如何在Xamarin iOS中将图标从UIBarButtonSystemItem转换为UIImage

Question

我正在尝试将图标从UIBarButtonSystemItem like UIBarButtonSystemItem.Camera类型转换为UIImage。用户可以在Swift and ObjC中做到这一点，但我不能理解它。

当我按照Swift和ObjC教程中的说明操作时，我的代码中出现null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。以前有没有人尝试过这样做？

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;
}

由@DaleXSoto提供的来自Xamarin iOS Gitter的解决方案

“问题是你在tempItem对象中调用了一个选择器(视图)，但这是一个技巧，因为UIBarButtonItem响应视图只是因为有人发现它不在头上。

所以在tempItem对象上调用IntPtr_objc_msgSend就是这样做的。“