startActivityForResult
Intent intent = new Intent(A.this,B.class);
startActivityForResult(Intent intent, Int requestCode)(A跳转到B还会回来的A)
setResult(int resultCode, Intent intent) (B调用这个函数回到前面页面A)
finish();
onActivityResult(int requestCode, int resultCode, Intent data) (A class自动执行onActivityResult)