【高级UI】【027】RecyclerView侧滑源码解析

前言

RecyclerView的侧滑功能主要是通过ItemTouchHelper这个类来完成的

这个类在androidx和support库里面都有

这里我们主要是手动拷贝一份，来讲解所有关键代码的实现思路

在阅读时，没必要每行代码都弄懂，那是不太现实的

关键是能读懂关键代码，能对自己想要的地方进行改动

实现思路

核心代码

	//这个控件的拖拽还有BUG，有时交换元素时，元素会消失
	//主要通过Demo来读源码流程，细节上不要太较真
	@SuppressWarnings("all")
	public class ItemTouchHelper extends RecyclerView.ItemDecoration implements RecyclerView.OnChildAttachStateChangeListener {
	
	    /**
	     * Up direction, used for swipe & drag control.
	     */
	    public static final int UP = 1;
	
	    /**
	     * Down direction, used for swipe & drag control.
	     */
	    public static final int DOWN = 1  (mActionState * DIRECTION_FLAG_COUNT);
	            mSelectedStartX = selected.itemView.getLeft();
	            mSelectedStartY = selected.itemView.getTop();
	            mSelected = selected;
	
	            if (actionState == ACTION_STATE_DRAG) {
	                mSelected.itemView.performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
	            }
	        }
	        final ViewParent rvParent = mRecyclerView.getParent();
	        if (rvParent != null) {
	            rvParent.requestDisallowInterceptTouchEvent(mSelected != null);//REMARK => 禁止RecyclerView父控件拦截事件
	        }
	        if (!preventLayout) {
	            mRecyclerView.getLayoutManager().requestSimpleAnimationsInNextLayout();
	        }
	        mCallback.onSelectedChanged(mSelected, mActionState); //REMARK => 新元素被选中时，执行回调
	        mRecyclerView.invalidate();
	    }
	
	    void postDispatchSwipe(final RecoverAnimation anim, final int swipeDir) {
	        // wait until animations are complete.
	        mRecyclerView.post(new Runnable() {
	            @Override
	            public void run() {
	                if (mRecyclerView != null && mRecyclerView.isAttachedToWindow()
	                        && !anim.mOverridden
	                        && anim.mViewHolder.getAdapterPosition() != RecyclerView.NO_POSITION) {
	                    final RecyclerView.ItemAnimator animator = mRecyclerView.getItemAnimator();
	                    // if animator is running or we have other active recover animations, we try
	                    // not to call onSwiped because DefaultItemAnimator is not good at merging
	                    // animations. Instead, we wait and batch.
	                    if ((animator == null || !animator.isRunning(null))
	                            && !hasRunningRecoverAnim()) {
	                        mCallback.onSwiped(anim.mViewHolder, swipeDir);
	                    } else {
	                        mRecyclerView.post(this);
	                    }
	                }
	            }
	        });
	    }
	
	    boolean hasRunningRecoverAnim() {
	        final int size = mRecoverAnimations.size();
	        for (int i = 0; i > (DIRECTION_FLAG_COUNT * ACTION_STATE_SWIPE);
	
	        if (swipeFlags == 0) {
	            return false;
	        }
	
	        // mDx and mDy are only set in allowed directions. We use custom x/y here instead of
	        // updateDxDy to avoid swiping if user moves more in the other direction
	        final float x = motionEvent.getX(pointerIndex);
	        final float y = motionEvent.getY(pointerIndex);
	
	        // Calculate the distance moved
	        final float dx = x - mInitialTouchX;
	        final float dy = y - mInitialTouchY;
	        // swipe target is chose w/o applying flags so it does not really check if swiping in that
	        // direction is allowed. This why here, we use mDx mDy to check slope value again.
	        final float absDx = Math.abs(dx);
	        final float absDy = Math.abs(dy);
	
	        if (absDx = 0; i--) {
	            final RecoverAnimation anim = mRecoverAnimations.get(i);
	            final View view = anim.mViewHolder.itemView;
	            if (hitTest(view, x, y, anim.mX, anim.mY)) {
	                return view;
	            }
	        }
	        return mRecyclerView.findChildViewUnder(x, y);
	    }
	
	    /**
	     * Starts dragging the provided ViewHolder. By default, ItemTouchHelper starts a drag when a
	     * View is long pressed. You can disable that behavior by overriding
	     * {@link Callback#isLongPressDragEnabled()}.
	     * 

