测滑菜单MenuDrawer的使用以及解析
2014-03-10 22:23 10379 0
在安卓中左右侧滑菜单的使用用的比ios多得多，可能是谷歌带的头吧，几乎所有的谷歌应用都有侧滑菜单。谷歌没有开放这个源码，在一个成熟的开源代码出现之前，大家都是各自为战，偶尔能看到一个勉强实现了的。MenuDrawer和其他的侧滑代码不同，他是一个性能高效且成熟的库。
在menuDraer出现之前我还用过slidemenu，效果差不多，但感觉没有MenuDrawer流畅，后来看了MenuDrawer和slidemenu的源码之后，才知道其实他们的实现思路是基本一致的，不过MenuDrawer的代码写的更好些。
MenuDrawer支持左、右、上、下四种侧滑方式，同时支持Overlay和Sliding两种模式。如下图：
但是MenuDrawer在github上的项目是gradle的android studio做的，直接将library导入到项目中会出现资源文件找不到的错误。因此要学会在eclipse中使用的话需要点耐心。官方已经有了其使用的方法，我这里就暂时不多说。其实要学会用这个库很简单，但是我的目的是要学会他实现滑动的基本方法，学会修改这个库，运用在更多的场景中。
我想弄明白下面的疑问：
在一个界面中如何自如的通过手势控制布局，想隐藏多少隐藏多少。
MenuDrawer的代码很多，但是其实我并不需要里面的很多功能，比如和actionbar的关联，甚至是上下滑动我也不需要，我只需要一个实现左滑菜单的范例，然后其他的我自己来，我不想一个项目下来用了很多别人的库，包含着一大堆跟业务无关的代码，而且我还不知道是拿来干嘛的。
经过研究，发现只需反复研究三个几个文件就能知道MenuDrawer的实现过程。
这三个类以及他们的的继承关系如下：
MenuDrawer.java -> DraggableDrawer.java -> SlidingDrawer.java
MenuDrawer.java 这个是基本的类，继承自ViewGroup，他定义了许多公共方法和属性变量，MenuDrawer初始化也是在这个类中。其中包含如何加载菜单和主内容的layout，如何根据xml中的属性以及样式来获取一些初始化值，比如菜单的宽度，是否要阴影以及当前菜单的箭头指示资源文件等，绘制箭头和阴影的实现也在这个文件中。
DraggableDrawer.java实现了menu滑动的一些动画过度效果，其实感觉这里面的东西并不多。
SlidingDrawer.java这个是实现滑动的主要类，要知道如何通过手势滑动布局，这里就可以找到答案。
下面是SlidingDrawer的代码，不过是我修改过了的。
package net.simonvt.menudrawer;
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.VelocityTracker;
public class SlidingDrawer extends DraggableDrawer {
private static final String TAG = "OverlayDrawer";
SlidingDrawer(Activity activity, int dragMode) {
super(activity, dragMode);
}
public SlidingDrawer(Context context) {
super(context);
}
public SlidingDrawer(Context context, AttributeSet attrs) {
super(context, attrs);
}
public SlidingDrawer(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
@Override
protected void initDrawer(Context context, AttributeSet attrs, int defStyle) {
super.initDrawer(context, attrs, defStyle);
super.addView(mMenuContainer, -1, new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
super.addView(mContentContainer, -1, new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
}
@Override
public void openMenu(boolean animate) {
int animateTo = 0;
switch (getPosition()) {
case LEFT:
case TOP:
animateTo = mMenuSize;
break;
case RIGHT:
case BOTTOM:
animateTo = -mMenuSize;
break;
}
animateOffsetTo(animateTo, 0, animate);
}
@Override
public void closeMenu(boolean animate) {
animateOffsetTo(mMenuCloseSize, 0, animate);
}
@Override
protected void onOffsetPixelsChanged(int offsetPixels) {
if (USE_TRANSLATIONS) {
switch (getPosition()) {
case TOP:
case BOTTOM:
mContentContainer.setTranslationY(offsetPixels);
break;
default:
mContentContainer.setTranslationX(offsetPixels);
break;
}
} else {
switch (getPosition()) {
case TOP:
case BOTTOM:
mContentContainer.offsetTopAndBottom(offsetPixels - mContentContainer.getTop());
break;
default:
mContentContainer.offsetLeftAndRight(offsetPixels - mContentContainer.getLeft());
break;
}
}
offsetMenu(offsetPixels);
invalidate();
}
@Override
protected void initPeekScroller() {
switch (getPosition()) {
case RIGHT:
case BOTTOM: {
final int dx = -mMenuSize / 3;
mPeekScroller.startScroll(0, 0, dx, 0, PEEK_DURATION);
break;
}
default: {
final int dx = mMenuSize / 3;
mPeekScroller.startScroll(0, 0, dx, 0, PEEK_DURATION);
break;
}
}
}
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
onOffsetPixelsChanged((int) mOffsetPixels);
