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public class MainActivity extends AppCompatActivity {
    private Button mSignUpBtn;
    private ImageView mLeftLogoImg;
    private ImageView mRightLogoImg;
    private TextView mDescTitleTextView;
    private LinearLayout mLettersLayout;
    private LinearLayout mSignInLayout;
    private ArrayList<SpringAnimation> mLetterAnims;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // hide the status ui.
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
        setContentView(R.layout.activity_main);
        // get the screen height.
        DisplayMetrics dm = getResources().getDisplayMetrics();
        int screenHeight = dm.heightPixels;
        // letters about 'Converse'
        mLettersLayout = (LinearLayout) findViewById(R.id.letter_layout);
        mLetterAnims = new ArrayList<>();
        for (int i = 0; i < mLettersLayout.getChildCount(); i++) {
            View letterView = mLettersLayout.getChildAt(i);
            letterView.setTranslationY(screenHeight);
            SpringAnimation letterAnimY = new SpringAnimation(letterView, SpringAnimation.TRANSLATION_Y, 0);
            letterAnimY.getSpring().setStiffness(SpringForce.STIFFNESS_VERY_LOW);
            letterAnimY.getSpring().setDampingRatio(SpringForce.DAMPING_RATIO_LOW_BOUNCY);
            mLetterAnims.add(letterAnimY);
        }
        
        // text about 'Native messaging'
        mDescTitleTextView = (TextView) findViewById(R.id.desc_title_textview);
        mDescTitleTextView.setTranslationY(500f);
        mDescTitleTextView.setAlpha(0f);
        final SpringAnimation descTitleAnimY = new SpringAnimation(mDescTitleTextView, DynamicAnimation.TRANSLATION_Y, 0);
        descTitleAnimY.getSpring().setStiffness(SpringForce.STIFFNESS_VERY_LOW);
        descTitleAnimY.getSpring().setDampingRatio(SpringForce.DAMPING_RATIO_LOW_BOUNCY);
        
        final ValueAnimator descTitleAlphaAnim = ObjectAnimator.ofFloat(0f, 1f);
        descTitleAlphaAnim.setDuration(300);
        descTitleAlphaAnim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator valueAnimator) {
                mDescTitleTextView.setAlpha((Float) valueAnimator.getAnimatedValue());
            }
        });
        
        // the button of sign up
        mSignUpBtn = (Button) findViewById(R.id.sign_up_btn);
        mSignUpBtn.setTranslationY(500f);
        final SpringAnimation signUpBtnAnimY = new SpringAnimation(mSignUpBtn, SpringAnimation.TRANSLATION_Y, 0);
        signUpBtnAnimY.getSpring().setStiffness(SpringForce.STIFFNESS_VERY_LOW);
        signUpBtnAnimY.getSpring().setDampingRatio(SpringForce.DAMPING_RATIO_LOW_BOUNCY);
        // the bottom text about 'Have an account? sign in'
        mSignInLayout = (LinearLayout) findViewById(R.id.signin_layout);
        mSignInLayout.setTranslationY(500f);
        final SpringAnimation signInLayoutAnimY = new SpringAnimation(mSignInLayout, SpringAnimation.TRANSLATION_Y, 0);
        signInLayoutAnimY.getSpring().setStiffness(SpringForce.STIFFNESS_VERY_LOW);
        signInLayoutAnimY.getSpring().setDampingRatio(SpringForce.DAMPING_RATIO_LOW_BOUNCY);
        // top logo by left
        mLeftLogoImg = (ImageView) findViewById(R.id.left_logo_imageview);
        mLeftLogoImg.setTranslationY(400f);
        mLeftLogoImg.setAlpha(0f);
        final SpringAnimation leftLogoAnimY = new SpringAnimation(mLeftLogoImg, SpringAnimation.TRANSLATION_Y, 0);
        leftLogoAnimY.getSpring().setStiffness(SpringForce.STIFFNESS_VERY_LOW);
        leftLogoAnimY.getSpring().setDampingRatio(SpringForce.DAMPING_RATIO_LOW_BOUNCY);
        leftLogoAnimY.setStartVelocity(-2000);
        // top logo by right
        mRightLogoImg = (ImageView) findViewById(R.id.right_logo_imageview);
        mRightLogoImg.setTranslationY(400f);
        mRightLogoImg.setAlpha(0f);
        final SpringAnimation rightLogoAnimY = new SpringAnimation(mRightLogoImg, SpringAnimation.TRANSLATION_Y, 0);
        rightLogoAnimY.getSpring().setStiffness(SpringForce.STIFFNESS_VERY_LOW);
        rightLogoAnimY.getSpring().setDampingRatio(SpringForce.DAMPING_RATIO_LOW_BOUNCY);
        rightLogoAnimY.setStartVelocity(-2000);
        
        final ValueAnimator logoAlphaAnim = ObjectAnimator.ofFloat(0f, 1f);
        logoAlphaAnim.setDuration(600);
        logoAlphaAnim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator valueAnimator) {
                mLeftLogoImg.setAlpha((Float) valueAnimator.getAnimatedValue());
                mRightLogoImg.setAlpha((Float) valueAnimator.getAnimatedValue());
            }
        });
        mRightLogoImg.postDelayed(new Runnable() {
            @Override
            public void run() {
                leftLogoAnimY.start();
                mRightLogoImg.postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        rightLogoAnimY.start();
                    }
                }, 150);
                mDescTitleTextView.postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        descTitleAlphaAnim.setStartDelay(100);
                        descTitleAlphaAnim.start();
                        descTitleAnimY.start();
                        signUpBtnAnimY.start();
                        signInLayoutAnimY.start();
                    }
                }, 300);
                for (final SpringAnimation letterAnim : mLetterAnims) {
                    mLettersLayout.postDelayed(new Runnable() {
                        @Override
                        public void run() {
                            letterAnim.start();
                        }
                    }, 12 * mLetterAnims.indexOf(letterAnim));
                }
                logoAlphaAnim.start();
            }
        }, 1000);
    }
}