In Rewtro there are some special objects you can test and manipulate in code commands to store scene and game variables, handle the game controls, etc.
Testing and changing the scene object keys you can manage a lot of scene features like global variables and timers, the screen background color, and more.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[
{"id":"A","x":0,"y":0,"noCamera":true,"backgroundColor":2,"width":160},
{"id":"B","textAlignment":"center"},
{"id":"C","backgroundColor":12}
],
"tilemaps":[{
"x":60,"y":60,
"map":[
"CCCCC",
"CAB C",
"CCCCC"
]
}],
"code":[
{
"when":[{"as":"scene","if":[{"itsAttribute":"timer","is":"%%","smallNumber":25}]}],
"then":[{"sum":[{"value0":[{"smallNumber":1}]}]}]
},
{
"when":[{"as":"scene","if":[{"itsAttribute":"value0","is":"%%","smallNumber":10}]}],
"then":[{"set":[{"backgroundColor":[{"list":[2,7],"randomNumber":true}]}]}]
},
{
"when":[{"as":"keyboard","attribute":"buttonA","if":[{"is":"hit"}]}],
"then":[{"runScene":[{"character":"A"}]}]
},
{
"then":[
{"id":"A","set":[{"text":[{"as":"scene","attribute":"value0","prefix":"TIME: "}]}]},
{"id":"B","set":[{"text":[{"as":"scene","attribute":"timer"}]}]},
{
"as":"scene",
"set":[{
"x":[{"attribute":"timer","sin":true}],
"y":[{"attribute":"timer","cos":true}]
}],
"multiply":[{
"x":[{"smallNumber":20}],
"y":[{"smallNumber":20}]
}]
}
]
}
]
}]
}
This cartridge shows a timer on the top of the screen and a faster red-framed counter spinning in the middle of the screen like crazy. Every time the top timer shows a number divisible by 10 the screen blinks for a while. When you hit the A button of the controller the scene is reset: both of the timers are reset to zero.
We'll learn how this example works in the next paragraphs.
The scene object is like an invisible sprite that's never destroyed until a new scene is loaded. As any sprite, it has a timer attribute that's increased every game frame. You can use this as a scene timer in your game since it holds how many frames have passed by the start of your scene.
In our example we print this timer in the spinning square:
{"id":"B","set":[{"text":[{"as":"scene","attribute":"timer"}]}]}
Pick the B sprite and set its text to the scene timer. We also use the scene timer to increase the scene variable value0.
{
"when":[{"as":"scene","if":[{"itsAttribute":"timer","is":"%%","smallNumber":25}]}],
"then":[{"sum":[{"value0":[{"smallNumber":1}]}]}]
}
when the scene timer is divisible by 25 then increase its value0 by 1. value0 is a scene variable. Let's talk about that.
Like standard sprites, the scene object has general-purpose attributes named from value0 to value4 to store any single decimal float number from -25 to 26.1 and from value5 to value9 to store a text string.
You can initialize these attributes at the scene start using the tilemap data block key and manipulate them using the as getter set to scene.
In our previous example, we've used the scene attribute value0 to store the slower timer value.
{
"when":[{"as":"scene","if":[{"itsAttribute":"timer","is":"%%","smallNumber":25}]}],
"then":[{"sum":[{"value0":[{"smallNumber":1}]}]}]
}
when the scene timer is divisible by 25 then increase its value0 by 1. This value0 attribute is also printed on screen using the A sprite
{"id":"A","set":[{"text":[{"as":"scene","attribute":"value0","prefix":"TIME: "}]}]}
Every frame this line picks the A sprite and set its text to the scene attribute named value0 and prefixes it with the text TIME:.
The background color of the scene object is used to set the screen background color. This color is kept until a new scene is started.
In our example we randomly change the scene backgroundColor every 10 seconds:
{
"when":[{"as":"scene","if":[{"itsAttribute":"value0","is":"%%","smallNumber":10}]}],
"then":[{"set":[{"backgroundColor":[{"list":[2,7],"randomNumber":true}]}]}]
}
when the scene custom attribute value0 is divisible by 10 then set its backgroundColor to a randomNumber between 2 and 7.
The x and y attributes of the scene object are used to move a virtual camera around. Rewtro simulates a camera that's placed at the x 0 and y 0 coordinates and that shows all of the sprites that fall in an area as big as the screen size.
To make a sprite appear on the screen you can both move the sprite in front of the camera or move the camera to frame that sprite. You can move the camera around changing the x and y attributes of the camera special object as you do with sprites.
In our example we used camera movement to trick the eye:
{
"as":"scene",
"set":[{
"x":[{"attribute":"timer","sin":true}],
"y":[{"attribute":"timer","cos":true}]
}],
"multiply":[{
"x":[{"smallNumber":20}],
"y":[{"smallNumber":20}]
}]
}
The scene x and y is set to the sine and cosine of the scene timer and then multiplied by the constant 20. That makes the camera moving in circle and that's why we see the red frame spinning: the sprites are not moving but it's the camera that is framing them from different positions! Aw, relativity.
The TIMER: sprite is not following the camera and that's because its A sprite has the noCamera attribute set to true. Learn more about sprite attributes here.
the game special object works like the scene special object except that it's created when your cartridge is started and is never destroyed.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[
{"id":"A","x":0,"y":0,"backgroundColor":2,"width":160},
{"id":"B","textAlignment":"center"},
{"id":"C","backgroundColor":12}
],
"tilemaps":[{
"x":60,"y":60,
"map":[
"CCCCC",
"CAB C",
"CCCCC"
]
}],
"code":[
{
"when":[{"as":"scene","if":[{"itsAttribute":"timer","is":"%%","smallNumber":25}]}],
"then":[{"as":"game","sum":[{"value0":[{"smallNumber":1}]}]}]
},
{
"when":[{"as":"keyboard","attribute":"buttonA","if":[{"is":"hit"}]}],
"then":[{"runScene":[{"character":"A"}]}]
},
{
"then":[
{"id":"A","set":[{"text":[{"as":"game","attribute":"value0","prefix":"TIME: "}]}]},
{"id":"B","set":[{"text":[{"as":"scene","attribute":"timer"}]}]}
]
}
]
}]
}
This cartridge is a lite version of our first example: still two timers but no screen blinking or spinning. Hitting the A button a new scene is loaded: the timer in the middle of the screen is reset as usual but the TIMER: value is not this time.
None of the game attributes are used by Rewtro so you can use them to store any game-wise values, like high scores, the player respawn coordinates, and more.
The keyboard special object allows your game to check the virtual controller status. You can set the as getter to the keyboard value, select a button picking the sub-attribute to any of up, down, left, right, buttonA, buttonB, buttonC, or buttonD and create a condition that checks if it's up, down, or hit. You can learn more about these conditions here.
Buttons' availability depends on the button layout you've chosen for your game. To learn more about that have a look at the systemConfiguration key controllerModel here.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[{"id":"A","x":76,"y":68}],
"tilemaps":[{"map":["A"]}],
"code":[
{
"when":[{"as":"keyboard","attribute":"down","if":[{"is":"down"}]}],
"then":[{"id":"A","sum":[{"y":[{"smallNumber":1}]}]}]
},{
"when":[{"as":"keyboard","attribute":"right","if":[{"is":"down"}]}],
"then":[{"id":"A","sum":[{"x":[{"smallNumber":1}]}]}]
},{
"when":[{"as":"keyboard","attribute":"up","if":[{"is":"down"}]}],
"then":[{"id":"A","subtract":[{"y":[{"smallNumber":1}]}]}]
},{
"when":[{"as":"keyboard","attribute":"left","if":[{"is":"down"}]}],
"then":[{"id":"A","subtract":[{"x":[{"smallNumber":1}]}]}]
},{
"when":[{"as":"keyboard","attribute":"buttonA","if":[{"is":"down"}]}],
"then":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":2}]}]}]
},{
"when":[{"as":"keyboard","attribute":"buttonA","if":[{"is":"up"}]}],
"then":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":5}]}]}]
},{
"when":[{"as":"keyboard","attribute":"buttonA","if":[{"is":"hit"}]}],
"then":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":3}]}]}]
}
]
}]
}
In this cartridge you move a red square around the screen using the up, down, left, and right buttons. Holding down the A button it becomes blue but flashes cyan when the button is just hit.
The keyboard special object is queried multiple times by the when commands asking for specific controller keys and key states. Once a when command condition is true the game reacts executing the then command and making it interactive.
The songRow special object knows which note has been played right now by the Rewtro song player and you can know it in the very same way we used for the keyboard. Have a look at the song row sub-getters from M0 to M9 into the getters documentation.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sounds":[
{"id":"A","wave":"triangle"},
{"id":"B","wave":"whitenoise"}
],
"music":[
{
"id":"A",
"notes":[
["C4-","D4-","E4-","F4-","G4-","F4-","E4-","D4-"],
["C4-"," ","G7-"," ","C4-"," ","G7-"," "]
],"instruments":"AB"}
],
"songs":[
{"id":"A","loopTo":0,"music":"A","tempo":5}
],
"sprites":[
{"id":"A","width":80,"height":72},
{"id":"B","x":80,"width":80,"height":72}
],
"tilemaps":[{"map":["AB"],"song":"A"}],
"code":[
{
"when":[{"as":"songRow","attribute":"M0","if":[{"is":"==","string":"AC4-"}]}],
"then":[{"as":"scene","set":[{"backgroundColor":[{"smallNumber":3}]}]}],
"else":[{"as":"scene","set":[{"backgroundColor":[{"smallNumber":1}]}]}]
},
{
"when":[{"as":"songRow","attribute":"M1","if":[{"is":"==","string":"BC4-"}]}],
"then":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":5}]}]}],
"else":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":0}]}]}]
},
{
"when":[{"as":"songRow","attribute":"M1","if":[{"is":"==","string":"BG7-"}]}],
"then":[{"id":"B","set":[{"backgroundColor":[{"smallNumber":2}]}]}],
"else":[{"id":"B","set":[{"backgroundColor":[{"smallNumber":0}]}]}]
}
]
}]
}
This cartrige plays a short scale of notes with a beat. The screen flashes green every time the loop is started, a red square on the top left of the screen blinks of red when a beat is played and a blue one on the top right flashes when the snare is played.
This example code has 3 when commands controlling the 3 flashes. They are very similar so let's have a look just to the first one.
The line when command conditions picks the songRow special object and its sub-attribute M0 and checks if its value is equal to the string AC4-.
The M0 represents the music track we want to query and in this case it's the first one. The tracks are the horizontal lines of notes defined in the music key of the data block - have a look at the data blocks chapter to learn more. The sub-getter M0 on the special object songRow returns the note that's currently playing on the first track of the current music.
songRow returns the currently played note prefixed with the instrument that's playing it. So AC4- means that the A instrument is playing the C4- note. A condition to this value on track M0 will trigger when the A instrument plays the C4- note on the first music track: the then statements will change the screen background to green else it's set back to black.