type Unmarshaler interface {
    UnmarshalJSON([]byte) error
}

package main

...

func (app *application) createMovieHandler(w http.ResponseWriter, r *http.Request) {
    var input struct {
        Title   string       `json:"title"`
        Year    int32        `json:"year"`
        Runtime data.Runtime `json:"runtime"` // Make this field a data.Runtime type.
        Genres  []string     `json:"genres"`
    }

    err := app.readJSON(w, r, &input)
    if err != nil {
        app.badRequestResponse(w, r, err)
        return
    }

    fmt.Fprintf(w, "%+v\n", input)
}

...

package data

import (
    "errors" // New import
    "fmt"
    "strconv"
    "strings" // New import
)

// Define an error that our UnmarshalJSON() method can return if we're unable to parse
// or convert the JSON string successfully.
var ErrInvalidRuntimeFormat = errors.New("invalid runtime format")

type Runtime int32

...

// Implement a UnmarshalJSON() method on the Runtime type so that it satisfies the
// json.Unmarshaler interface. IMPORTANT: Because UnmarshalJSON() needs to modify the
// receiver (our Runtime type), we must use a pointer receiver for this to work 
// correctly. Otherwise, we will only be modifying a copy (which is then discarded when 
// this method returns).
func (r *Runtime) UnmarshalJSON(jsonValue []byte) error {
    // We expect that the incoming JSON value will be a string in the format 
    // "<runtime> mins", and the first thing we need to do is remove the surrounding 
    // double quotes from this string. If we can't unquote it, then we return the 
    // ErrInvalidRuntimeFormat error.
    unquotedJSONValue, err := strconv.Unquote(string(jsonValue))
    if err != nil {
        return ErrInvalidRuntimeFormat
    }

    // Split the string to isolate the part containing the number. 
    parts := strings.Split(unquotedJSONValue, " ")

    // Sanity check the parts of the string to make sure it was in the expected format. 
    // If it isn't, we return the ErrInvalidRuntimeFormat error again.
    if len(parts) != 2 || parts[1] != "mins" {
        return ErrInvalidRuntimeFormat
    }

    // Otherwise, parse the string containing the number into an int32. Again, if this
    // fails return the ErrInvalidRuntimeFormat error.
    i, err := strconv.ParseInt(parts[0], 10, 32)
    if err != nil {
        return ErrInvalidRuntimeFormat
    }

    // Convert the int32 to a Runtime type and assign this to the receiver. Note that we
    // use the * operator to dereference the receiver (which is a pointer to a Runtime 
    // type) in order to set the underlying value of the pointer.
    *r = Runtime(i)

    return nil
}