How to explain the Coordinate system on Android animation?

Question

As we know , the android coordinate system is start from the top left corner of the android screen. The x-axis is down growth and the y-axis is right growth.But I found it's not right for the animation.

For example, I initialized the TranslateAnimation using the constructed function:

TranslateAnimation ta = new TranslateAnimation(0.0f, 200, 0.0f, 200);  

Does the coordinate system have changed ? I found it didn't start from the top left corner.

Then I initialized the other translateAnimation for moving up and right direction :

TranslateAnimation ta = new TranslateAnimation(0.0f, 200, 0.0f, -200); 
ta.setReaptMode(Animation.REVERSE);

The same behavior would be found. I am confused about it.

Answer 1

I believe that constructor for TranslateAnimation uses deltas. See this. Or look at the constructor sig. : (float fromXDelta, float toXDelta, float fromYDelta, float toYDelta). So if you want your anim. to jump up first, you could use a negative third ctor param.

More precisely:

An animation can never start until after the layout has been measured. One usually shouldn't have to worry about how this works beyond that the algorithm is mostly very good and you can take control of its strategies by setting layout parameters. In short, by the time an animation might be started, we know where you want the view to be on the screen, because you set layout parameters.

Translate animation then takes deltas from that position. So your current animation shouldn't start from the top left, but rather wherever those layout params were evaluated by onMeasure.

Some would say- how annoying. It's gonna get complicated even if you just want to do some simple up-down type animations... Well, here's an advisable development strategy; it snould make android animation development a breeeze. Set an animationListener on every animation. In onAnimationEnd, in possibly a parametized way, reset the layout parameters on the view your animating to where you expect it to be. That way, you'll get no surprising "jumps" when you re-apply an animation again. You may need to invalidate in some circumstances, or clearAnimation. The reason that this works is that the measure pass will be caused to come round again and you'll have a new offset for your TranslateAnimation. Finally, you may want to do all this resetting posted to the message queue of a view using post(Runnable runnable) in the listener, so you're off the last pass of the animation draw.

I too found android Animations can occasionally surprise you and cause jumpy behaviour. But if you do it like this, constructors taking delta params shouldn't be confusing again.