viewmodel和view控件的执行顺序

Question

I have a serious problem with the order of execution in MVVM: this code instaceiate the view (XAML):

<local:ExecutionView x:Name="ExecutionView" DataContext="{Binding ExecutionViewNodel}"/>

ExecutionView has inner field of viewmodel ( _viewModel = DataContext as ExecutionViewModel; ). In its CTOR it uses the viewmodel- _viewModel.DoSomthing();

problem is that it is null..

I cant control the timing of the binding- i have read a bit and there ways of handling the issue with (Loaded event and datacontextchanged)- still i think it is a workaround and NOT the way it should be done.

i need a way to control the steps where i can write a function that will be called once after the DataContext is certain to be valid.

BR, Mosh.

Answer 1

Instead of using the DataContext in your constructor, when it isn't quite available yet, subscribe to the DataContextChanged event in the constructor, and when your even handler is called, grab the reference to the new DataContext and perform whatever operations you need.

It's also possible to declare an event handler in your XAML, so the subscription to the event doesn't need to be in the constructor.

Here is a link to MSDN regarding that event.