Material Design动画（一）reveal effect

   Animator animator = ViewAnimationUtils.createCircularReveal(btn2,
                        btn2.getWidth() / 2,
                        btn2.getHeight() / 2, 0,
                        btn2.getHeight());

                animator.addListener(new AnimatorListenerAdapter() {
                    @Override
                    public void onAnimationStart(Animator animator) {
                        super.onAnimationStart(animator);
                        Log.d(TAG, "onAnimationStart");
                        btn2.setVisibility(View.VISIBLE);
                    }

                    @Override
                    public void onAnimationResume(Animator animation) {
                        super.onAnimationResume(animation);
                        Log.d(TAG, "onAnimationResume");

                    }

                    @Override
                    public void onAnimationEnd(Animator animation) {
                        super.onAnimationEnd(animation);
                        Log.d(TAG, "onAnimationEnd");
                    }

                    @Override
                    public void onAnimationPause(Animator animation) {
                        super.onAnimationPause(animation);
                        Log.d(TAG, "onAnimationPause");
                    }
                });

                animator.setInterpolator(new LinearInterpolator());
                animator.setDuration(3000);
                animator.start();
            }

btn2这个view从无到有 以 btn2.getWidth() / 2 和 btn2.getHeight() / 2为圆心，初始半径为 0 终止半径为 btn2.getHeight()展示出来

需要注意的一点: btn2一定要设置成invisible而不能设置为gone