Lesson 11 - Displaying the Score
Further to the discussion of puppets in the previous lesson, we now move onto scripting another kind of Director object 'from the outside', a field cast member. For displaying text. Director relies on two cast member types, fields and richtext. Director 7 has superceded richtext cast members with text cast members. So that those with Director 6 can benefit, and to avoid obsolete information I have chosen to use fields to display the players score and lives.
Fields are basically texts stored in cast members. They also contain some font and style information but they are most useful as low bandwidth storage for text and data in ASCII form. In Director 6 and earlier, fields are essential for user input, editable text, hypertext and text which should be generated or manipulated at runtime by Lingo. Fields use fonts installed on the system, so you'd be wise to choose a standard system font like Arial/Helvetica. The standard system fonts are automatically mapped over to equivalent fonts on the other platform. In Director7, text cast members are editable at runtime and can be rendered with antialiased edges. Director7 can also include embedded fonts.
We are going to want to have some kind of scoring system so that when the bullet hits an invader, the score increases. Additionally, we will probably want to give the user several chances at getting onto the next level, so we will want to display how many lives there are left and indeed which level we have reached.
Starting with the score, to keep it simple, we will need a new field cast member. This can be created in various ways:
In every case a new field cast member will be created in the cast. Give it the name "Score Display". If you did not create the field on the stage with the tool palette, drag the field onto the stage. Make sure the field is in sprite channel 20. Move the field to the top or the bottom of the stage, or wherever you would like it to be, then make a new script for the sprite.
A new score script will open which says
on mouseUp end
As before. We are not interested in mouse messages here so you can delete this and replace it with the following.
property displayField, score on beginsprite me set displayField to the member of sprite the spritenum of me end
Now you should notice a difference here from what we have been doing before. Previously we have been wanting to manipulate sprite properties (locH, locV etc.) so we stored the value of the spritenum of me as a property in our sprite scripts. Here we want to manipulate the contents of a field cast member, so we are storing a reference to that cast member in the property displayField. Once again, this is just for convenience so that we do not have to refer to the member of sprite the spritenum of me continuously throughout the script. It is a perfectly adequate object reference, but would you as a human being feel secure that you would always type it accurately?
Let's think about the kind of messages we would like to send to the score display. We want it to display the score, sure, but once it is on screen it is doing that already. We want to increase the score, and we want to reset the score to 0 at the beginning of a new game. Fields do not get reset automatically when you rewind a movie. This is true of any modifications you might make to your cast members with lingo.
So our score display script might look like this:
property displayField, val on beginsprite me set displayField to the member of sprite the spritenum of me reset me end on reset me set val to 0 update me end on update me put val into field displayField end on inc me, amount set val to val + amount update me end
The put command is often used with the message window because the default output of put is the message window. If you specify the destination of your put as
put abc into field xyz
...the abc value you are referring to will appear in the field xyz. Here, the latest score will be put into the "Score Display" field cast member which, being on stage as a sprite will be visible to the user immediately. It is recommended that readers check the manuals or online help for a full description of the many ways that the put command can be used. (It can even be used instead of set, though I would discourage this.)
The word inc is often used in programming, right down to the machine code level. It is short for increment, and incrementation is something you do an awful lot when you write programs. Here, the inc handler increments the score by the amount passed to the amount parameter. Obviously if this value is negative, the val property will decrease.
Now we have a script designed for field sprites. Because I
have taken some care to make it reuseable. We can use the same
script for lives and level display! Right now, when you run the
movie, the score will display 0 no matter what. Before we proceed
and get it working better, lets take another brief theoretical
detour...
Public and Private
Notice the way the handlers beginsprite, reset and inc call the update handler. A procedure which is only called internally is referred to as private. As you go on to create scripts of your own you will realise that your handlers and properties can be seperated into two conceptual groups, public (also known as 'exported') and private, used only by the script itself.
Public handlers and properties are those which are designed to be exported or made available outside the object. Those that are essential to the inner workings of the object but are not to be messed around with are private.
In Lingo there is no integrated way of distinguishing public from private, it is more of a conceptual distinction. There are some special functions, getPropertyDescriptionList and getBehaviorDescription which go some of the way, but they are not implemented automatically and rely a great deal on the good will of the programmer to encapsulate their code. We will look at these later. Nevertheless, the distinction between public and private is a very important one which can make your thinking about your object designs much clearer.
Now in order to get the score to increase when an invader gets killed, we need to send an appropriate message from the hit handler of the invader:
on hit me set alive to false set the locH of sprite mySprite to -999 puppetsound 1, "explosion" inc sprite 20, 10 end
This means that the score will increase by 10 when the invader gets hit. If you replace the 10 with a different value, the score will increase more with each hit.
Public and Private (reprise)
Let's assume that we wanted each invader to have a different score. Maybe some of them are worth 10 points and some are worth 50. We might even want to relate the score to their speed. Perhaps different invaders should explode with different sounds. None of this should affect the scripting, it's just a matter of different values for the properties.
If we add a points property to the invader script this would be quite easy. Then if points were public, appearing in a authoring dialog box alongside speed and explosionSound for example, it would be easy to assign different amounts of points to each invader without touching the script. Keep that thought in mind. We will be implementing it later...