Behavior子类SwipeDismissBehavior简单分析

r囧r小猫 2022-07-28 11:44 161阅读 0赞

在前一篇文章[CoordinatorLayout里Behavior简单分析][CoordinatorLayout_Behavior]的基础上继续简单分析SwipeDismissBehavior的简单实现。

SwipeDismissBehavior作用，如果View设置SwipeDismissBehavior那么这个View是可以设置左右滑动的，同时往左边滑动的时候会去改变View的透明度，往右边滑动不会改变View的透明度。(注意View并不会消失的，只是有的时候设置了透明度你看不见而已)。

下面从SwipeDismissBehavior源码的角度看看SwipeDismissBehavior是怎么工作的。SwipeDismissBehavior继承自CoordinatorLayout.Behavior(我们知道CoordinatorLayout.Behavior的使用我们分了三种情况)。这里SwipeDismissBehavior只是关注CoordinatorLayout.Behavior 的 onInterceptTouchEvent和onTouchEvent方法。也就是[CoordinatorLayout里Behavior简单分析][CoordinatorLayout_Behavior]里面说的Behavior的第一种情况。

SwipeDismissBehavior里面都是使用ViewDragHelper帮助类来处理我们SwipeDismissBehavior对应View的拖拽手势操作。按照ViewDragHelper的使用规则我们就直接看ViewDragHelper的Callback类。

private final ViewDragHelper.Callback mDragCallback = new ViewDragHelper.Callback() {
            private int mOriginalCapturedViewLeft;
    
            @Override
            public boolean tryCaptureView(View child, int pointerId) {
                mOriginalCapturedViewLeft = child.getLeft();
                return true;
            }
    
            @Override
            public void onViewDragStateChanged(int state) {
                if (mListener != null) {
                    mListener.onDragStateChanged(state);
                }
            }
    
            @Override
            public void onViewReleased(View child, float xvel, float yvel) {
                final int childWidth = child.getWidth();
                int targetLeft;
                boolean dismiss = false;
    
                if (shouldDismiss(child, xvel)) {
                    targetLeft = child.getLeft() < mOriginalCapturedViewLeft
                                 ? mOriginalCapturedViewLeft - childWidth
                                 : mOriginalCapturedViewLeft + childWidth;
                    dismiss = true;
                } else {
                    // Else, reset back to the original left
                    targetLeft = mOriginalCapturedViewLeft;
                }
    
                if (mViewDragHelper.settleCapturedViewAt(targetLeft, child.getTop())) {
                    ViewCompat.postOnAnimation(child, new SettleRunnable(child, dismiss));
                } else if (dismiss && mListener != null) {
                    mListener.onDismiss(child);
                }
            }
    
            private boolean shouldDismiss(View child, float xvel) {
                if (xvel != 0f) {
                    final boolean isRtl = ViewCompat.getLayoutDirection(child)
                                          == ViewCompat.LAYOUT_DIRECTION_RTL;
    
                    if (mSwipeDirection == SWIPE_DIRECTION_ANY) {
                        // We don't care about the direction so return true
                        return true;
                    } else if (mSwipeDirection == SWIPE_DIRECTION_START_TO_END) {
                        // We only allow start-to-end swiping, so the fling needs to be in the
                        // correct direction
                        return isRtl ? xvel < 0f : xvel > 0f;
                    } else if (mSwipeDirection == SWIPE_DIRECTION_END_TO_START) {
                        // We only allow end-to-start swiping, so the fling needs to be in the
                        // correct direction
                        return isRtl ? xvel > 0f : xvel < 0f;
                    }
                } else {
                    final int distance = child.getLeft() - mOriginalCapturedViewLeft;
                    final int thresholdDistance = Math.round(child.getWidth() * mDragDismissThreshold);
                    return Math.abs(distance) >= thresholdDistance;
                }
    
                return false;
            }
    
            @Override
            public int getViewHorizontalDragRange(View child) {
                return child.getWidth();
            }
    
            @Override
            public int clampViewPositionHorizontal(View child, int left, int dx) {
                final boolean isRtl = ViewCompat.getLayoutDirection(child)
                                      == ViewCompat.LAYOUT_DIRECTION_RTL;
                int min, max;
    
                if (mSwipeDirection == SWIPE_DIRECTION_START_TO_END) {
                    if (isRtl) {
                        min = mOriginalCapturedViewLeft - child.getWidth();
                        max = mOriginalCapturedViewLeft;
                    } else {
                        min = mOriginalCapturedViewLeft;
                        max = mOriginalCapturedViewLeft + child.getWidth();
                    }
                } else if (mSwipeDirection == SWIPE_DIRECTION_END_TO_START) {
                    if (isRtl) {
                        min = mOriginalCapturedViewLeft;
                        max = mOriginalCapturedViewLeft + child.getWidth();
                    } else {
                        min = mOriginalCapturedViewLeft - child.getWidth();
                        max = mOriginalCapturedViewLeft;
                    }
                } else {
                    min = mOriginalCapturedViewLeft - child.getWidth();
                    max = mOriginalCapturedViewLeft + child.getWidth();
                }
    
                return clamp(min, left, max);
            }
    
            @Override
            public int clampViewPositionVertical(View child, int top, int dy) {
                return child.getTop();
            }
    
