD.ToString("0.00");




                } 

                    var attributeKod1 = node.Attributes["Kod"].Value;
                    if (attributeKod1.Equals("EUR"))
                    {
                        var GETbanknoteSellingEU = node.SelectNodes("BanknoteSelling")[0].InnerText;
                        var GETbanknoteBuyingEU = node.SelectNodes("BanknoteBuying")[0].InnerText;
                        var banknoteSellingEU = float.Parse(GETbanknoteSellingEU);
                        var banknoteBuyingEU = float.Parse(GETbanknoteBuyingEU);
                        labelEUSelling.Text = banknoteSellingEU.ToString("0.00");
                        labelEUBuying.Text = banknoteBuyingEU.ToString("0.00");





                    }

                }

            }
        }
    }

Answer 1

print green and red arrows depending on if rates went up or down since the last time user opened the app

To achieve this, you will have to store the previous value. The easiest way may be to use the properties dictionary ( see here ). You can store simple properties within that.

You could capsule the behavior in a class

public class ExchangeCourseSource : IExchangeCourseSource
{
    public ExchangeCourseSource(XmlDocument sourceDocument)
    {
        this.sourceDocument = sourceDocument;
    }

    public ExchangeCourse GetCourse(string currency)
    {
        // parse from XML (see your code)
    }

}

class ExchangeCourse
{
    public string Currency { get; set; }
    public double ExchangeRate { get; set; }
    public double Difference { get; set; }
}

and decorate this with a class that stores and retrieved the courses to and fro the properties dictionary

public class StoredExchangeCourseSourceDecorator : IExchangeCourseSource
{
    public ExchangeCourseSource(IExchangeCourceSource source, Application application)
    {
        this.source = source;
        this.application = application;
    }

    public ExchangeCourse GetCourse(string currency)
    {
        var exchangeCourse = source.GetCourse(currency);
        if(HasStoredCourse())
        {
            var storedCourse = GetStoredCourse(currency);
            exchangeCourse.Difference = exchangeCourse.ExchangeRate - storedCourse;
        }
        StoreCourse(exchangeCourse);
        return exchangeCourse;
    }

    private bool HasStoredCourse(string currency)
    {
        return application.Properties.ContainsKey(currency);
    }

    private double GetStoredCourse(string currency)
    {
        return (double)application.Properties[currency];
    }

    private void StoreCourse(ExchangeCourse exchangeCourse)
    {
        application.Properties[exchangeCourse.Currency] = exchangeCourse.ExchangeRate;
        application.SavePropertiesAsync().Wait();
    }
}

Answer 2

OK, so to answer the question, You have to store data somewhere, the easiest method will be in ISharedPreferences to save and restore data.

From AndroidDeveloper :

If you don't need to store a lot of data and it doesn't require structure, you should use SharedPreferences. The SharedPreferences APIs allow you to read and write persistent key-value pairs of primitive data types: booleans, floats, ints, longs, and strings.

The key-value pairs are written to XML files that persist across user sessions, even if your app is killed. You can manually specify a name for the file or use per-activity files to save your data.

So it's a good place to store some info and retrieve them.

All you have to do is to get an instance from ISharedPreferences and use ISharedPreferencesEditor to insert and retrieve data.

You find it in Android.Content Namespace

To save your data you can apply this code :

ISharedPreferences preference = PreferenceManager.GetDefaultSharedPreferences(this);
ISharedPreferencesEditor editor = preference.Edit();
editor.PutString("key", "Value");
editor.Apply();

In your case, you can PutFloat

So your data which is "Value" is saved with a key named "key" is now saved

then you can retrieve data by :

ISharedPreferences preference = PreferenceManager.GetDefaultSharedPreferences(this);
var a = preference .GetString("key", "null");//"null" is the default value if the value not found. and the key, it to retrieve a specific data as we stored the data with the key named "key"

In your case, use GetFloat So you get your value stored in a variable a.

All you have to do is : Store your data in the Sharedpreference when a new data changed or OnSleep() method which will be called when the app closed, then in OnCreate() method in your app, call the data saved in the SharedPreference and compare it with the new data.