为什么不直接在animatorset上调用cancel方法，而该animator到底是什么呢？

Question

I have three questions that I really need to clarify one them is in the code provided by the android developer at this link: http://developer.android.com/training/animation/zoom.html#animate (I didn't want to overwhelm you with the code so I didn't copy and paste the code here and the code is under the section called "Zoom the View")

1) So my question is why does the programmer do the following? mCurrentAnimator = set and then write

if (mCurrentAnimator != null) {
            mCurrentAnimator.cancel();
        }

Let me quickly mention that mCurrentAnimator is an Animator object and set is an AnimatorSet object. My confusion lies in the fact that why does the developer feel the need to make the an Animator object = to a AnimatorSet object before calling the cancel() because from my perspective, since AnimatorSet extends Animator then shouldn't I be able to call the cancel method directly. Why feel the need to make a superclass = to the subclass to call a method when the subclass already can call that method?

2) My question is what exactly is an Animator because when I check at the documentation there are some method of this class such as public void setTarget (Object target) which made me confuse. What made me confuse was the fact that the description stated this method "Sets the target object whose property will be animated by this animation". Precisely how can an animator tells what property to animate just simply by providing a target?

3) Also, one of the animator method was public void setupStartValues () and the description stated "This method tells the object to use appropriate information to extract starting values for the animation. For example, a AnimatorSet object will pass this call to its child objects to tell them to set up the values." However, what I don't understand is how can a method be pass into a child object and how does the method tells the object to use what information to use so that it is appropriate??

Answer 1

For your first question, the main function of that example is to zoom the image thumbnail when the user clicks on it. You'll notice that the mCurrentAnimator.cancel() method is called at the very beginning of the onClick method, and mCurrentAnimator = set is at the end. What this is doing is, once an animation is created and started, the program is saving a reference to it. This is useful because what if, say, the user clicks the thumbnail, the image starts to animate, and while the animation is running, the user clicks the image again? the mCurrentAnimator will hold the reference to the animation currently in progress, and the mCurrentAnimator.cancel() stops that animation so that the onClick method can create another animator in response to the most recent click.

For the second question, simply put, an Animator is used to animate a given view by changing its properties. To address the confusion around the setTarget method, that method is not a constructor. If you want to create a new animator, you can use the ofFloat, ofArgb, or similar static methods from the ObjectAnimator class. You can read about them here . Using those methods, you tell the animator what properties it needs to change. The setTarget method is only to change what object you are animating in an existing animator , which can be useful if you would like to apply the same animator to multiple views.

Finally, an AnimatorSet is essentially a container to hold different animations you want to play either together or in sequence. Very simply, "passing the call" means that, when you call the setupStartValues() on the AnimatorSet, it calls the setupStartValues() method on each of the individual Animators belonging to that set.

I hope this helps!