}
@Override
protected void drawOverlay(Canvas canvas) {
final int width = getWidth();
final int height = getHeight();
final int offsetPixels = (int) mOffsetPixels;
final float openRatio = Math.abs(mOffsetPixels) / mMenuSize;
switch (getPosition()) {
case LEFT:
mMenuOverlay.setBounds(0, 0, offsetPixels, height);
break;
case RIGHT:
mMenuOverlay.setBounds(width + offsetPixels, 0, width, height);
break;
case TOP:
mMenuOverlay.setBounds(0, 0, width, offsetPixels);
break;
case BOTTOM:
mMenuOverlay.setBounds(0, height + offsetPixels, width, height);
break;
}
mMenuOverlay.setAlpha((int) (MAX_MENU_OVERLAY_ALPHA * (1.f - openRatio)));
mMenuOverlay.draw(canvas);
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
final int width = r - l;
final int height = b - t;
if (USE_TRANSLATIONS) {
mContentContainer.layout(0, 0, width, height);
} else {
final int offsetPixels = (int) mOffsetPixels;
if (getPosition() == Position.LEFT || getPosition() == Position.RIGHT) {
mContentContainer.layout(offsetPixels, 0, width + offsetPixels, height);
} else {
mContentContainer.layout(0, offsetPixels, width, height + offsetPixels);
}
}
switch (getPosition()) {
case LEFT:
mMenuContainer.layout(0, 0, mMenuSize, height);
break;
case RIGHT:
mMenuContainer.layout(width - mMenuSize, 0, width, height);
break;
case TOP:
mMenuContainer.layout(0, 0, width, mMenuSize);
break;
case BOTTOM:
mMenuContainer.layout(0, height - mMenuSize, width, height);
break;
}
}
/**
* Offsets the menu relative to its original position based on the position of the content.
*
* @param offsetPixels The number of pixels the content if offset.
*/
private void offsetMenu(int offsetPixels) {
if (!mOffsetMenu || mMenuSize == 0) {
return;
}
final int width = getWidth();
final int height = getHeight();
final int menuSize = mMenuSize;
final int sign = (int) (mOffsetPixels / Math.abs(mOffsetPixels));
final float openRatio = Math.abs(mOffsetPixels) / menuSize;
final int offset = (int) (-0.25f * ((1.0f - openRatio) * menuSize) * sign);
switch (getPosition()) {
case LEFT: {
if (USE_TRANSLATIONS) {
if (offsetPixels > 0) {
mMenuContainer.setTranslationX(offset);
} else {
mMenuContainer.setTranslationX(-menuSize);
}
} else {
mMenuContainer.offsetLeftAndRight(offset - mMenuContainer.getLeft());
mMenuContainer.setVisibility(offsetPixels == 0 ? INVISIBLE : VISIBLE);
}
break;
}
case RIGHT: {
if (USE_TRANSLATIONS) {
if (offsetPixels != 0) {
mMenuContainer.setTranslationX(offset);
} else {
mMenuContainer.setTranslationX(menuSize);
}
} else {
final int oldOffset = mMenuContainer.getRight() - width;
final int offsetBy = offset - oldOffset;
mMenuContainer.offsetLeftAndRight(offsetBy);
mMenuContainer.setVisibility(offsetPixels == 0 ? INVISIBLE : VISIBLE);
}
break;
}
case TOP: {
if (USE_TRANSLATIONS) {
if (offsetPixels > 0) {
mMenuContainer.setTranslationY(offset);
} else {
mMenuContainer.setTranslationY(-menuSize);
}
} else {
mMenuContainer.offsetTopAndBottom(offset - mMenuContainer.getTop());
mMenuContainer.setVisibility(offsetPixels == 0 ? INVISIBLE : VISIBLE);
}
break;
}
case BOTTOM: {
if (USE_TRANSLATIONS) {
if (offsetPixels != 0) {
mMenuContainer.setTranslationY(offset);
} else {
mMenuContainer.setTranslationY(menuSize);
}
} else {
