要在YouTube播放器上启用全屏按钮，WebChromeClient必须实现OnShowCustomView和OnHideCustomView，因此您有责任为您的应用定义什么是“全屏”，因为它不必由设备的屏幕大小定义。
注意：您仍然需要在iFrame html源代码中使用allowfullscreen的HTML5标记
假设你有这样的布局：

LinearLayout (id = linearLayout)
   LinearLayout (id = contentLayout)
      Button
      WebView

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您可以子类化WebChromeClient并定义您希望如何显示“全屏”内容，在此示例中，我们将假设最外部的LinearLayout是我们希望显示YouTube视频的位置，内部的LinearLayout包含您希望在全屏视频播放时隐藏的所有Activity内容。

WebChromeClient实现：

public class FullScreenClient : WebChromeClient
{
    readonly FrameLayout.LayoutParams matchParentLayout = new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MatchParent,
                                                                                                     ViewGroup.LayoutParams.MatchParent);
    readonly ViewGroup content;
    readonly ViewGroup parent;
    View customView;

    public FullScreenClient(ViewGroup parent, ViewGroup content)
    {
        this.parent = parent;
        this.content = content;
    }

    public override void OnShowCustomView(View view, ICustomViewCallback callback)
    {
        customView = view;
        view.LayoutParameters = matchParentLayout;
        parent.AddView(view);
        content.Visibility = ViewStates.Gone;
    }

    public override void OnHideCustomView()
    {
        content.Visibility = ViewStates.Visible;
        parent.RemoveView(customView);
        customView = null;
    }
}

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SetWebChromeClient实现：

webView.SetWebChromeClient(new FullScreenClient(linearLayout, contentLayout));

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我知道答案已经被接受了，但我花了一些时间来完成所提供的代码。我从来没有见过“readonly”关键字，“override”不太应该在那里，看起来他回答了LinearLayout并提供了FrameLayout. LayoutParams。我希望这只是真的很好的psudocode笑。如果没有，请告诉我代码语法！
我在一个非常有用的YouTube video的评论部分发布了这个链接。如果你是从那里来的，你需要向VideoAdapter类构造函数添加2个参数。一个用于LinearLayout（parent），一个用于RecyclerView（content）。
感谢@SushiHangover提供代码。你会帮助很多人！别忘了对他的回答投赞成票。
我的父布局是LinearLayout**（父），子布局是RecyclerView（内容）**，可以容纳多个视频。

// Custom Web View Class to allow for full screen
private class CustomWebView extends WebChromeClient {

    LinearLayout.LayoutParams layoutParams = new LinearLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
                                                                           ViewGroup.LayoutParams.MATCH_PARENT);

    ViewGroup parent;
    ViewGroup content;
    View customView;

    public CustomWebView(ViewGroup parent, ViewGroup content){
        this.parent = parent;
        this.content = content;
    }

    @Override
    public void onShowCustomView(View view, CustomViewCallback callback) {
        super.onShowCustomView(view, callback);

        customView = view;
        view.setLayoutParams(layoutParams);
        parent.addView(view);
        content.setVisibility(View.GONE);
    }

    @Override
    public void onHideCustomView() {
        super.onHideCustomView();

        content.setVisibility(View.VISIBLE);
        parent.removeView(customView);
        customView = null;
    }

}

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webView.webChromeClient = object：public void run（）{

private var customViewCallback: CustomViewCallback? = null
        var layoutParams = LinearLayout.LayoutParams(
            ViewGroup.LayoutParams.MATCH_PARENT,
            ViewGroup.LayoutParams.MATCH_PARENT
        )

        var parent: ViewGroup? = findViewById(android.R.id.content)
        var content: ViewGroup? = null
        var customView: View? = null
        private var originalOrientation = 0

        

        override fun onShowCustomView(view: View?, callback: CustomViewCallback?) {
            super.onShowCustomView(view, callback)
            this@MainActivity.window.apply {
                clearFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS)
                addFlags(WindowManager.LayoutParams.FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS)

                if (Build.VERSION.SDK_INT > 11 && Build.VERSION.SDK_INT < 19) { // lower api
                    val v: View = getWindow().getDecorView()
                    v.systemUiVisibility = View.GONE
                } else if (Build.VERSION.SDK_INT >= 19) {
                    //for new api versions.
                    val decorView = window.decorView
                    val uiOptions =
                        View.SYSTEM_UI_FLAG_HIDE_NAVIGATION or View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY or View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                    decorView.systemUiVisibility = uiOptions
                }

                statusBarColor = Color.TRANSPARENT
            }
            originalOrientation = this@MainActivity.requestedOrientation;
            customView = view;
            view?.setLayoutParams(layoutParams);
            parent?.addView(view);
            content?.setVisibility(View.GONE);
          
        }

        override fun onHideCustomView() {
            super.onHideCustomView()
            content?.visibility = View.VISIBLE;
            parent?.removeView(customView);
            customView = null;
           
            this@MainActivity.window.decorView.systemUiVisibility = View.SYSTEM_UI_FLAG_VISIBLE;
            this@MainActivity.requestedOrientation = ActivityInfo.SCREEN_ORIENTATION_PORTRAIT;
           
        }

        
    }

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