一、背景
ViewDragHelper是处理手势事件的神兵利器，在此使用它来实现一个可上下滑动抽屉式的自定义控件--VDragLayout。
实现功能：
1、可以根据手势的速度快慢开关抽屉；
2、松开手指滑动速度小于设定值时，当显示的区域超过可滑动的区域一半，就打开抽屉，否则关闭；
3、开关的方向，可以向上或向下；
4、当手指点击时不在子View的范围内，不获取处理事件；
二、实现
第一步：初始化ViewDragHelper
使用静态方法：ViewDragHelper.create(ViewGroup forParent, Callback cb)
//初始化ViewDragHelper
mDragHelper = ViewDragHelper.create(this, new ViewDragCallback());
第二步：ViewDragHelper获取事件
在ViewGroup的onTouchEvent方法里传入事件，注意要返回true，表示消费了事件。
@Override
public boolean onTouchEvent(MotionEvent event) {
//ViewDragHelper传入事件
mDragHelper.processTouchEvent(event);
return true;
}
第三步：创建ViewDragHelper.Callback的实例
实例即第一步中的Callback，复写必要的方法：
private class ViewDragCallback extends ViewDragHelper.Callback {
/**
* 确定是否分析view的touch
*/
@Override
public boolean tryCaptureView(@NonNull View view, int i) {
return view == mChildView;
}
/**
* touch移动之后的垂直方向回调分析
*
* @param child capture的view/移动的view
* @param top child的顶部距离
* @param dy 垂直方向增量y
* @return 返回要移动的距离
*/
@Override
public int clampViewPositionVertical(@NonNull View child, int top, int dy) {
if (child == mChildView) {
//向上滑动
if (mDirection == UP) {
if (top > 0) {
return 0;
} else if (-top > mCanOffsetHeight) {
return -mCanOffsetHeight;
}
} else if (mDirection == DOWN) {
if (top < 0) {
return 0;
} else if (top > mCanOffsetHeight) {
return mCanOffsetHeight;
}
}
}
return top;
}
/**
* 当控件位置移动时的回调
* @param changedView 位置改变的View
* @param left 左侧距离
* @param top 顶部距离
* @param dx 水平方向增量
* @param dy 垂直方向增量
*/
@Override
public void onViewPositionChanged(@NonNull View changedView, int left, int top, int dx, int dy) {
ViewCompat.postInvalidateOnAnimation(VDragLayout.this);
}
/**
* 手势释放时回调：松开，判断超过一半，则打开，没超过一半则回归原位
* @param releasedChild
* @param xvel 释放时水平方向速度
* @param yvel 释放时垂直方向速度
*/
@Override
public void onViewReleased(@NonNull View releasedChild, float xvel, float yvel) {
if (Math.abs(yvel) > mVelocityLimit) {
boolean isShow = (yvel > 0 && mDirection == UP) || (yvel < 0 && mDirection == DOWN);
if (isShow) {
toggle(true);
} else {
toggle(false);
}
return;
}
int top = mChildView.getTop();
boolean isShow = (mDirection == UP && -top < mCanOffsetHeight / 2)
|| (mDirection == DOWN && top < mCanOffsetHeight / 2);
if (isShow) {
toggle(true);
} else {
toggle(false);
}
}
}
/**
* 显示隐藏切换方法
*
* @param isShow 是否显示
*/
public void toggle(boolean isShow) {
if (mOnDragListener != null) {
mOnDragListener.onDragChanged(VDragLayout.this, isShow);
}
mDragHelper.smoothSlideViewTo(mChildView, 0, isShow ? 0
: (mDirection == UP ? -mCanOffsetHeight : mCanOffsetHeight));
ViewCompat.postInvalidateOnAnimation(VDragLayout.this);
}
第四步：手势释放时，平滑过渡到目标位置
调用方法smoothSlideViewTo(@NonNull View child, int finalLeft, int finalTop)来平滑过渡到目标位置，注意要在ViewGroup的computeScroll方法中，调用ViewDragHelper的方法continueSettling(true)，并当返回true时重新渲染，否则不会移动。
@Override
public void computeScroll() {
if (mDragHelper.continueSettling(true)) {
ViewCompat.postInvalidateOnAnimation(VDragLayout.this);
}
}
第五步：onTouchEvent方法
继续处理，当手指点击时触摸点不在子View的范围内，不获取处理事件返回false，否则传入ViewDragHelper，返回true。
@Override
public boolean onTouchEvent(MotionEvent event) {
