code: private void startNewTransition() {
          if (!hasBounds()) {
              return; // Can't start transition if the drawable has no bounds.
          }
          mCurrentTrans = mTransGen.generateNextTransition(mDrawableRect, mViewportRect);
          mElapsedTime = 0;
          mLastFrameTime = System.currentTimeMillis();
          fireTransitionStart(mCurrentTrans);
      }
docs: Generates and starts a transition.