Android 可伸缩的下拉过度HeadZoomScrollView

思路

1. 获得要放大的控件，并获得其宽高； 
2. 在顶部时继续往下拉，通过LayoutParams改变控件的宽高； 
3. 手指抬起时初始化各项参数，通过属性动画回弹控件。

坑（ScrollView 在xml在最顶层要包LinearLayout 不然会报错误crollView can host only one direct child）

public class HeadZoomScrollView  extends ScrollView {


public HeadZoomScrollView(Context context) {
            super(context);
        }

public HeadZoomScrollView(Context context, AttributeSet attrs) {
            super(context, attrs);
        }

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

// 用于记录下拉位置
        private float y = 0f;
// zoomView原本的宽高
        private int zoomViewWidth = 0;
        private int zoomViewHeight = 0;

// 是否正在放大
        private boolean mScaling = false;

// 放大的view，默认为第一个子view
        private View zoomView;
        public void setZoomView(View zoomView) {
            this.zoomView = zoomView;
        }

// 滑动放大系数，系数越大，滑动时放大程度越大
        private float mScaleRatio = 0.4f;
        public void setmScaleRatio(float mScaleRatio) {
            this.mScaleRatio = mScaleRatio;
        }

// 最大的放大倍数
        private float mScaleTimes = 2f;
        public void setmScaleTimes(int mScaleTimes) {
            this.mScaleTimes = mScaleTimes;
        }

// 回弹时间系数，系数越小，回弹越快
        private float mReplyRatio = 0.5f;
        public void setmReplyRatio(float mReplyRatio) {
            this.mReplyRatio = mReplyRatio;
        }

        @Override
        protected void onFinishInflate() {
            super.onFinishInflate();
// 不可过度滚动，否则上移后下拉会出现部分空白的情况
            setOverScrollMode(OVER_SCROLL_NEVER);
// 获得默认第一个view
            if (getChildAt(0) != null && getChildAt(0) instanceof ViewGroup && zoomView == null) {
                ViewGroup vg = (ViewGroup) getChildAt(0);
                if (vg.getChildCount() > 0) {
                    zoomView = vg.getChildAt(0);
                }
            }
        }

        @Override
        public boolean onTouchEvent(MotionEvent ev) {
            if (zoomViewWidth <= 0 || zoomViewHeight <=0) {
                zoomViewWidth = zoomView.getMeasuredWidth();
                zoomViewHeight = zoomView.getMeasuredHeight();
            }
            if (zoomView == null || zoomViewWidth <= 0 || zoomViewHeight <= 0) {
                return super.onTouchEvent(ev);
            }
            switch (ev.getAction()) {
                case MotionEvent.ACTION_MOVE:
                    if (!mScaling) {
                        if (getScrollY() == 0) {
                            y = ev.getY();//滑动到顶部时，记录位置
                        } else {
                            break;
                        }
                    }
                    int distance = (int) ((ev.getY() - y)*mScaleRatio);
                    if (distance < 0) break;//若往下滑动
                    mScaling = true;
                    setZoom(distance);
                    return true;
                case MotionEvent.ACTION_UP:
                    mScaling = false;
                    replyView();
                    break;
            }
            return super.onTouchEvent(ev);
        }

/**放大view*/
        private void setZoom(float s) {
            float scaleTimes = (float) ((zoomViewWidth+s)/(zoomViewWidth*1.0));
// 如超过最大放大倍数，直接返回
            if (scaleTimes > mScaleTimes) return;

            ViewGroup.LayoutParams layoutParams = zoomView.getLayoutParams();
            layoutParams.width = (int) (zoomViewWidth + s);
            layoutParams.height = (int)(zoomViewHeight*((zoomViewWidth+s)/zoomViewWidth));
// 设置控件水平居中
            ((MarginLayoutParams) layoutParams).setMargins(-(layoutParams.width - zoomViewWidth) / 2, 0, 0, 0);
            zoomView.setLayoutParams(layoutParams);
        }

/**回弹*/
        private void replyView() {
            final float distance = zoomView.getMeasuredWidth() - zoomViewWidth;
// 设置动画
            ValueAnimator anim = ObjectAnimator.ofFloat(distance, 0.0F).setDuration((long) (distance * mReplyRatio));
            anim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
                @Override
                public void onAnimationUpdate(ValueAnimator animation) {
                    setZoom((Float) animation.getAnimatedValue());
                }
            });
            anim.start();
        }

        @Override
        protected void onScrollChanged(int l, int t, int oldl, int oldt) {
            super.onScrollChanged(l, t, oldl, oldt);
            if (onScrollListener!=null) onScrollListener.onScroll(l,t,oldl,oldt);
        }

        private OnScrollListener onScrollListener;
        public void setOnScrollListener(OnScrollListener onScrollListener) {
            this.onScrollListener = onScrollListener;
        }

/**滑动监听*/
        public interface OnScrollListener{
            void onScroll(int scrollX,int scrollY,int oldScrollX, int oldScrollY);
        }


    }

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <com.example.myapplication.HeadZoomScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        >
        <LinearLayout android:id="@+id/linearLayout1"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">
    <ImageView
        android:layout_width="match_parent"
        android:layout_height="800dp"
        android:id="@+id/tvssss"
        android:src="@mipmap/ic_launcher"
         />

            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Hello World!"
                android:id="@+id/tv"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintLeft_toLeftOf="parent"
                app:layout_constraintRight_toRightOf="parent"
                app:layout_constraintTop_toTopOf="parent" />

        </LinearLayout>



    </com.example.myapplication.HeadZoomScrollView>

</android.support.constraint.ConstraintLayout>