如何在 Xamarin 中删除 ViewTreeObserver? [英] How to remove ViewTreeObserver in Xamarin?

问题描述

假设我需要获取并设置视图的高度.在 Android 中，众所周知，只有在绘制后才能获得视图高度.如果您使用的是 Java，则有很多答案，其中最广为人知的方法之一如下所示，取自 this answer:

Let's just say I need to get and set a View's height. In Android, it's known you can get a view height only after it's drawn. If you're using Java, many answers, one of the most well-known way is like this one below, taken from this answer:

view.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
        @Override
        public void onGlobalLayout() {
            view.getViewTreeObserver().removeOnGlobalLayoutListener(this);
            view.getHeight(); //height is ready
        }
    });

因此我搜索 C#/Xamarin 版本，并发现此方法有效:

Thus I search C#/Xamarin version, and found this works:

int viewHeight = 0;
ViewTreeObserver vto = view.ViewTreeObserver;
vto.GlobalLayout += (sender, args) =>
{
    viewHeight = view.Height;
};

事情是，它一次又一次地开火.在 Java 版本中，它可以删除view.getViewTreeObserver().removeOnGlobalLayoutListener(this);

Thing is, it fired again and again. In Java version, it can be removed with view.getViewTreeObserver().removeOnGlobalLayoutListener(this);

如何在 C#\Xamarin 中实现?我是否应该使用 boolean 属性来知道它是否被执行?有没有办法像android那样做?

How to do it in C#\Xamarin? Should I resort to using boolean properties to know whether it's executed or not? Is there not way to do it like the android one?

推荐答案

如果您正在使用 C# 事件，如果您需要取消订阅，请避免使用匿名事件，或者您可以实现 IOnGlobalLayoutListener 并添加/移除监听器:

If you are using C# Events, avoid using anonymous events if you need to unsubscribe, or you can implement the IOnGlobalLayoutListener and add/remove the listener:

为要调用的事件创建一个 EventHandler 方法:

Create an EventHandler method for the event to invoke:

void Globallayout_handler(object sender, EventArgs e)
{
    // ViewTreeObserver.IOnGlobalLayoutListener events
}

订阅:

var viewTreeObserver = aView.ViewTreeObserver;
viewTreeObserver.GlobalLayout += Globallayout_handler;

取消订阅:

var viewTreeObserver = aView.ViewTreeObserver;
viewTreeObserver.GlobalLayout -= Globallayout_handler;

C# 中的 Java 侦听器样式:

添加并实现ViewTreeObserver.IOnGlobalLayoutListener:

public class CustomButtonRenderer : Xamarin.Forms.Platform.Android.AppCompat.ButtonRenderer,
ViewTreeObserver.IOnGlobalLayoutListener
{
   ~~~~

    public void OnGlobalLayout()
    {
       // ViewTreeObserver.IOnGlobalLayoutListener events
    }
}

现在您可以使用 Java 方式添加和删除此侦听器:

Now you can use the Java way to add and remove this listener:

aView.ViewTreeObserver.RemoveOnGlobalLayoutListener(this); 

aView.ViewTreeObserver.AddOnGlobalLayoutListener(this); 

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