当我尝试从 ViewModel 以编程方式设置工具栏徽章计数时，它不起作用

Question

我已经在 Xamarin 平台上的共享库中创建了两端和接口的依赖服务。 当我在 xaml 中使用它时.cs硬编码它可以工作，但是当我在 ViewModel 中使用它使用消息传递中心在应用程序工具栏中设置通知计数时，它不起作用 有没有人知道发生了什么或我应该做什么？

此代码适用于 Xaml.cs 页面

DependencyService.Get<IToolbarItemBadgeService>().SetBadge(this, ToolbarItems[0], "2", Color.Red, Color.White);

下面的代码不起作用

From ViewModel 
" MessagingCenter.Send(new MessagingCenterModel { }, "NotificationCount", NotificationListCount);
At Xaml.cs Page
MessagingCenter.Subscribe<MessagingCenterModel, string>(this, "NotificationCount", (sender, args) =>
{
DependencyService.Get<IToolbarItemBadgeService>().SetBadge(this, ToolbarItems[0], args, Color.Red, Color.White);
});

接口代码

public interface IToolbarItemBadgeService
{
void SetBadge(Page page, ToolbarItem item, string value, Color backgroundColor, Color textColor);
}

依赖关系服务

public class ToolbarItemBadgeService : IToolbarItemBadgeService
{
public void SetBadge(Page page, ToolbarItem item, string value, Color backgroundColor, Color textColor)
{
Device.BeginInvokeOnMainThread(() =>
{
var toolbar = CrossCurrentActivity.Current.Activity.FindViewById(Resource.Id.toolbar) as Android.Support.V7.Widget.Toolbar;
if (toolbar != null)
{
if (!string.IsNullOrEmpty(value))
{
var idx = page.ToolbarItems.IndexOf(item);
if (toolbar.Menu.Size() > idx)
{
var menuItem = toolbar.Menu.GetItem(idx);
BadgeDrawable.SetBadgeText(CrossCurrentActivity.Current.Activity, menuItem, value, backgroundColor.ToAndroid(), textColor.ToAndroid());
}
}
}
});
}
}

徽章可抽取类

public class BadgeDrawable : Drawable
{
private const string BadgeValueOverflow = "*";
private Paint _badgeBackground;
private Paint _badgeText;
private Rect _textRect = new Rect();
private string _badgeValue = "";
private bool _shouldDraw = true;
Context _context;
public override int Opacity => (int)Format.Unknown;
public BadgeDrawable(Context context, Color backgroundColor, Color textColor)
{
_context = context;
float textSize = context.Resources.GetDimension(Resource.Dimension.textsize_badge_count);
_badgeBackground = new Paint();
_badgeBackground.Color = backgroundColor;
_badgeBackground.AntiAlias = true;
_badgeBackground.SetStyle(Paint.Style.Fill);
_badgeText = new Paint();
_badgeText.Color = textColor;
_badgeText.SetTypeface(Typeface.Default);
_badgeText.TextSize = textSize;
_badgeText.AntiAlias = true;
_badgeText.TextAlign = Paint.Align.Center;
}
public override void Draw(Canvas canvas)
{
if (!_shouldDraw)
{
return;
}
Rect bounds = Bounds;
float width = bounds.Right - bounds.Left;
float height = bounds.Bottom - bounds.Top;
float oneDp = 1 * _context.Resources.DisplayMetrics.Density;
// Position the badge in the top-right quadrant of the icon.
float radius = ((Java.Lang.Math.Max(width, height) / 2)) / 2;
float centerX = (width - radius - 1) + oneDp * 2;
float centerY = radius - 2 * oneDp;
canvas.DrawCircle(centerX, centerY, (int)(radius + oneDp * 5), _badgeBackground);
// Draw badge count message inside the circle.
_badgeText.GetTextBounds(_badgeValue, 0, _badgeValue.Length, _textRect);
float textHeight = _textRect.Bottom - _textRect.Top;
float textY = centerY + (textHeight / 2f);
canvas.DrawText(_badgeValue.Length > 2 ? BadgeValueOverflow : _badgeValue,
centerX, textY, _badgeText);
}
// Sets the text to display. Badge displays a '*' if more than 2 characters
private void SetBadgeText(string text)
{
_badgeValue = text;
// Only draw a badge if the value isn't a zero
_shouldDraw = !text.Equals("0");
InvalidateSelf();
}
public override void SetAlpha(int alpha)
{
// do nothing
}
public override void SetColorFilter(ColorFilter cf)
{
// do nothing
}
public static void SetBadgeCount(Context context, IMenuItem item, int count, Color backgroundColor, Color textColor)
{
SetBadgeText(context, item, $"{count}", backgroundColor, textColor);
}
public static void SetBadgeText(Context context, IMenuItem item, string text, Color backgroundColor, Color textColor)
{

if (item.Icon == null)
{
return;
}
BadgeDrawable badge = null;
Drawable icon = item.Icon;

if (item.Icon is LayerDrawable)
{
LayerDrawable lDrawable = item.Icon as LayerDrawable;
if (string.IsNullOrEmpty(text) || text == "0")
{
icon = lDrawable.GetDrawable(0);
lDrawable.Dispose();
}
else
{
for (var i = 0; i < lDrawable.NumberOfLayers; i++)
{
if (lDrawable.GetDrawable(i) is BadgeDrawable)
{
badge = lDrawable.GetDrawable(i) as BadgeDrawable;
break;
}
}
if (badge == null)
{
badge = new BadgeDrawable(context, backgroundColor, textColor);
icon = new LayerDrawable(new Drawable[] { item.Icon, badge });
}
}
}
else
{
badge = new BadgeDrawable(context, backgroundColor, textColor);
icon = new LayerDrawable(new Drawable[] { item.Icon, badge });
}
badge?.SetBadgeText(text);
item.SetIcon(icon);
icon.Dispose();
}
}

Answer 1

首先，确保我们在发送消息之前已经Subscribe了消息。否则，将永远不会调用订阅中的代码。

此外，依赖服务中的代码只能在导航栏中设置ToolbarItem徽章(在选项卡式栏上它永远不会起作用(。

如果要设置选项卡式页面图标的徽章，可以使用插件 Plugin.Badge 。