需要在绑定时设置属性变更触发

UpdateSourceTrigger=PropertyChanged

例如:

<Border CornerRadius="20">

            <Border.Background>

                <ImageBrush

                    x:Name="objBackImg"

                    ImageSource="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=local:AnimationVideoPlayer},Path=CoverImage,UpdateSourceTrigger=PropertyChanged}"/>

            </Border.Background>

</Border>