如何以及为什么使用Android Visibility Listeners

The Android UI is built up from Views, and in a regular application, there are usually several of them. To find out which View the user is currently looking at, you need to install Visibility Listeners.

Android UI是从Views构建的，在常规应用程序中，通常有几种。要了解用户当前正在查看哪个View，您需要安装Visibility Listeners 。

Read below to find out about the different options you have to identify the visibility status of a View.

请阅读以下内容，了解确定视图的可见性状态所必须使用的不同选项。

如何变得可见 (How To Become Visible)

In order for our listeners to work, we must first make sure our View is found in the layout hierarchy. There are two ways this happens:

为了使我们的监听器正常工作，我们必须首先确保在布局层次结构中找到我们的View。发生这种情况的方式有两种：

Your View is already part of your layout as it is defined in an XML file您的视图已经是布局的一部分，因为它是在XML文件中定义的
You created a View dynamically, and you need to add it using the addView method您动态创建了一个View，需要使用addView方法添加它

A View’s visibility status is of Integer type and can have one of three options:

视图的可见性状态为整数类型，可以具有以下三个选项之一：

VISIBLE (0) - The View is visible to the user

可见(0) -视图对用户可见
INVISIBLE (4) - The View is invisible to the user, but still takes up space in the layout

看不见的(4) -视图对用户不可见，但仍占据布局中的空间
GONE (8) - The View is invisible, and it does not take up space in the layout

消失(8) -视图是不可见的，并且不占用布局中的空间

Once inside our layout hierarchy, there are a few native options to help us know when our View’s visibility has changed.

进入布局层次结构后，有一些本机选项可帮助我们了解View的可见性已更改的时间。

onVisibilityChanged (onVisibilityChanged)

protected void onVisibilityChanged (View changedView, int visibility)

This method is triggered when the visibility of the view or of an ancestor of the view has changed. The status of the visibility is found inside the visibility parameter.

当视图或视图祖先的可见性已更改时，将触发此方法。可见性的状态位于可见性参数内。

onWindowVisibilityChanged (onWindowVisibilityChanged)

protected void onWindowVisibilityChanged (int visibility)

This method is triggered when the containing window of our View has changed its visibility. This does not guarantee that the window your View is in is visible to the user, as it may be obscured by another window.

当视图的包含窗口更改其可见性时，将触发此方法。 这不能保证用户可以看到您的视图所在的窗口，因为它可能被另一个窗口遮盖。

行动中的可见性侦听器 (Visibility Listeners In Action)

To see these two listeners in action, let us create a simple project. We will have a LinearLayout with a TextView and a button. We’ll make the button’s on click action add our custom view to the layout.

要查看这两个侦听器的运行情况，让我们创建一个简单的项目。我们将使用带有TextView和按钮的LinearLayout。我们将使按钮的单击动作添加我们的自定义视图到布局。

Our custom view:

我们的自定义视图：

package com.tomerpacific.viewvisibility;import android.content.Context;
import android.graphics.Color;
import android.util.Log;
import android.view.View;
import android.widget.LinearLayout;
import android.widget.TextView;import static android.view.Gravity.CENTER;public class MyCustomView extends LinearLayout {private final String TAG = MyCustomView.class.getSimpleName();public MyCustomView(Context context) {super(context);this.setBackgroundColor(Color.GREEN);this.setGravity(CENTER);TextView myTextView = new TextView(context);myTextView.setText("My Custom View");addView(myTextView);}@Overridepublic void onVisibilityChanged(View changedView, int visibility) {super.onVisibilityChanged(changedView, visibility);Log.d(TAG, "View " + changedView + " changed visibility to " + visibility);}@Overridepublic void onWindowVisibilityChanged(int visibility) {super.onWindowVisibilityChanged(visibility);Log.d(TAG, "Window visibility changed to " + visibility);}}

And finally, the code in our MainActivity:

最后，我们MainActivity中的代码：

When we run the application and press the button we get:

当我们运行该应用程序并按下按钮时，我们得到：

https://giphy.com/gifs/8JZA6Djt7DmYpEXj2h/html5

You can get the sample project here.

您可以在此处获得示例项目。

ViewTreeObserver (ViewTreeObserver)

This is a native object that has a wide range of listeners that are notified of various visibility changes to the view tree. Some prominent ones to take notice of are:

这是一个本机对象，具有广泛的侦听器，这些侦听器会收到有关视图树的各种可见性更改的通知。需要注意的一些著名的是：

OnGlobalLayoutListener

OnGlobalLayoutListener
OnWindowAttachListener

OnWindowAttachListener
OnWindowFocusChangeListener

OnWindowFocusChangeListener

To attach a ViewTreeObserver, you need to do the following:

要附加ViewTreeObserver，您需要执行以下操作：

The line linearLayout.getViewTreeObserver().removeOnGlobalLayoutListener(this) makes sure that the listener will only get called once. If you want to continue listening in on changes, remove it.

行linearLayout.getViewTreeObserver().removeOnGlobalLayoutListener(this) 确保侦听器仅被调用一次。如果要继续收听更改，请将其删除。

If you have any comments or suggestions, feel free to let me know.

如果您有任何意见或建议，请随时告诉我。

翻译自: https://www.freecodecamp.org/news/how-and-why-to-use-android-visibility-listeners-971e3b6511ec/