//点击时是否在子View的范围内，在范围内就传入事件，返回true拦截事件，否则返回false，不处理事件
int action = event.getAction();
if (action == MotionEvent.ACTION_DOWN) {
int[] location = new int[2];
mChildView.getLocationOnScreen(location);
int left = location[0];
int top = location[1];
int right = left + mChildView.getMeasuredWidth();
int bottom = top + mChildView.getMeasuredHeight();
int x = (int) event.getRawX();
int y = (int) event.getRawY();
isDownInbound = x >= left && x <= right && y >= top && y <= bottom;
if (!isDownInbound) {
return false;
}
}
if (isDownInbound) {
//ViewDragHelper传入事件
mDragHelper.processTouchEvent(event);
return true;
}
if (action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_CANCEL) {
isDownInbound = false;
}
return false;
}
三、全部代码
public class VDragLayout extends ViewGroup {
private final int UP = 0;
private final int DOWN = 1;
private ViewDragHelper mDragHelper;
private View mChildView;
private float mOffsetPercent;
private int mCanOffsetHeight;
private int mDirection = UP;
private int mVelocityLimit = 1000;
private boolean isDownInbound = false;
private OnDragListener mOnDragListener;
public VDragLayout(Context context) {
this(context, null);
}
public VDragLayout(Context context, @Nullable AttributeSet attrs) {
this(context, attrs, 0);
}
public VDragLayout(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
//初始化ViewDragHelper
mDragHelper = ViewDragHelper.create(this, new ViewDragCallback());
//获取滑动的百分比
TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.VDragLayout);
mOffsetPercent = typedArray.getFloat(R.styleable.VDragLayout_OffsetPercent, 0.9f);
mDirection = typedArray.getInt(R.styleable.VDragLayout_OffsetDirection, UP);
int dragLevel = typedArray.getInt(R.styleable.VDragLayout_DragLevel, 2);
if (mOffsetPercent < 0) {
mOffsetPercent = 0;
}
mOffsetPercent = mOffsetPercent > 1 ? 1.0f : mOffsetPercent;
mVelocityLimit = dragLevel == 2 ? 1000 : (dragLevel == 0 ? 2000 : 1500);
}
@Override
protected void onFinishInflate() {
//只能有一个孩子
int childCount = getChildCount();
if (childCount > 1) {
throw new UnsupportedOperationException("VDragLayout cannot contain more than two child view!");
}
mChildView = getChildAt(0);
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
mChildView.layout(0, 0, mChildView.getMeasuredWidth(), mChildView.getMeasuredHeight());
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
mChildView.measure(widthMeasureSpec, heightMeasureSpec);
setMeasuredDimension(MeasureSpec.getSize(widthMeasureSpec), mChildView.getMeasuredHeight());
//可以滑动的距离
mCanOffsetHeight = (int) (mChildView.getMeasuredHeight() * mOffsetPercent);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
int action = event.getAction();
if (action == MotionEvent.ACTION_DOWN) {
int[] location = new int[2];
mChildView.getLocationOnScreen(location);
int left = location[0];
int top = location[1];
int right = left + mChildView.getMeasuredWidth();
int bottom = top + mChildView.getMeasuredHeight();
int x = (int) event.getRawX();
int y = (int) event.getRawY();
isDownInbound = x >= left && x <= right && y >= top && y <= bottom;
