Three callbacks:

1 PropertyChangedCallback value callback

2 CoerceValueCallback mandatory callback

3 ValidateValueCallback validation callback

The dependency property contains three callbacks

public int YL{get { return (int)GetValue(YLProperty); }set { SetValue(YLProperty, value); }}// Using a DependencyProperty as the backing store for YL. This enables animation, styling, binding, etc...public static readonly DependencyProperty YLProperty =DependencyProperty.Register("YL", typeof(int), typeof(TestMode),new PropertyMetadata(0,new PropertyChangedCallback(OnPropertyChanged), // first value callback positionnew CoerceValueCallback(OnCoerceValueCallback) // third forced callback position),new ValidateValueCallback(OnValidateValueCallBack)//The second value validation callback position);static void OnPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e){MessageBox.Show($"The 3rd value callback old:{e.OldValue} new:{e.NewValue}");}static object OnCoerceValueCallback(DependencyObject d, object obj){MessageBox.Show($"The second mandatory callback ({obj})");if (int.Parse(obj.ToString()) > 1000)return 2000;return obj;}static bool OnValidateValueCallBack(object obj){var r = int.Parse(obj.ToString());MessageBox.Show($"The first value validation callback val:{r}");if (r > 100 && r < 200)return false;// When binding, the exception can be caught for promptingreturn true;}

Value callback, PropertyChangedCallback

The callback function is triggered when the value of the property changes.You can get e.OldValue and e.NewValue through the parameter e

At this point, we can also provide an "insurance" for PropertyChangedCallback: even if the property value changes,

However, this time may be outrageous, so don't trigger a "value callback" because it doesn't make sense.

The insurance at this time is, ValidateValueCallback validation callback.

(The value callback and validation callback are both parameters of the PropertyMetadata constructor.)

It needs to pass in a function that returns a bool type. If the function returns false, it means that the value callback should not be alerted.

The starting condition of ValidateValueCallback is that the property is assigned a value.

Last look at CoerceValueCallback force callback

It is a parameter in DependencyProperty.Register.

Before introducing this callback, let's take a look at the order of these three callbacks. When the property is assigned, the first is the verification callback,

If the validation callback returns false, nothing else happens to the other callbacks.If it returns true, the second trigger is Force callback,

It returns a value for its purpose, and this value will be directly assigned to the attribute (at this time, the attribute assignment will not start itself again. If the returned value is different from the previous one, the verification callback will be started again at this time.!And the trigger is also before the value callback)

The last is the value callback. If the verification callback returns true at this time, and the value of the mandatory callback is different from the previous value, then the value callback will be triggered at this time.

You can write a program to verify: