使用 DrawString (MonoTouch) 向 UIImageView 添加文本时遇到问题

Question

我想使用 C# 和 MonoTouch 在 iPhone 上的图像上写入文本。 DrawImage 方法看起来很简单，但是当我运行它时会出错。我错过了什么？非常感谢任何帮助！
提前致谢。

错误：

Thu Feb  3 12:40:34 MacMini.local ImageTextMT[3877] <Error>: CGContextSetFont: invalid context 0x0  
Thu Feb  3 12:40:34 MacMini.local ImageTextMT[3877] <Error>: CGContextSetTextMatrix: invalid context 0x0  
Thu Feb  3 12:40:34 MacMini.local ImageTextMT[3877] <Error>: CGContextSetFontSize: invalid context 0x0  
Thu Feb  3 12:40:34 MacMini.local ImageTextMT[3877] <Error>: CGContextSetTextPosition: invalid context 0x0  
Thu Feb  3 12:40:34 MacMini.local ImageTextMT[3877] <Error>: CGContextShowGlyphsWithAdvances: invalid context 0x0 

MonoDevelop 2.4.2，MonoTouch

1. 我创建了一个新解决方案：C#、iPhone 和 iPad、基于 iPhone 窗口的项目

2. 编辑了 MainWindow.xib：
   2a。在窗口中添加了 UIImageView 和 UIButton；
   2b。我在 AppDelegate 中添加了两个出口，btnAddText（引用到 UIButton）和 imgImage（引用到 UIImageView）；

3. 代码（抱歉——代码格式受到质疑...）：

   using System;
   using System.Collections.Generic;
   using MonoTouch.Foundation;
   using MonoTouch.UIKit;
   using MonoTouch.CoreGraphics;
   using System.Drawing;
   
   namespace ImageTextMT;
   
   {
   public class Application
       {
           static void Main (string[] args)
           {
       UIApplication.Main (args);
           }
       }
   
   // The name AppDelegate is referenced in the MainWindow.xib file.
       public partial class AppDelegate : UIApplicationDelegate
       {
       // This method is invoked when the application has loaded its UI and its ready to run
       public override bool FinishedLaunching (UIApplication app, NSDictionary options)
           {
               // If you have defined a view, add it here:
               // window.AddSubview (navigationController.View);
           window.MakeKeyAndVisible ();
   
           // Add the events for the controls
               btnAddText.TouchDown += btnAddText_Click;
   
               imgImage.Image = UIImage.FromFile("/Users/ParisRat/Projects/ImageTextMT/ImageTextMT/images/testblock.png");
               return true;
           }
   
           protected void btnAddText_Click (object sender, EventArgs e)
           {
               // Define a rectangle
               System.Drawing.RectangleF rect = new System.Drawing.RectangleF(10.0f, 5.0f, 20.0f, 25.0f);
   
               // *** Write text on to the image ***
               this.imgImage.DrawString("HELLO",rect,UIFont.SystemFontOfSize(15.0f), UILineBreakMode.CharacterWrap, UITextAlignment.Left);
           }
   
   
           // This method is required in iPhoneOS 3.0
           public override void OnActivated (UIApplication application)
           {
           }
       }
       }
   

Answer 1

你有两个选择。如果你想使用 imgImage.DrawString 方法，你需要先继承 ImageView，重写 Draw 方法并在那里调用 imgImage.DrawString 方法。

第二个选项是创建您自己的位图上下文并在那里绘制。在 monotouch.dialog 中可以找到一个很好的例子。有一个自定义绘制的日历。这是那里的代码。

public static UIImage MakeCalendarBadge (UIImage template, string smallText, string bigText) {
        using (var cs = CGColorSpace.CreateDeviceRGB ()){
            using (var context = new CGBitmapContext (IntPtr.Zero, 57, 57, 8, 57*4, cs, CGImageAlphaInfo.PremultipliedLast)){
                //context.ScaleCTM (0.5f, -1);
                context.TranslateCTM (0, 0);
                context.DrawImage (new RectangleF (0, 0, 57, 57), template.CGImage);
                context.SetRGBFillColor (1, 1, 1, 1);

                context.SelectFont ("Helvetica", 10f, CGTextEncoding.MacRoman);

                // Pretty lame way of measuring strings, as documented:
                var start = context.TextPosition.X;                 
                context.SetTextDrawingMode (CGTextDrawingMode.Invisible);
                context.ShowText (smallText);
                var width = context.TextPosition.X - start;

                var ns = new NSString (smallText);
                UIFont ff = UIFont.FromName ("Helvetica", 10);

                context.SetTextDrawingMode (CGTextDrawingMode.Fill);
                context.ShowTextAtPoint ((57-width)/2, 46, smallText);

                // The big string
                context.SelectFont ("Helvetica-Bold", 32, CGTextEncoding.MacRoman);                 
                start = context.TextPosition.X;
                context.SetTextDrawingMode (CGTextDrawingMode.Invisible);
                context.ShowText (bigText);
                width = context.TextPosition.X - start;

                context.SetRGBFillColor (0, 0, 0, 1);
                context.SetTextDrawingMode (CGTextDrawingMode.Fill);
                context.ShowTextAtPoint ((57-width)/2, 9, bigText);

                context.StrokePath ();

                return UIImage.FromImage (context.ToImage ());
            }
        }
    }