I am making a fitness app so I need a timer.I have done this in code-behind:

public partial class Timer : ContentView
    {
        private int seconds = 30;
        private System.Timers.Timer timer;
        public Timer()
        {
            InitializeComponent();
            timer = new System.Timers.Timer();
            timer.Interval = 1000;
            timer.AutoReset = true;
            timer.Elapsed  = new System.Timers.ElapsedEventHandler(OneSecondPassed);
            timer.Enabled = true;
        }
        private void OneSecondPassed(object source, System.Timers.ElapsedEventArgs e)
        {
            seconds--;
        }
       public string Time
        {
            get => seconds.ToString();
          
        }
   
    }

And then for the UI, I made a label and bound its text property to my Time property:

<Label  BindingContext ="{x:Reference this}"
              Text="{Binding Time}"/>
//"this" is a reference to my class

When I start the app, the timer remains 30. I know that "seconds" is for sure decreasing, so there must be a problem with the binding.I know I could've just updated the text property of the label inside OneSecondPassed , but I'd like to learn more about data binding.Help?

CodePudding user response：

As Jason said, implementing the INotifyPropertyChanged interface is a good choice.

For your better understanding, I wrote a runnable project that meets your requirements for your reference.

Here is the xaml code:

<StackLayout>
    <Label Text="{Binding DateTime}"
       FontSize="Large"
       HorizontalOptions="Center"
       VerticalOptions="Center"/>
</StackLayout>

Here is the cs code:

public partial class MainPage : ContentPage, INotifyPropertyChanged
{
    int dateTime;
    public event PropertyChangedEventHandler PropertyChanged;   
    public MainPage()
    {
        InitializeComponent();

        this.DateTime = 30;

        Device.StartTimer(TimeSpan.FromSeconds(1), () =>
        {
            if (DateTime > 0)
            {
                DateTime--;
            }
            return true;
        });
        BindingContext = this;
    }
    public int DateTime
    {
        set
        {
            if (dateTime != value)
            {
                dateTime = value;

                if (PropertyChanged != null)
                {
                    PropertyChanged(this, new PropertyChangedEventArgs("DateTime"));
                }
            }
        }
        get
        {
            return dateTime;
        }
    }
}