Francesco Francesco - 10 months ago 53
Android Question

How does Android ObjectAnimator work?

how ObjectAnimator is able to call the appropriate method

if the attribute
is specified as a string. What i mean is, what technique is used to recognize that i want to animate the attribute "rotation" of my view and call the appropriate method setRotation of that view?

I already understood how ObjectAnimator works and managed to use it, it is quite simple, i am just curious about the operating principles.

Sorry for my english, i never use it :)


There are a number of ways to animate the rotation of a view:

1. ObjectAnimator.ofFloat(view, "rotation", 0f, 90f).start();

This uses reflection to call the setRotation(float f) and float getRotation() methods of the view.

You can use this method to animate any property of a class as long as that class has implemented the appropriate getter and setter methods for that property.

But reflection is a slow operation, so there is a second method that doesn't use reflection.

2. ObjectAnimator.ofFloat(view, View.ROTATION, 0f, 90f).start();

This uses the rotation Property of the view. Property is an abstract class that defines the setValue(T) and the T get() methods which in turn call the actual getter and setter of the supplied object. For example, the rotation property on the View class uses the following code:

public static final Property<View, Float> ROTATION = new FloatProperty<View>("rotation") {
    public void setValue(View object, float value) {

    public Float get(View object) {
        return object.getRotation();

If you want to animate a custom property of an object, you can implement your own Property like the one above.

Then there is a third method, which also doesn't use reflection.

3. view.animate().rotation(90f);

This one has a fluent interface so it's easier to use. You can also chain multiple animations to run together, for example: view.animate().rotation(90f).translationX(10f);

The downside of this method is that you can only animate the standard properties of a View and not custom properties or properties on your own classes.