Monday, January 11, 2016

Tweens in Phaser

I was playing around with another simple game-in-a-comic idea today and I started using Phaser's Tweens. No, Phaser doesn't actually have children between the ages of 10 and 12. Phaser's Tweens are a convenient way of updating one or more object properties over a specified time range.

I'd never used a Tween before, but after playing around with them for a little while I decided that I could use them to replace an effect I had been doing "the hard way" in my platformer. Recall that when the frog gets hurt, I temporarily set his alpha value to 0.5 and then start a timer to set it back. Along with the alpha value, I also set/unset an immune flag on the frog. I can do this same thing better using a Tween.

I've replaced my frog-hurting-alpha-adjusting code with the following...

var t = game.add.tween(frog);
t.onComplete.add(function() {
    frog.immune = false;
}); { alpha: 0.5 }, 50, Phaser.Easing.Linear.None, true, 0, 2, true);

On line 1, I create the Tween object using the familiar GameObjectFactory (game.add). When creating a Tween, you pass in the object on which it will work. In my case, that's the frog sprite.

Before starting any tweening, I add a listener for the onComplete event. I do this because I want to turn off the frog's immunity when the alpha animation is over.

The final step is to create and start the tween. The last line above calls the to function, creating a tween that moves the specified properties of the object TO the given values. The properties and the target values are given by the object that is the first argument to the function. In my example above, that's { alpha: 0.5 }, which means I want to move the frog.alpha property from whatever it is currently to 0.5. The second argument is 50, which means I want the transition to the new alpha value to take 50 milliseconds. The third argument specifies the easing function to use in the transition. The fourth argument specifies auto-start. I want this tween to start immediately upon creation, so I specified true. The 5th argument is start delay, which is none because I used 0. The 6th argument, which I have set to 2, is the number of times to repeat. The final argument, which I set to true, is the yoyo parameter. Turning on yoyo means that the tween with play backwards automatically at the end. This means that after reaching the alpha value 0.5, it will start transitioning back to the original alpha value. For me, this created a nice flashing effect.

So how long does the whole thing take? I specified 50 milliseconds, but that's just for the one-way transition from 1 to 0.5. Since I turned on yoyo, it doubles to 100 milliseconds. And I also set it for 2 repeats, so the whole flashing alpha thing should complete around 300 milliseconds after it starts. Just remember to do all that math if the length of time is important to you.

The frog's alpha flashes after being hit - thanks to Tweens!
Tweens give me the ability to do some nice things with animating Sprite properties which would be very difficult to do in other ways. I'm glad I discovered them. You can discover the complete source code for the game on GitHub. Just click on this link!

And speaking of discovering things, have you discovered how much you like reading my comics yet? Look at today's, and maybe look at some other ones while you're there. comic for 11 January 2016

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