final int oldOffset = mMenuContainer.getBottom() - height;
final int offsetBy = offset - oldOffset;
mMenuContainer.offsetTopAndBottom(offsetBy);
mMenuContainer.setVisibility(offsetPixels == 0 ? INVISIBLE : VISIBLE);
}
break;
}
}
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
if (widthMode == MeasureSpec.UNSPECIFIED || heightMode == MeasureSpec.UNSPECIFIED) {
throw new IllegalStateException("Must measure with an exact size");
}
final int width = MeasureSpec.getSize(widthMeasureSpec);
final int height = MeasureSpec.getSize(heightMeasureSpec);
/** by hejie */
mMenuSize = width;
mMenuCloseSize = (int) (mMenuSize * 0.3f);;
//mMenuCloseSize = (int) (mMenuSize * 0.4f);;
// if (mOffsetPixels == -1) openMenu(false);
openMenu(false);
/** */
int menuWidthMeasureSpec;
int menuHeightMeasureSpec;
switch (getPosition()) {
case TOP:
case BOTTOM:
menuWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, width);
menuHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec, 0, mMenuSize);
break;
default:
// LEFT/RIGHT
menuWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, mMenuSize);
menuHeightMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, height);
}
mMenuContainer.measure(menuWidthMeasureSpec, menuHeightMeasureSpec);
final int contentWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, width);
final int contentHeightMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, height);
mContentContainer.measure(contentWidthMeasureSpec, contentHeightMeasureSpec);
setMeasuredDimension(width, height);
updateTouchAreaSize();
}
private boolean isContentTouch(int x, int y) {
boolean contentTouch = false;
switch (getPosition()) {
case LEFT:
contentTouch = ViewHelper.getLeft(mContentContainer) < x;
break;
case RIGHT:
contentTouch = ViewHelper.getRight(mContentContainer) > x;
break;
case TOP:
contentTouch = ViewHelper.getTop(mContentContainer) < y;
break;
case BOTTOM:
contentTouch = ViewHelper.getBottom(mContentContainer) > y;
break;
}
return contentTouch;
}
protected boolean onDownAllowDrag(int x, int y) {
switch (getPosition()) {
case LEFT:
return (!mMenuVisible && mInitialMotionX <= mTouchSize)
|| (mMenuVisible && mInitialMotionX >= mOffsetPixels);
case RIGHT:
final int width = getWidth();
final int initialMotionX = (int) mInitialMotionX;
return (!mMenuVisible && initialMotionX >= width - mTouchSize)
|| (mMenuVisible && initialMotionX <= width + mOffsetPixels);
case TOP:
return (!mMenuVisible && mInitialMotionY <= mTouchSize)
|| (mMenuVisible && mInitialMotionY >= mOffsetPixels);
case BOTTOM:
final int height = getHeight();
return (!mMenuVisible && mInitialMotionY >= height - mTouchSize)
|| (mMenuVisible && mInitialMotionY <= height + mOffsetPixels);
}
return false;
}
protected boolean onMoveAllowDrag(int x, int y, float dx, float dy) {
switch (getPosition()) {
case LEFT:
return (!mMenuVisible && mInitialMotionX <= mTouchSize && (dx > 0))
|| (mMenuVisible && x >= mOffsetPixels);
case RIGHT:
final int width = getWidth();
return (!mMenuVisible && mInitialMotionX >= width - mTouchSize && (dx < 0))
|| (mMenuVisible && x <= width + mOffsetPixels);
case TOP:
return (!mMenuVisible && mInitialMotionY <= mTouchSize && (dy > 0))
|| (mMenuVisible && y >= mOffsetPixels);
case BOTTOM:
final int height = getHeight();
