Android滑动Scrollview标题栏渐变效果(仿京东toolbar)

/**
 * @ClassName MyScrollView
 * @Author Rex
 * @Date 2021/1/27 17:38
 */
public class MyScrollView extends ScrollView {
 private TranslucentListener mTranslucentListener;

 public void setTranslucentListener(TranslucentListener translucentListener) {
  this.mTranslucentListener = translucentListener;
 }
 public MyScrollView(Context context) {
  this(context, null);
 }

 public MyScrollView(Context context, AttributeSet attrs) {
  this(context, attrs, 0);
 }

 public MyScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
  this(context, attrs, defStyleAttr, 0);
 }

 public MyScrollView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
  super(context, attrs, defStyleAttr, defStyleRes);
 }

 @Override
 protected void onScrollChanged(int l, int t, int oldl, int oldt) {
  super.onScrollChanged(l, t, oldl, oldt);
  if (mTranslucentListener != null) {
   //ScrollView滑出高度
   int scrollY = getScrollY();
   //屏幕高度
   int screenHeight = getContext().getResources().getDisplayMetrics().heightPixels;
   //有效滑动距离为屏幕2分之一
   // alpha = 滑动高度/（screenHeight/3f）
   if (scrollY <= screenHeight / 2f) {
    Log.d(">>>>>>>>>", "ScrollView划出高度：" + scrollY);
    Log.d(">>>>>>>>>", "屏幕高度：" + screenHeight);
    Log.d(">>>>>>>>>", "渐变值：" + (0 + scrollY / (screenHeight / 4f)));
    // 渐变的过程 1~0
    mTranslucentListener.onTranslucent(0 + scrollY / (screenHeight /4f));
   }
  }
 }
}

public class ToolbarActivity extends AppCompatActivity implements TranslucentListener {
 private Toolbar mToolBar;
 private MyScrollView mScrollView;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_toobar);
  mToolBar = findViewById(R.id.id_toolbar);
  mScrollView = findViewById(R.id.id_scrollView);
  //初始化渐变为0
  mToolBar.setAlpha(0);
  //设置渐变回调
  mScrollView.setTranslucentListener(this);
 }

 @Override
 public void onTranslucent(float alpha) {
  mToolBar.setAlpha(alpha);
 }
}

/**
 * @ClassName TranslucentListener
 * @Author rex
 * @Date 2021/1/27 17:38
 */
public interface TranslucentListener {
 /**
  * 透明度的回调监听
  *
  * @param alpha 0~1 透明度
  */
 public void onTranslucent(float alpha);
}

/**
 * @ClassName MyScrollView
 * @Author Rex
 * @Date 2021/1/27 17:38
 */
public class MyScrollView extends ScrollView {
 private TranslucentListener mTranslucentListener;

 public void setTranslucentListener(TranslucentListener translucentListener) {
  this.mTranslucentListener = translucentListener;
 }
 public MyScrollView(Context context) {
  this(context, null);
 }

 public MyScrollView(Context context, AttributeSet attrs) {
  this(context, attrs, 0);
 }

 public MyScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
  this(context, attrs, defStyleAttr, 0);
 }

 public MyScrollView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
  super(context, attrs, defStyleAttr, defStyleRes);
 }

 @Override
 protected void onScrollChanged(int l, int t, int oldl, int oldt) {
  super.onScrollChanged(l, t, oldl, oldt);
  if (mTranslucentListener != null) {
   //ScrollView滑出高度
   int scrollY = getScrollY();
   //屏幕高度
   int screenHeight = getContext().getResources().getDisplayMetrics().heightPixels;
   //有效滑动距离为屏幕2分之一
   // alpha = 滑动高度/（screenHeight/3f）
   if (scrollY <= screenHeight / 2f) {
    Log.d(">>>>>>>>>", "ScrollView划出高度：" + scrollY);
    Log.d(">>>>>>>>>", "屏幕高度：" + screenHeight);
    Log.d(">>>>>>>>>", "渐变值：" + (1 - scrollY / (screenHeight / 4f)));
    // 渐变的过程 1~0
    mTranslucentListener.onTranslucent(1 - scrollY / (screenHeight /4f));
   }
  }
 }
}