            @Override
            public void onViewPositionChanged(View child, int left, int top, int dx, int dy) {
                final float startAlphaDistance = mOriginalCapturedViewLeft
                                                 + child.getWidth() * mAlphaStartSwipeDistance;
                final float endAlphaDistance = mOriginalCapturedViewLeft
                                               + child.getWidth() * mAlphaEndSwipeDistance;
    
                if (left <= startAlphaDistance) {
                    ViewCompat.setAlpha(child, 1f);
                } else if (left >= endAlphaDistance) {
                    ViewCompat.setAlpha(child, 0f);
                } else {
                    // We're between the start and end distances
                    final float distance = fraction(startAlphaDistance, endAlphaDistance, left);
                    ViewCompat.setAlpha(child, clamp(0f, 1f - distance, 1f));
                }
            }
        };

直接看里面主要的几个函数了  
1. tryCaptureView函数，保存了我们触摸到的View距离父控件左边的距离。return true。接收手势了。  
2. onViewDragStateChanged 直接给listener了。  
3. onViewReleased 手势释放的时候调用。找到View最终要到达的位置(指定位置还是回到原位置)。看下shouldDismiss函数具体干了什么事情，第41行如果水平方向有速度(fling的时候不为0，scroll的时候为0)判断是哪个方向的要不要直接到制定的位置去。如果水平方向速度为0(scroll) 则是去判断滑动的距离是否超过了一半(默认是一半可以自己设置)。  
4. clampViewPositionHorizontal 水平方向移动。  
5. clampViewPositionVertical 垂直方向不移动。  
6. onViewPositionChanged 设置在移动的过程中设置透明度。

SwipeDismissBehavior的onInterceptTouchEvent函数。CoordinatorLayout里面是怎么调用到这个函数里面来的[CoordinatorLayout里Behavior简单分析][CoordinatorLayout_Behavior]有简单的介绍。

@Override
        public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {
            switch (MotionEventCompat.getActionMasked(event)) {
                case MotionEvent.ACTION_UP:
                case MotionEvent.ACTION_CANCEL:
                    // Reset the ignore flag
                    if (mIgnoreEvents) {
                        mIgnoreEvents = false;
                        return false;
                    }
                    break;
                default:
                    mIgnoreEvents = !parent.isPointInChildBounds(child,
                                                                 (int) event.getX(), (int) event.getY());
                    break;
            }
    
            if (mIgnoreEvents) {
                return false;
            }
    
            ensureViewDragHelper(parent);
            return mViewDragHelper.shouldInterceptTouchEvent(event);
        }

SwipeDismissBehavior的onInterceptTouchEvent函数会在CoordinatorLayout的onInterceptTouchEvent函数中调用到。先把intercept的权利交给了SwipeDismissBehavior的onInterceptTouchEvent。SwipeDismissBehavior会对触摸点判断是否在SwipeDismissBehavior对应的View位置之内。判断是否走到mViewDragHelper.shouldInterceptTouchEvent(event);

同样SwipeDismissBehavior的onTouchEvent

@Override
        public boolean onTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {
            if (mViewDragHelper != null) {
                mViewDragHelper.processTouchEvent(event);
                return true;
            }
            return false;
        }

调用了mViewDragHelper.processTouchEvent(event);这样所有的处理都到了mViewDragHelper的Callback函数里面去了，在里面做一些相应的处理如前面对Callback的描述。

几个相关函数的解释

/** * 设置View滑动的方向 SWIPE_DIRECTION_START_TO_END(右),SWIPE_DIRECTION_END_TO_START(左),SWIPE_DIRECTION_ANY(两个方向都可以) 三个当中的一个 */
        public void setSwipeDirection(@SwipeDirection int direction)
    
        /** * 设置当手指放开的时候滑动的距离达到改设置值*View的宽度的时候 会自动偏移View的宽度的位置 */
        public void setDragDismissDistance(float distance)
    
        /** * 透明度开始变化的位置(当滑动的距离达到改设置值*View的宽度的时候透明度开始变化) */
        public void setStartAlphaSwipeDistance(float fraction)
    
        /** * 透明度结束变化的位置(当滑动的距离达到改设置值*View的宽度的时候透明度结束变化) */
        public void setEndAlphaSwipeDistance(float fraction)
    
        /** * 用于设置ViewDragHelper的touchSlop */
        public void setSensitivity(float sensitivity)

总结SwipeDismissBehavior具体干了什么事情。  
当触摸到SwipeDismissBehavior对应的View的时候，View会更随手指滑动。如果是往左边滑动的时候View的透明度会改变，如果是右边滑动透明度则是不会变的。

注：这里有发现一个问题，如果CoordinatorLayout里面有A View和B View。如果我们只给A View设置了SwipeDismissBehavior。当我们触摸B View的时候B View也会有滑动消失的效果。因为在ViewDragHelper的Callback的tryCaptureView所有的View都是返回的ture。

最后给个简单的DEMO[下载地址][Link 1]

[CoordinatorLayout_Behavior]: http://blog.csdn.net/wuyuxing24/article/details/51074317
[Link 1]: http://download.csdn.net/detail/wuyuxing24/9491932