如何在 Xamarin iOS 中将 UIBarButtonSystemItem 中的图标转换为 UIImage

Question

我正在尝试将 UIBarButtonSystemItem like UIBarButtonSystemItem.Camera 类型的图标转换为 UIImage。 用户可以在 Swift and ObjC 中做到这一点，但我无法弄清楚。

当我按照 Swift 和 ObjC 教程中的说明进行操作时，我的代码中出现空引用异常错误

{
    UIImage image = null;

    viewController.InvokeOnMainThread(() =>
    {
        UIBarButtonItem oldItem = viewController.NavigationItem.RightBarButtonItem;
        var buttonItem = new UIBarButtonItem(item);

        // Add to toolbar and render it
        viewController.NavigationItem.RightBarButtonItem = buttonItem;
        var x = new ObjCRuntime.Selector("view");
        var y = viewController.NavigationItem.RightBarButtonItem.PerformSelector(x);
        UIView itemView = (UIView)y;

        // Get the image from real UIButton
        foreach (var view in itemView.Subviews)
        {
            if (view.GetType() == typeof(UIButton))
                image = ((UIButton)view).ImageView.Image;
        }
                viewController.NavigationItem.RightBarButtonItem = oldItem;
    });

    return image;
}

我也试过了，

public static UIImage ToUIImage2(UIBarButtonSystemItem item, UIViewController viewController)
{
    UIImage image = null;

    viewController.InvokeOnMainThread(() =>
    {
        var buttonItem = new UIBarButtonItem(item);
        // Add to toolbar and render it
        var tempItem = new UIBarButtonItem[1] { buttonItem };
        var bar = new UIToolbar();
        bar.SetItems(tempItem, false);
        bar.SnapshotView(true);

        // Get the image from real UIButton
        var buttonItemView = bar.ValueForKey(new Foundation.NSString("view"));
        var itemView = (UIView)buttonItemView;

        foreach (var view in itemView.Subviews)
        {
            if (view.GetType() == typeof(UIButton))
                image = ((UIButton)view).ImageView.Image;
        }
    });
    return image;
}

在 StackOverflow 中找不到 C# 的任何内容。 以前有人试过吗？

Answer 1

public extern static IntPtr IntPtr_objc_msgSend (IntPtr receiver, IntPtr selector);

public static UIImage GetImage (UIBarButtonSystemItem systemItem)
{
    var tempItem = new UIBarButtonItem (systemItem);

    // Add to toolbar and render it
    var bar = new UIToolbar ();
    bar.SetItems (new [] { tempItem }, false);
    bar.SnapshotView (true);

    // Get image from real UIButton
    var selHandle = Selector.GetHandle ("view");
    var itemView = Runtime.GetNSObject<UIView>(IntPtr_objc_msgSend (tempItem.Handle, selHandle));
    foreach    (var view in itemView?.Subviews) {
        if (view is UIButton button)
            return button.ImageForState (UIControlState.Normal);
    }

    return null;
}

Xamarin iOS Gitter 中 @DaleXSoto 提供的解决方案

“问题是你正在调用一个选择器（视图）到 tempItem 对象 但这是一个 hack，因为 UIBarButtonItem 响应视图只是因为有人发现了，嘿，它不在标题上

所以 IntPtr_objc_msgSend 是您在 tempItem 对象上调用视图的方式"