return (!mMenuVisible && mInitialMotionY >= height - mTouchSize && (dy < 0))
|| (mMenuVisible && y <= height + mOffsetPixels);
}
return false;
}
protected void onMoveEvent(float dx, float dy) {
switch (getPosition()) {
case LEFT:
setOffsetPixels(Math.min(Math.max(mOffsetPixels + dx, mMenuCloseSize), mMenuSize));
break;
case RIGHT:
setOffsetPixels(Math.max(Math.min(mOffsetPixels + dx, 0), -mMenuSize));
break;
case TOP:
setOffsetPixels(Math.min(Math.max(mOffsetPixels + dy, 0), mMenuSize));
break;
case BOTTOM:
setOffsetPixels(Math.max(Math.min(mOffsetPixels + dy, 0), -mMenuSize));
break;
}
}
protected void onUpEvent(int x, int y) {
final int offsetPixels = (int) mOffsetPixels;
switch (getPosition()) {
case LEFT: {
if (mIsDragging) {
mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);
final int initialVelocity = (int) getXVelocity(mVelocityTracker);
mLastMotionX = x;
animateOffsetTo(initialVelocity > 0 ? mMenuSize : mMenuCloseSize, initialVelocity, true);
// Close the menu when content is clicked while the menu is visible.
} else if (mMenuVisible && x > offsetPixels) {
closeMenu();
}
break;
}
case TOP: {
if (mIsDragging) {
mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);
final int initialVelocity = (int) getYVelocity(mVelocityTracker);
mLastMotionY = y;
animateOffsetTo(initialVelocity > 0 ? mMenuSize : 0, initialVelocity, true);
// Close the menu when content is clicked while the menu is visible.
} else if (mMenuVisible && y > offsetPixels) {
closeMenu();
}
break;
}
case RIGHT: {
final int width = getWidth();
if (mIsDragging) {
mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);
final int initialVelocity = (int) getXVelocity(mVelocityTracker);
mLastMotionX = x;
animateOffsetTo(initialVelocity > 0 ? 0 : -mMenuSize, initialVelocity, true);
// Close the menu when content is clicked while the menu is visible.
} else if (mMenuVisible && x < width + offsetPixels) {
closeMenu();
}
break;
}
case BOTTOM: {
if (mIsDragging) {
mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);
final int initialVelocity = (int) getYVelocity(mVelocityTracker);
mLastMotionY = y;
animateOffsetTo(initialVelocity < 0 ? -mMenuSize : 0, initialVelocity, true);
// Close the menu when content is clicked while the menu is visible.
} else if (mMenuVisible && y < getHeight() + offsetPixels) {
closeMenu();
}
break;
}
}
}
protected boolean checkTouchSlop(float dx, float dy) {
switch (getPosition()) {
case TOP:
case BOTTOM:
return Math.abs(dy) > mTouchSlop && Math.abs(dy) > Math.abs(dx);
default:
return Math.abs(dx) > mTouchSlop && Math.abs(dx) > Math.abs(dy);
}
}
public boolean onInterceptTouchEvent(MotionEvent ev) {
final int action = ev.getAction() & MotionEvent.ACTION_MASK;
if (action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_CANCEL) {
mActivePointerId = INVALID_POINTER;
mIsDragging = false;
if (mVelocityTracker != null) {
mVelocityTracker.recycle();
mVelocityTracker = null;
}
if (Math.abs(mOffsetPixels) > mMenuSize / 2) {
openMenu();
} else {
closeMenu();
}
return false;
}
if (action == MotionEvent.ACTION_DOWN && mMenuVisible && isCloseEnough()) {
setOffsetPixels(0);
stopAnimation();
endPeek();
setDrawerState(STATE_CLOSED);
mIsDragging = false;
}
// Always intercept events over the content while menu is visible.
