动画 -- 按钮 --- 从无到有（中心）

//        sa = new ScaleAnimation(0, 1, 0, 1,100,50);
//        sa = new ScaleAnimation(0, 1, 0, 1, Animation.RELATIVE_TO_SELF, 0.5f, Animation.RELATIVE_TO_SELF, 0.5f)
//        sa.setDuration(1000);
        
        findViewById(R.id.btnScaleMe).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View arg0) {
                arg0.startAnimation(AnimationUtils.loadAnimation(MainActivity.this, R.anim.sa));
            }
        });

<?xml version="1.0" encoding="utf-8"?>
<scale xmlns:android="http://schemas.android.com/apk/res/android"
    android:fromXScale="0"
    android:toXScale="1"
    android:fromYScale="0"
    android:toYScale="1"
    android:duration="1000"
    android:pivotX="50%"
    android:pivotY="50%" >

</scale>