- Getting to Know the Looks Blocks
- Interacting with the Player
- Getting Sprites to "Talk" to Each Other
- Summary
- Workshop
- Challenge
This chapter is from the book
This chapter is from the book
This chapter is from the book
Getting Sprites to “Talk” to Each Other
A common question among Scratchers who have made some progress in their work is, “How can I get sprites to affect each other?” For instance, what if you want Sprite B to move when Sprite A touches it?
In Scratch, you use broadcasts to pass messages among sprites. You can even communicate between sprites and the Stage by sending and receiving broadcasts.
You will find the 
and 
blocks in the Events palette.
You’ll also see a 
Hat block that you use to catch outgoing broadcasts.
Trust me—before too long, you’ll appreciate how powerful broadcasts are; they will be an indispensable addition to your Scratch programming toolkit.
Here are a few key points to keep in mind regarding Scratch broadcast messages:
A sprite can both send and receive the same broadcast message.
Broadcast messages can be received by all sprites (and the Stage).
Broadcasts are most commonly used to (1) connect different events, (2) run two scripts in the same frame, and/or (3) prepare a scene with multiple sprites.
To get set up for the Move from Room to Room Try It Yourself exercise, fire up a new, blank project that contains the following assets:
Sprite: Default Scratch Cat; rename sprite to Cat.
Sprite: Magic Carpet (look in the Transportation section of the Sprite Library); rename to Carpet.
Stage backdrops: Add the room1 and room2 backdrops from the Indoors category of the Backdrop Library. You can delete the default backdrop for this exercise.
Try It Yourself: Move from Room to Room
For this Try It Yourself exercise, you try your hand at a very brief and very simple interactive story. You first have the Scratch Cat move from one room to another (a very cool trick that you’ll enjoy). Next, you have the Cat ask the user to press a key on the keyboard to move a second sprite (the Carpet) out of the way.
The Cat and Carpet scripts are illustrated in Figures 5.6 and 5.7, respectively.
)
FIGURE 5.6 Cat sprite source code for the Move from Room to Room Try It Yourself exercise.
)
FIGURE 5.7 Carpet sprite source code for the Move from Room to Room Try It Yourself exercise.
The completed solution file is named Hour05c.sb2. Work through the following steps in order to complete this exercise. Let’s get to work!
Begin by coding the Cat’s scripts. Reset the story environment by ensuring that the room1 backdrop is active and the Cat is placed appropriately on the Stage, as shown in the code section labeled A in Figure 5.6.
Have the Cat say something to the player to help him or her feel more engaged with the story (see the code section labeled B in Figure 5.6).
The repeat block shown in the code section labeled C in Figure 5.6 defines the Cat’s animation across the first backdrop. How to do this was covered earlier in the book; simply move the sprite 10 steps, switch its costume, and insert a short pause.
Now you get to the heart of the matter. Bring out a
block, open up the drop-down, and select New Message from the menu. You can name a broadcast anything; for this purpose, make it SwitchScreen. (See the code section labeled D in Figure 5.6.)In the code section labeled E in Figure 5.6, you see something that might strike you as surprising; namely, that sprites can actually “listen for” and receive their own messages! In just a moment, you have the Carpet sprite listen for this broadcast as well.
For now, switch the Stage backdrop to the second room and place the sprite appropriately on the Stage. This action, combined with the previously defined sprite movement, gives the player the impression that the sprite traveled from one room to another.
Add in some animation blocks to get the Cat moved to the Carpet (see the code section labeled F in Figure 5.6). (We haven’t addressed the Carpet yet, I realize that.)
Insert an
C block to test for the Sensing condition where the Cat sprite touches the Carpet sprite. If that statement evaluates to True, then bring the Cat sprite to the very top layer on the Stage and have it say “I’m touching the carpet.” (See the code section labeled G in Figure 5.6.)After a 2-second pause, have the Cat sprite say “Press the space bar to get rid of the carpet, please.” You then need to create a second broadcast named EndGame that will be picked up by the Carpet. (See the code section labeled H in Figure 5.6.)
Now switch your focus to the Carpet sprite and cross-reference these steps with Figure 5.7. Place the Carpet sprite at (4, -114) on the Stage. Reminder: Coordinates are given as (x,y). Simply hide the Carpet until the Cat reaches the second room (see the code section labeled I in Figure 5.7).
Next, listen for the SwitchScreen broadcast. At that time, reset the style and position of the Carpet sprite, and explicitly show it on the Stage. (See the code section labeled J in Figure 5.7.)
Finally, listen for the EndGame broadcast, which is, you’ll remember, the broadcast that is triggered if the Cat sprite touches the Carpet sprite. (See the code section labeled K in Figure 5.7.)
Use the wait until block to test for the key space pressed Boolean condition (that block is found in the Sensing palette). If True, then code the carpet to glide horizontally across the screen to an X value that lies beyond the 480 pixels of the Stage. You then invoke the
Control block to end the interaction.