if (mMenuVisible) {
int index = 0;
if (mActivePointerId != INVALID_POINTER) {
index = ev.findPointerIndex(mActivePointerId);
index = index == -1 ? 0 : index;
}
final int x = (int) ev.getX(index);
final int y = (int) ev.getY(index);
if (isContentTouch(x, y)) {
return true;
}
}
if (!mMenuVisible && !mIsDragging && mTouchMode == TOUCH_MODE_NONE) {
return false;
}
if (action != MotionEvent.ACTION_DOWN && mIsDragging) {
return true;
}
switch (action) {
case MotionEvent.ACTION_DOWN: {
mLastMotionX = mInitialMotionX = ev.getX();
mLastMotionY = mInitialMotionY = ev.getY();
final boolean allowDrag = onDownAllowDrag((int) mLastMotionX, (int) mLastMotionY);
mActivePointerId = ev.getPointerId(0);
if (allowDrag) {
setDrawerState(mMenuVisible ? STATE_OPEN : STATE_CLOSED);
stopAnimation();
endPeek();
mIsDragging = false;
}
break;
}
case MotionEvent.ACTION_MOVE: {
final int activePointerId = mActivePointerId;
if (activePointerId == INVALID_POINTER) {
// If we don't have a valid id, the touch down wasn't on content.
break;
}
final int pointerIndex = ev.findPointerIndex(activePointerId);
if (pointerIndex == -1) {
mIsDragging = false;
mActivePointerId = INVALID_POINTER;
endDrag();
closeMenu(true);
return false;
}
final float x = ev.getX(pointerIndex);
final float dx = x - mLastMotionX;
final float y = ev.getY(pointerIndex);
final float dy = y - mLastMotionY;
if (checkTouchSlop(dx, dy)) {
if (mOnInterceptMoveEventListener != null && (mTouchMode == TOUCH_MODE_FULLSCREEN || mMenuVisible)
&& canChildrenScroll((int) dx, (int) dy, (int) x, (int) y)) {
endDrag(); // Release the velocity tracker
requestDisallowInterceptTouchEvent(true);
return false;
}
final boolean allowDrag = onMoveAllowDrag((int) x, (int) y, dx, dy);
if (allowDrag) {
setDrawerState(STATE_DRAGGING);
mIsDragging = true;
mLastMotionX = x;
mLastMotionY = y;
}
}
break;
}
case MotionEvent.ACTION_POINTER_UP:
onPointerUp(ev);
mLastMotionX = ev.getX(ev.findPointerIndex(mActivePointerId));
mLastMotionY = ev.getY(ev.findPointerIndex(mActivePointerId));
break;
}
if (mVelocityTracker == null) mVelocityTracker = VelocityTracker.obtain();
mVelocityTracker.addMovement(ev);
return mIsDragging;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
if (!mMenuVisible && !mIsDragging && mTouchMode == TOUCH_MODE_NONE) {
return false;
}
final int action = ev.getAction() & MotionEvent.ACTION_MASK;
if (mVelocityTracker == null) mVelocityTracker = VelocityTracker.obtain();
mVelocityTracker.addMovement(ev);
switch (action) {
case MotionEvent.ACTION_DOWN: {
mLastMotionX = mInitialMotionX = ev.getX();
mLastMotionY = mInitialMotionY = ev.getY();
final boolean allowDrag = onDownAllowDrag((int) mLastMotionX, (int) mLastMotionY);
mActivePointerId = ev.getPointerId(0);
if (allowDrag) {
stopAnimation();
endPeek();
startLayerTranslation();
}
break;
}
case MotionEvent.ACTION_MOVE: {
final int pointerIndex = ev.findPointerIndex(mActivePointerId);
if (pointerIndex == -1) {
mIsDragging = false;
mActivePointerId = INVALID_POINTER;
