In this article we talk about animation, specifically property animation.

ViewPropertyAnimator

View has many properties, like position on screen, color and size. Property animation is the type of animation that animate on those properties.

In other words, the animator’s job is to set a specific set of properties of the view to different values many many times in a short period of time.

For example, the following code will move the view’s position 500px to the right:

view . animate (). translationX ( 500 );

Now let’s list all the setter methods of the view’s properties and its corresponding method in the ViewPropertyAnimator .

// TRANSLATION_X view . setTranslationX () view . animate (). translationX () // TRANSLATION_Y view . setTranslationY () view . animate (). translationY () // TRANSLATION_Z view . setTranslationZ () view . animate (). translationZ () // X view . setX () view . animate (). x () // Y view . setY () view . animate (). y () // Z view . setZ () view . animate (). z () // ROTATION view . setRotation () view . animate (). rotation () // ROTATION_X view . setRotationX () view . animate (). rotationX () // SCALE_X view . setScaleX () view . animate (). scaleX () // SCALE_Y view . setScaleY () view . animate (). scaleY () // ALPHA view . setAlpha () view . animate (). alpha ()

One thing to note is that these animations can be run in parallel:

imageView . animate () . translationX ( Utils . dpToPixel ( 200 )) . rotation ( 360 ) . scaleX ( 1 ) . scaleY ( 1 ) . alpha ( 1 );

These animation are pretty intuitive, so I will not give examples on them.

ObjectAnimator

ViewPropertyAnimator can be used to animate the existing properties of the view, but what if you have a custom view and you want to animate on a special property? Then you can use ObjectAnimator .

Let’s say you have a custom progress view like this:

public class ProgressView extends View { final float radius = dpToPixel ( 80 ); // Property progress float progress = 0 ; RectF arcRectF = new RectF (); Paint paint = new Paint ( Paint . ANTI_ALIAS_FLAG ); public ProgressView ( Context context ) { super ( context ); } public ProgressView ( Context context , @Nullable AttributeSet attrs ) { super ( context , attrs ); } public ProgressView ( Context context , @Nullable AttributeSet attrs , int defStyleAttr ) { super ( context , attrs , defStyleAttr ); } { paint . setTextSize ( dpToPixel ( 40 )); paint . setTextAlign ( Paint . Align . CENTER ); } // For object animator's use public float getProgress () { return progress ; } // For object animator's use public void setProgress ( float progress ) { this . progress = progress ; invalidate (); } @Override public void onDraw ( Canvas canvas ) { super . onDraw ( canvas ); float centerX = getWidth () / 2.0f ; float centerY = getHeight () / 2.0f ; paint . setColor ( Color . parseColor ( "#E91E63" )); paint . setStyle ( Paint . Style . STROKE ); paint . setStrokeCap ( Paint . Cap . ROUND ); paint . setStrokeWidth ( dpToPixel ( 15 )); arcRectF . set ( centerX - radius , centerY - radius , centerX + radius , centerY + radius ); canvas . drawArc ( arcRectF , 135 , progress * 2.7f , false , paint ); paint . setColor ( Color . WHITE ); paint . setStyle ( Paint . Style . FILL ); canvas . drawText (( int ) progress + "%" , centerX , centerY - ( paint . ascent () + paint . descent ()) / 2 , paint ); } }

Now we can use the objectAnimtor to animate the property progress in the view.

ObjectAnimator animator = ObjectAnimator . ofFloat ( view , "progress" , 0 , 65 ); animator . setDuration ( 1000 ); animator . setInterpolator ( new FastOutSlowInInterpolator ()); animator . start ();

The object animator here will automatically call setProgress() to set the progress, and getProgress() to get its initial value. So if your property is XXX, then in your custom view, you have to define setXXX() and getXXX() methods. Also be careful that in setXXX() you have to manually update the view by calling invalidate() .

Interpolator

An interpolator defines the rate of change of an animation. This allows the basic animation effects (alpha, scale, translate, rotate) to be accelerated, decelerated, repeated, etc.

Let’s explore different types of interpolator that Android provides.

AccelerateDecelerateInterpolator

An interpolator where the rate of change starts and ends slowly but accelerates through the middle.

This is the default interpolator, and also probably the most natural in the physical world. This should work for you most of the time.

LinearInterpolator

An interpolator where the rate of change is constant

AccelerateInterpolator

An interpolator where the rate of change starts out slowly and then accelerates.

This is a perfect choice if you want to simulate the effect of a rocket taking off.

DecelerateInterpolator

An interpolator where the rate of change starts out quickly and then decelerates.

This is the opposite of AccelerateInterpolator , it is useful when you want to show something flies into the screen.

AnticipateInterpolator

An interpolator where the change starts backward then flings forward.

OvershootInterpolator

An interpolator where the change flings forward and overshoots the last value then comes back.

AnticipateOvershootInterpolator

An interpolator where the change starts backward then flings forward and overshoots the target value and finally goes back to the final value.

This is the combination of AnticipateInterpolator and OvershootInterpolator .

BounceInterpolator

An interpolator where the change bounces at the end.

CycleInterpolator

Repeats the animation for a specified number of cycles. The rate of change follows a sinusoidal pattern.

Let’s try to understand the cycle. When we specify the 0.5 cycle:

new CycleInterpolator ( 0.5f );

Then 1.0f cycle:

new CycleInterpolator ( 1f );

This looks pretty useful for certain type of loading animation.

FastOutLinearInInterpolator

Uses a lookup table for the Bezier curve from (0,0) to (1,1) with control points: P0 (0, 0) P1 (0.4, 0) P2 (1.0, 1.0) P3 (1.0, 1.0)

The defination makes not much sense from the doc. But basically this is just like AccelerateInterpolator , the difference is that AccelerateInterpolator uses Logarithm function, and FastOutLinearInInterpolator uses Bezier function.

The red line is FastOutSlowInInterpolator and the green line is AccelerateInterpolator , see the difference? Not much right?

FastOutSlowInInterpolator

Interpolator corresponding to {@link android.R.interpolator#fast_out_slow_in}. Uses a lookup table for the Bezier curve from (0,0) to (1,1) with control points: P0 (0, 0) P1 (0.4, 0) P2 (0.2, 1.0) P3 (1.0, 1.0)

OK, basically this is similar to AccelerateDecelerateInterpolator . Let’s compare them:

The green line is AccelerateDecelerateInterpolator and the red line is FastOutLinearInInterpolator .

LinearOutSlowInInterpolator

This is basically similar to DecelerateInterpolator .

Let’s do the comparison here: