golang / urfave.cli: can't get to set flags manually

We have an app which receives an urfave cli context object in a func:

import "gopkg.in/urfave/cli.v1"

func runApp(ctx *cli.Context) {
   cache := ctx.GlobalInt(MyFlag.Name)
   .... // do more stuff
}

I am trying to call runApp without the whole cli stuff, directly from code. Hence I have to build the ctx manually:

func MyTest() {
  fs := flag.NewFlagSet("", flag.ExitOnError)
  app := cli.NewApp()
  fs.Set(MyFlag.Name, "5000")
  ctx := cli.NewContext(app, fs, nil)
  runApp(ctx)
}

But it actually looks like the flag is never set, runApp never sees the value I set by hand.

What am I doing wrong? Can this actually be done at all?

It looks like MyFlag.Name this field is not actually defined as a flag.
To define it as a flag, you need do this: fs.String(MyFlag.Name, "5000", "usageOfTheFlag")
After this you can use fs.Set for MyFlag.Name.

var MyFlag = flag.Flag{
	Name: "TestFlag",
}

func MyTest() {
  fs := flag.NewFlagSet("", flag.ExitOnError)
  app := cli.NewApp()
  // define a flag with default value
  fs.String(MyFlag.Name, "5000", "usageOfTheFlag")
  // set a new value
  err := fs.Set(MyFlag.Name, "5000")

  if err != nil {
     fmt.Println(err)
     return
  }

  ctx := cli.NewContext(app, fs, nil)
  runApp(ctx)
}

func runApp(ctx *cli.Context) {
   cache := ctx.GlobalInt(MyFlag.Name)
   .... // do more stuff
}

If you would do an error checking on fs.Set with your flag name without define it as a flag:

// fs.String(MyFlag.Name, "5000", "usageOfTheFlag")
err := fs.Set(MyFlag.Name, "5000")

	if err != nil {
		fmt.Println(err)
		return
	}

You would get this error with the name of the flag that is currently not defined as a flag.
In this example it is "TestFlag":

no such flag -TestFlag

Thus MyFlag.Name didn't get the new value and because of it your program didn't see your new value.