当我使用propertychanged将字符串绑定到我的XAML时,为什么我没有得到任何文本？C#Xamarin

Question

尝试通过字符串将文本绑定到我的xaml,并使用propertychanged函数根据int的值更改文本.这是我的代码:

XAML:

<Label Text = "{Binding Forename}" />

码:

public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged (string propertyName)
    {
        var changed = PropertyChanged;
        if (changed != null) {

            PropertyChanged (this, new PropertyChangedEventArgs (propertyName));    
        }

    }

    string info;
    int myInt = 0;

    public string Forename {

        get {
            return info;
        }
        set {
            if (myInt == 0) {
                info = value;
                OnPropertyChanged ("TextOne");
            }

            else if (myInt == 1)
            {
                OnPropertyChanged ("TextTwo");

            }
            else 
            {
                OnPropertyChanged ("TextThree");
            }
        }
    }

我现在没有文字.

Answer 1

那不是多么OnPropertyChanged()有效.您不应传递要作为参数显示的新文本,而应将新文本分配给info字段,并将属性的名称作为参数.

什么OnPropertyChanged(name)是告诉XAML属性name已更改,XAML将在该属性上获取该属性,并相应地更新显示的值.

public string Forename {

    get {
        return info;
    }
    set {
        if (myInt == 0) {
            info = value;
            OnPropertyChanged ("Forename");
        }

        else if (myInt == 1)
        {
            info = "TextTwo";
            OnPropertyChanged ("Forename");

        }
        else 
        {
            info = "TextThree";
            OnPropertyChanged ("Forename");
        }
    }
}

编辑

当你对应该投入的内容有一些疑问时protected virtual void OnPropertyChanged(string propertyName),这个功能就好了.当它被调用时,它将使用PropertyChangedEventHandler PropertyChangedto让事件的所有子程序员知道属性propertyName已经改变.XAML {Binding Forename}充当它的订阅者/侦听者PropertyChanged,并且一旦被告知它绑定的属性("Forename")已经改变就会更新它的文本.拨打电话时会发生什么OnPropertyChanged("Forename").

编辑2 似乎您可能忘记分配您的DataContext在XAML的代码隐藏中(将命名为mainwindow.xaml.cs)

InitializeComponent();
ViewModel vm = new ViewModel();
DataContext = vm;
vm.Forename = "TestString"

并将Forename-code的东西移到一个单独的类中,命名为ViewModel.像这样:

public class ViewModel{
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged (string propertyName)
    {
        var changed = PropertyChanged;
        if (changed != null) {
            PropertyChanged (this, new PropertyChangedEventArgs (propertyName));    
        }

    }

string info;
int myInt = 0;

public string Forename {

    get {
        return info;
    }
    set {
        if (myInt == 0) {
            info = value;
            OnPropertyChanged ("Forename");
        }

        else if (myInt == 1)
        {
            info = "TextTwo";
            OnPropertyChanged ("Forename");

        }
        else 
        {
            info = "TextThree";
            OnPropertyChanged ("Forename");
        }
    }
}
}