如何在 Mvvmcross 中使用 CanExecute [英] How to use CanExecute with Mvvmcross

问题描述

我有一个按钮

<Button 
      android:id="@+id/ButtonConnect"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:text="Disconnect"
      local:MvxBind="{'Click':{'Path':'DisconnectCommand'}}" />

我有一个命令

public IMvxCommand DisconnectCommand
{
    get
    {
        return new MvxRelayCommand(this.GetService<IConnectionService>().Disconnect);
    }
}

然后我想启用/禁用 DisconnectCommand 使用

Then I want to enable/disable the DisconnectCommand using

DisconnectCommand.CanExecute(this.GetService<IConnectionService>().UsbConnected);

但这显然是错误的(它不起作用)，我将检查作为参数输入，但通常我会这样做

But thats clearly wrong (It is not working), I put the check in as a a parameter, but normally I would do

DisconnectCommand.CanExecute = someBool;

但是没有要设置的属性，那么如何处理呢?

But there is no property to set, so how to go about this?

推荐答案

要了解如何使用 CanExecute，请查看 Silverlight 或 WPF - 有很多博客讨论如何使用使用 ICommand - 例如http://weblogs.asp.net/nmarun/archive/2009/12/02/using-icommand-silverlight-4.aspx 或 http://blog.galasoft.ch/archive/2009/09/26/using-relaycommands-in-silverlight-and-wpf.aspx

To work out how to use CanExecute, take a look at Silverlight or WPF - there's lots of blogs out there which talk about how to use ICommand - e.g. http://weblogs.asp.net/nmarun/archive/2009/12/02/using-icommand-silverlight-4.aspx or http://blog.galasoft.ch/archive/2009/09/26/using-relaycommands-in-silverlight-and-wpf.aspx

一个例子是:

private MvxRelayCommand _disconnectCommand;
public IMvxCommand DisconnectCommand
{
    get
    {
        if (_disconnectCommand == null)
            _disconnectCommand = new MvxRelayCommand(this.GetService<IConnectionService>().Disconnect, item => this.IsItemConnected(item));
        return _disconnectCommand;
    }
}

private void SomeServiceNotificationHandler()
{
    _disconnectCommand.RaisePropertyChanged();
}

private bool IsItemConnected(object thing)
{
    return /* your code */;
}

<小时>

不过有一个小问题......

---

There is one small problem though....

CanExecute 并没有真正在所有平台上的所有 MvxBindings 中完全实现......它适用于其中一些，但对于其中一些它不会 - 而我没有真的知道目前有哪些！如果您遇到问题，请告诉我(通过 GitHub 问题)，他们会得到解决...

CanExecute isn't really fully implemented across all the MvxBindings across all platforms... It will work for some of them, but for some of them it won't - and I don't really know which ones at present! If you come across issues, then please let me know (via GitHub issues) and they will get fixed...

就个人而言...我不倾向于使用 CanExecute - 我倾向于使用单独的布尔属性，然后我将其绑定到控件上可用的任何属性 - 例如大多数控件都有类似 Enabled、IsEnabled、Disabled、IsDisabled 等.

Personally... I don't tend to use CanExecute - I tend instead to use a separate Boolean property which I then bind to whatever property is available on the control - e.g. most controls have something like Enabled, IsEnabled, Disabled, IsDisabled, etc.

我通常发现设置布尔属性比调用RaiseCanExecuteChanged

I generally find it easier (and more readable) to set the Boolean property rather than to call RaiseCanExecuteChanged

例如我会使用类似的东西:

e.g. I'd use something like:

<Button 
  android:id="@+id/ButtonConnect"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:text="Disconnect"
  local:MvxBind="{'Click':{'Path':'DisconnectCommand'},'Enabled':{'Path':'UsbConnected'}}" />

您可以肯定地认为 CanExecute 方法具有优势 - 因为它将命令逻辑全部保存在一个对象中，并且因为它可用于防止发生 Execute 调用在 RelayCommand 中.这就是为什么我很乐意尝试修复 mvvmcross 绑定中的 CanExecute 错误.

You can definitely argue that the CanExecute approach has advantages - because it keeps the Command logic all in one object, and because it can be used to prevent Execute calls happening within the RelayCommand. That's why I'm happy to try to fix CanExecute bugs in mvvmcross bindings as we find them.

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