为何在渲染后分配ItemsControl项目的DataContext

<UserControl.Resources>
    <DataTemplate x:Key=\"myDataTemplate\">
        <Grid Margin=\"20\">

            <View:MyView Width=\"123\" Height=\"212\"/>

        </Grid>
    </DataTemplate>
</UserControl.Resources>

<Grid>
    <ItemsControl ItemsSource=\"{Binding MyVMs}\" ItemTemplate=\"{StaticResource myDataTemplate}\">
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <WrapPanel />
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
    </ItemsControl>
</Grid>