问题场景：

笔者自定义了一个GridContainer控件，并给GridContainer定义了一个Items依赖属性，把要放到gridContainer里显示的内容传递过去。当GridContainer所在的UserControl通过Xaml创建时一切都正常，但是当UserControl在通过代码创建时，发现总是找不到style里的Grid元素。后来查找资料才知道，这个时候是因为强制使用ApplyTemplate()方法。

//用于传递内容的依赖属性
public static readonly DependencyProperty ItemsProperty = DependencyProperty.Register(nameof(Items), 
            typeof(IEnumerable<GridElement>), typeof(GridContainer), 
            new PropertyMetadata(default(IEnumerable<GridElement>), new PropertyChangedCallback(CallMeWhenItemsFilled)));

//grid container style
<Style TargetType="{x:Type customControls:GridContainer}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate>
                    <Grid x:Name="Container" />
                </ControlTemplate>
            </Setter.Value>
        </Setter>
</Style>

private static void CallMeWhenItemsFilled(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
            var items = e.NewValue as IEnumerable<GridElement>;
            var gridElements = (items ?? Array.Empty<GridElement>()).ToList();
            if (d is GridContainer gridContainer)
            {
//代码会卡在这里，总是找不到"container"元素
//gridContainer.GetTemplateChild("Container") 总是返回空值
//但是在gridContainer.Template通过Visual Studio调试的时候可以看到ChildNames里有这个元素


gridContainer.ApplyTemplate(); //这句是关键，加上这个后就可以再次找到"Container"元素

if (gridContainer.GetTemplateChild("Container") is Grid grid)
                {
                    grid.ColumnDefinitions.Add(new ColumnDefinition {Width = GridLength.Auto});
                    grid.ColumnDefinitions.Add(new ColumnDefinition
                    {
                        Width = new GridLength(1,GridUnitType.Star)
                    });
                    var counter = 0;
                    foreach (var frameworkElement in gridElements.Select(ResolveGridElementControls))
                    {
                        grid.RowDefinitions.Add(new RowDefinition
                        {
                            Height = GridLength.Auto,
                        });
                        frameworkElement.Item1.SetValue(Grid.RowProperty, counter);
                        frameworkElement.Item1.SetValue(Grid.ColumnProperty, 0);
                        frameworkElement.Item2.SetValue(Grid.RowProperty, counter);
                        frameworkElement.Item2.SetValue(Grid.ColumnProperty, 1);

                        grid.Children.Add(frameworkElement.Item1);
                        grid.Children.Add(frameworkElement.Item2);
                        counter++;
                    }
                }
}