Custom animation in Android

Question

I've written a custom View. Now I want to do a little custom animation when the user touches it.

When I say custom, I mean I basically want to render each frame myself, and not use a "predefined" animation like described here.

What is the proper way of implementing this?

Answer 1

Most flexible (and pretty easy) way to create custom animation is to extend Animation class.

In general:

1. Set duration of your animation using setDuration() method.
2. Optionally set the interpolator for your animation using setInterpolator() (for exapmle you can use LinearInterpolator or AccelerateInterpolator etc.)
3. Override applyTransformation method. Here we interested in interpolatedTime variable which changes between 0.0 and 1.0 and represent the your animation progress.

Here is an example (I'm using this class to change ofsset of my Bitmap. Bitmap itself is drawn in draw method):

public class SlideAnimation extends Animation {

    private static final float SPEED = 0.5f;

    private float mStart;
    private float mEnd;

    public SlideAnimation(float fromX, float toX) {
        mStart = fromX;
        mEnd = toX;

        setInterpolator(new LinearInterpolator());

        float duration = Math.abs(mEnd - mStart) / SPEED;
        setDuration((long) duration);
    }

    @Override
    protected void applyTransformation(float interpolatedTime, Transformation t) {
        super.applyTransformation(interpolatedTime, t);

        float offset = (mEnd - mStart) * interpolatedTime + mStart;
        mOffset = (int) offset;
        postInvalidate();
    }

}

Also you can modify View by using Transformation#getMatrix().

UPDATE

In case if you're using Android Animator framework (or compatibility implementation - NineOldAndroids) you can just declare setter and getter for your custom View property and animate it directly. Here is an another example:

public class MyView extends View {

    private int propertyName = 50;

    /* your code */

    public int getPropertyName() {
        return propertyName;
    }

    public void setPropertyName(int propertyName) {
        this.propertyName = propertyName;
    }

    /*
    There is no need to declare method for your animation, you 
    can, of course, freely do it outside of this class. I'm including code
    here just for simplicity of answer.
    */
    public void animateProperty() {
        ObjectAnimator.ofInt(this, "propertyName", 123).start();
    }

}