WPF -- PasswordBox数据绑定方法

本文介绍下passwordbox进行数据绑定的方法，。

本文完整示例程序见github。

问题描述

passwordbox的password属性不是依赖属性，因此无法进行数据绑定。

解决办法

该问题的解决办法有多种，本文介绍如何通过添加附加属性解决该问题。

附加属性是说一个属性本不属于某个对象，但由于某种需求附加到该对象上，通过附加属性可以实现将属性与宿主解耦的目的。附加属性本质上就是依赖属性，只是它们在属性包装器和注册时有区别。注册附加属性使用registerattached方法，注册依赖属性使用register方法，这两个方法的参数差别并不大。

首先添加一个passwordboxbindinghelper类，该类包含一个附加属性（snippet：propa+两次tab），通过设置该属性的propertychangedcallback将改变通知到passwordbox.password，并通过添加对passwordbox.passwordchanged事件的响应来响应passwordbox.password的改变。有了该附加属性，即可进行数据绑定。

public static string getpasswordcontent(dependencyobject obj) => (string)obj.getvalue(passwordcontentproperty);

public static void setpasswordcontent(dependencyobject obj, string value) => obj.setvalue(passwordcontentproperty, value);

public static readonly dependencyproperty passwordcontentproperty =
    dependencyproperty.registerattached("passwordcontent", typeof(string), typeof(passwordboxbindinghelper),
    new propertymetadata(string.empty, onpasswordcontentpropertychanged));

private static void onpasswordcontentpropertychanged(dependencyobject d, dependencypropertychangedeventargs e)
{
    var box = d as passwordbox;
    box.passwordchanged -= onpasswordchanged;
    var password = (string)e.newvalue;
    if (box != null && box.password != password)
        box.password = password;
    box.passwordchanged += onpasswordchanged;
}

private static void onpasswordchanged(object sender, routedeventargs e)
{
    var box = sender as passwordbox;
    setpasswordcontent(box, box.password);
}

然后在view中使用该附加属性进行数据绑定，本文示例中主窗口包含一个passwordbox控件及一个button按钮：

// xaml 绑定附加属性
<window ...
        xmlns:local="clr-namespace:passwordboxbinding"
        title="passwordboxbinding" height="300" width="450" windowstartuplocation="centerscreen">

    <grid>
        <stackpanel horizontalalignment="center" orientation="horizontal">
            <passwordbox minwidth="200" height="30" borderbrush="lightgray" borderthickness="2"
                         local:passwordboxbindinghelper.passwordcontent="{binding password,mode=twoway}"/>
            <rectangle width="20"/>
            <button width="80" height="30" content="查看密码" command="{binding clickedcommand}"/>
        </stackpanel>
    </grid>
</window>

//xaml.cs 设置绑定源
public mainwindow()
{
    initializecomponent();
    this.datacontext = new mainwindowviewmodel();
}

最后创建viewmodel进行逻辑处理：

// viewmodel
public class mainwindowviewmodel : inotifypropertychanged
{
    public string password
    {
        get => _password;
        set
        {
            _password = value;
            onpropertychanged();
        }
    }

    public delegatecommand clickedcommand => _clickedcommand ?? (_clickedcommand = new delegatecommand { executeaction = onclicked });

    // 使用callermembername特性简化代码，并可以避免手动输入错误
    public void onpropertychanged([callermembername] string name = "") => propertychanged?.invoke(this, new propertychangedeventargs(name));

    private void onclicked(object o) => messagebox.show($"password: {password}");

    public event propertychangedeventhandler propertychanged;

    private delegatecommand _clickedcommand;
    private string _password;
}

// 实现icommand
public class delegatecommand : icommand
{
    public bool canexecute(object parameter) => canexecuteaction?.invoke(parameter) ?? true;

    public void execute(object parameter) => executeaction?.invoke(parameter);

    public event eventhandler canexecutechanged;

    public action<object> executeaction { get; set; }
    public func<object, bool> canexecuteaction { get; set; }
}