endDrag();
closeMenu(true);
return false;
}
if (!mIsDragging) {
final float x = ev.getX(pointerIndex);
final float dx = x - mLastMotionX;
final float y = ev.getY(pointerIndex);
final float dy = y - mLastMotionY;
if (checkTouchSlop(dx, dy)) {
final boolean allowDrag = onMoveAllowDrag((int) x, (int) y, dx, dy);
if (allowDrag) {
setDrawerState(STATE_DRAGGING);
mIsDragging = true;
mLastMotionX = x;
mLastMotionY = y;
} else {
mInitialMotionX = x;
mInitialMotionY = y;
}
}
}
if (mIsDragging) {
startLayerTranslation();
final float x = ev.getX(pointerIndex);
final float dx = x - mLastMotionX;
final float y = ev.getY(pointerIndex);
final float dy = y - mLastMotionY;
mLastMotionX = x;
mLastMotionY = y;
onMoveEvent(dx, dy);
}
break;
}
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP: {
int index = ev.findPointerIndex(mActivePointerId);
index = index == -1 ? 0 : index;
final int x = (int) ev.getX(index);
final int y = (int) ev.getY(index);
onUpEvent(x, y);
mActivePointerId = INVALID_POINTER;
mIsDragging = false;
break;
}
case MotionEvent.ACTION_POINTER_DOWN:
final int index = (ev.getAction() & MotionEvent.ACTION_POINTER_INDEX_MASK)
>> MotionEvent.ACTION_POINTER_INDEX_SHIFT;
mLastMotionX = ev.getX(index);
mLastMotionY = ev.getY(index);
mActivePointerId = ev.getPointerId(index);
break;
case MotionEvent.ACTION_POINTER_UP:
onPointerUp(ev);
mLastMotionX = ev.getX(ev.findPointerIndex(mActivePointerId));
mLastMotionY = ev.getY(ev.findPointerIndex(mActivePointerId));
break;
}
return true;
}
private void onPointerUp(MotionEvent ev) {
final int pointerIndex = ev.getActionIndex();
final int pointerId = ev.getPointerId(pointerIndex);
if (pointerId == mActivePointerId) {
final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
mLastMotionX = ev.getX(newPointerIndex);
mActivePointerId = ev.getPointerId(newPointerIndex);
if (mVelocityTracker != null) {
mVelocityTracker.clear();
}
}
}
}
onInterceptTouchEvent和onTouchEvent是捕捉用户滑动事件的方法，其中onInterceptTouchEvent的用途是在onTouchEvent之前截获事件，并提供一次改变事件传递方向的机会。当用户滑动屏幕，onTouchEvent会被调用并执行case MotionEvent.ACTION_MOVE:下面的代码，通过比较当前触摸点和上次触摸点的位置记录x和y方向上的偏移量，然后根据这个偏移量移动继承自ViewGroup的MenuDrawer。其实MenuDrawer的这个过程和ViewPager非常相似。
final float x = ev.getX(pointerIndex);
final float dx = x - mLastMotionX;
final float y = ev.getY(pointerIndex);
final float dy = y - mLastMotionY;
mLastMotionX = x;
mLastMotionY = y;
onMoveEvent(dx, dy)
onMoveEvent就是移动ViewGroup的方法。
protected void onMoveEvent(float dx, float dy) {
switch (getPosition()) {
case LEFT:
setOffsetPixels(Math.min(Math.max(mOffsetPixels + dx, mMenuCloseSize), mMenuSize));
break;
case RIGHT:
setOffsetPixels(Math.max(Math.min(mOffsetPixels + dx, 0), -mMenuSize));
break;
case TOP:
setOffsetPixels(Math.min(Math.max(mOffsetPixels + dy, 0), mMenuSize));
break;
case BOTTOM:
setOffsetPixels(Math.max(Math.min(mOffsetPixels + dy, 0), -mMenuSize));
break;
}
}
根据上面的代码我们知道手指滑动屏幕的过程中onTouchEvent onMoveEvent和setOffsetPixels相继被调用。setOffsetPixels之后最终会在offsetMenu中用offsetLeftAndRight（）来移动ViewGroup。