if (!isDownInbound) {
return false;
}
}
if (isDownInbound) {
mDragHelper.processTouchEvent(event);
return true;
}
if (action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_CANCEL) {
isDownInbound = false;
}
return false;
}
private class ViewDragCallback extends ViewDragHelper.Callback {
/**
* 确定是否分析view的touch
*/
@Override
public boolean tryCaptureView(@NonNull View view, int i) {
return view == mChildView;
}
/**
* touch移动之后的垂直方向回调分析
*
* @param child capture的view/移动的view
* @param top child的顶部距离
* @param dy 垂直方向增量y
* @return 返回要移动的距离
*/
@Override
public int clampViewPositionVertical(@NonNull View child, int top, int dy) {
if (child == mChildView) {
//向上滑动
if (mDirection == UP) {
if (top > 0) {
return 0;
} else if (-top > mCanOffsetHeight) {
return -mCanOffsetHeight;
}
} else if (mDirection == DOWN) {
if (top < 0) {
return 0;
} else if (top > mCanOffsetHeight) {
return mCanOffsetHeight;
}
}
}
return top;
}
/**
* 当控件位置移动时的回调
* @param changedView 位置改变的View
* @param left 左侧距离
* @param top 顶部距离
* @param dx 水平方向增量
* @param dy 垂直方向增量
*/
@Override
public void onViewPositionChanged(@NonNull View changedView, int left, int top, int dx, int dy) {
ViewCompat.postInvalidateOnAnimation(VDragLayout.this);
}
/**
* 手势释放时回调：松开，判断超过一半，则打开，没超过一半则回归原位
* @param releasedChild
* @param xvel 释放时水平方向速度
* @param yvel 释放时垂直方向速度
*/
@Override
public void onViewReleased(@NonNull View releasedChild, float xvel, float yvel) {
if (Math.abs(yvel) > mVelocityLimit) {
boolean isShow = (yvel > 0 && mDirection == UP) || (yvel < 0 && mDirection == DOWN);
if (isShow) {
toggle(true);
} else {
toggle(false);
}
return;
}
int top = mChildView.getTop();
boolean isShow = (mDirection == UP && -top < mCanOffsetHeight / 2)
|| (mDirection == DOWN && top < mCanOffsetHeight / 2);
if (isShow) {
toggle(true);
} else {
toggle(false);
}
}
}
@Override
public void computeScroll() {
if (mDragHelper.continueSettling(true)) {
ViewCompat.postInvalidateOnAnimation(VDragLayout.this);
}
}
/**
* 显示隐藏切换方法
*
* @param isShow 是否显示
*/
public void toggle(boolean isShow) {
if (mOnDragListener != null) {
mOnDragListener.onDragChanged(VDragLayout.this, isShow);
}
mDragHelper.smoothSlideViewTo(mChildView, 0, isShow ? 0
: (mDirection == UP ? -mCanOffsetHeight : mCanOffsetHeight));
ViewCompat.postInvalidateOnAnimation(VDragLayout.this);
}
/**
* 设置显示隐藏状态更改监听
*
* @param listener
*/
public void setOnDragListener(OnDragListener listener) {
this.mOnDragListener = listener;
}
public interface OnDragListener {
/**
* 拖动回调
*
* @param view 容器view
* @param isShowed 是否显示
*/
void onDragChanged(VDragLayout view, boolean isShowed);
}
}
//attrs.xml
<declare-styleable name="VDragLayout">
<!--滑动偏移百分比，0.0-1.0-->
<attr name="OffsetPercent" format="float"/>
<!--滑动方向-->
<attr name="OffsetDirection" format="enum">
<enum name="up" value="0"/>
<enum name="down" value="1"/>
</attr>
<!--滑动手势释放时，显示/隐藏的困难程度，high-->
<attr name="DragLevel" format="enum">
<enum name="high" value="0"/>
<enum name="middle" value="1"/>
<enum name="low" value="2"/>
</attr>
</declare-styleable>
