自定义滚动的线性布局主要需要完成下面3个功能
1 计算子view及本身的尺寸
2 把子view布局到指定的位置
3 添加滑动事件
1 计算尺寸
需要重写下面这个方法
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
参数中的widthMeasureSpec和heightMeasureSpec是包含宽和高的信息。里面放了测量模式和尺寸大小，具体获取方法
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
MeasureSpec.AT_MOST对应布局中的WRAP_CONTENT,MeasureSpec.EXACTLY对应MATCH_PARENT和固定的尺寸，MeasureSpec.UNSPECIFIED指未指定尺寸，这种情况很少
如果宽高是自适应的、就需要我们自己来计算实际尺寸大小
要实现可以滚动的水平线性布局viewGroup的最大宽度实际上就是所有子view中最宽的view的宽度，最大高度是所有子view高度的和
private int getMaxChildWidth() {
int childCount = getChildCount();
int maxWidth = 0;
for (int i = 0; i < childCount; i++) {
View childView = getChildAt(i);
if (childView.getMeasuredWidth() > maxWidth)
maxWidth = childView.getMeasuredWidth();
}
return maxWidth
}
private int getTotleHeight() {
int childCount = getChildCount();
int height = 0;
for (int i = 0; i < childCount; i++) {
View childView = getChildAt(i);
height += childView.getMeasuredHeight();
}
return height;
}
重写onMeasure方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
measureChildren(widthMeasureSpec, heightMeasureSpec);
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
//如果宽高都是包裹内容
if (widthMode == MeasureSpec.AT_MOST && heightMode == MeasureSpec.AT_MOST) {
//我们将高度设置为所有子View的高度相加，宽度设为子View中最大的宽度
int height = getTotleHeight();
int width = getMaxChildWidth();
setMeasuredDimension(width, height);
} else if (heightMode == MeasureSpec.AT_MOST) {//如果只有高度是包裹内容
//宽度设置为ViewGroup自己的测量宽度，高度设置为所有子View的高度总和
setMeasuredDimension(widthSize, getTotleHeight());
} else if (widthMode == MeasureSpec.AT_MOST) {//如果只有宽度是包裹内容
//宽度设置为子View中宽度最大的值，高度设置为ViewGroup自己的测量值
setMeasuredDimension(getMaxChildWidth(), heightSize);
} else {
setMeasuredDimension(widthSize, heightSize);
}
}
尺寸计算好了，下面就要布局子view了
2 布局子view
实现起来很简单
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
int childCount = getChildCount();
int top = 0;
for (int i = 0; i < childCount; i++) {
View child = getChildAt(i);
int measuredHeight = child.getMeasuredHeight();
int measuredWidth = child.getMeasuredWidth();
child.layout(0, top, measuredWidth, top + measuredHeight);
top += measuredHeight;
}
}
3 添加滑动事件
添加滑动事件就要重写onTouchEvent方法了，可以用手势帮助类GestureDetector来实现
@Override
public boolean onTouchEvent(MotionEvent event) {
mGesture.onTouchEvent(event);
return true;
}
private void init() {
mGesture = new GestureDetector(getContext(), new GestureDetector.SimpleOnGestureListener() {
@Override
public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
scrollBy(0, (int) distanceY);
return true;
}
});
}
到此，可以滑动的线性布局就基本完成了，但是还存在一些缺陷，滑动会超过边界、没有惯性滑动
首先先来解决滑动超过边界的问题，思路是在手指抬起的时候如果滑动超过边界，就重新滚动到边界，为了使滑动流畅，使用OverScroller实现,修改之前的onTouchEvent
@Override
public boolean onTouchEvent(MotionEvent event) {
mGesture.onTouchEvent(event);
if (event.getAction() == MotionEvent.ACTION_UP || event.getAction() == MotionEvent.ACTION_CANCEL) {
if (getScrollY() < 0) {
mScroller.startScroll(0, getScrollY(), 0, -getScrollY());
invalidate();
}
View lastChild = getChildAt(getChildCount() - 1);
int bottomY = (int) (lastChild.getY() + lastChild.getMeasuredHeight() - screenHeight);
if (getScrollY() > bottomY) {
mScroller.startScroll(0, getScrollY(), 0, bottomY - getScrollY());
invalidate();
}
}
return true;
}
另外不要忘了重写computeScroll方法
@Override
public void computeScroll() {
//判断滚动时候停止
if (mScroller.computeScrollOffset()) {
//滚动到指定的位置
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
//这句话必须写，否则不能实时刷新
postInvalidate();
}
}
剩下的问题就是惯性滑动的添加了，这里需要用到VelocityTracker这个类来计算速度，然后表现到滑动上面,再修改onTouchEvent方法
@Override
public boolean onTouchEvent(MotionEvent event) {
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
mVelocityTracker.addMovement(event);
mGesture.onTouchEvent(event);
if (event.getAction() == MotionEvent.ACTION_UP || event.getAction() == MotionEvent.ACTION_CANCEL) {
mVelocityTracker.computeCurrentVelocity(1000, maxFlingVelocity);
float velocityX = mVelocityTracker.getXVelocity(event.getPointerId(0));
float velocityY = mVelocityTracker.getYVelocity(event.getPointerId(0));
completeMove(-velocityX, -velocityY);
if (mVelocityTracker != null) {
mVelocityTracker.recycle();
mVelocityTracker = null;
}
if (getScrollY() < 0) {
mScroller.startScroll(0, getScrollY(), 0, -getScrollY());
invalidate();
}
View lastChild = getChildAt(getChildCount() - 1);
int bottomY = (int) (lastChild.getY() + lastChild.getMeasuredHeight() - screenHeight);
if (getScrollY() > bottomY) {
mScroller.startScroll(0, getScrollY(), 0, bottomY - getScrollY());
invalidate();
}
}
return true;
}
最后完成惯性滑动的方法封装到completeMove方法中
private void completeMove(float v, float velocityY) {
View lastChild = getChildAt(getChildCount() - 1);
int bottomY = (int) (lastChild.getY() + lastChild.getMeasuredHeight() - screenHeight);
mScroller.fling(0, getScrollY(), 0, (int) (velocityY ), 0, getMeasuredWidth(), 0, bottomY);
invalidate();
}