	     * For this method to work:
	     * 

	     * 
The provided ViewHolder must be a child of the RecyclerView to which this
	     * ItemTouchHelper
	     * is attached.
	     * 
{@link Callback} must have dragging enabled.
	     * 
There must be a previous touch event that was reported to the ItemTouchHelper
	     * through RecyclerView's ItemTouchListener mechanism. As long as no other ItemTouchListener
	     * grabs previous events, this should work as expected.
	     * 
	     * 

	     * For example, if you would like to let your user to be able to drag an Item by touching one
	     * of its descendants, you may implement it as follows:
	     * 
	     *     viewHolder.dragButton.setOnTouchListener(new View.OnTouchListener() {
	     *         public boolean onTouch(View v, MotionEvent event) {
	     *             if (MotionEvent.getActionMasked(event) == MotionEvent.ACTION_DOWN) {
	     *                 mItemTouchHelper.startDrag(viewHolder);
	     *             }
	     *             return false;
	     *         }
	     *     });
	     * 
	     * 

	     *
	     * @param viewHolder The ViewHolder to start dragging. It must be a direct child of
	     *                   RecyclerView.
	     * @see Callback#isItemViewSwipeEnabled()
	     */
	    public void startDrag(ViewHolder viewHolder) {
	        if (!mCallback.hasDragFlag(mRecyclerView, viewHolder)) {
	            return;
	        }
	        if (viewHolder.itemView.getParent() != mRecyclerView) {
	            return;
	        }
	        obtainVelocityTracker();
	        mDx = mDy = 0f;
	        select(viewHolder, ACTION_STATE_DRAG);
	    }
	
	    /**
	     * Starts swiping the provided ViewHolder. By default, ItemTouchHelper starts swiping a View
	     * when user swipes their finger (or mouse pointer) over the View. You can disable this
	     * behavior
	     * by overriding {@link Callback}
	     * 

	     * For this method to work:
	     * 

	     * 
The provided ViewHolder must be a child of the RecyclerView to which this
	     * ItemTouchHelper is attached.
	     * 
{@link Callback} must have swiping enabled.
	     * 
There must be a previous touch event that was reported to the ItemTouchHelper
	     * through RecyclerView's ItemTouchListener mechanism. As long as no other ItemTouchListener
	     * grabs previous events, this should work as expected.
	     * 
	     * 

	     * For example, if you would like to let your user to be able to swipe an Item by touching one
	     * of its descendants, you may implement it as follows:
	     * 
	     *     viewHolder.dragButton.setOnTouchListener(new View.OnTouchListener() {
	     *         public boolean onTouch(View v, MotionEvent event) {
	     *             if (MotionEvent.getActionMasked(event) == MotionEvent.ACTION_DOWN) {
	     *                 mItemTouchHelper.startSwipe(viewHolder);
	     *             }
	     *             return false;
	     *         }
	     *     });
	     * 
	     *
	     * @param viewHolder The ViewHolder to start swiping. It must be a direct child of
	     *                   RecyclerView.
	     */
	    public void startSwipe(ViewHolder viewHolder) {
	        if (!mCallback.hasSwipeFlag(mRecyclerView, viewHolder)) {
	            return;
	        }
	        if (viewHolder.itemView.getParent() != mRecyclerView) {
	            return;
	        }
	        obtainVelocityTracker();
	        mDx = mDy = 0f;
	        select(viewHolder, ACTION_STATE_SWIPE);
	    }
	
	    RecoverAnimation findAnimation(MotionEvent event) {
	        if (mRecoverAnimations.isEmpty()) {
	            return null;
	        }
	        View target = findChildView(event);
	        for (int i = mRecoverAnimations.size() - 1; i >= 0; i--) {
	            final RecoverAnimation anim = mRecoverAnimations.get(i);
	            if (anim.mViewHolder.itemView == target) {
	                return anim;
	            }
	        }
	        return null;
	    }
	
	    void updateDxDy(MotionEvent ev, int directionFlags, int pointerIndex) {
	        final float x = ev.getX(pointerIndex);
	        final float y = ev.getY(pointerIndex);
	
	        // Calculate the distance moved
	        mDx = x - mInitialTouchX;
	        mDy = y - mInitialTouchY;
	        if ((directionFlags & LEFT) == 0) {
	            mDx = Math.max(0, mDx);
	        }
	        if ((directionFlags & RIGHT) == 0) {
	            mDx = Math.min(0, mDx);
	        }
	        if ((directionFlags & UP) == 0) {
	            mDy = Math.max(0, mDy);
	        }
	        if ((directionFlags & DOWN) == 0) {
	            mDy = Math.min(0, mDy);
	        }
	    }
	
	    private int swipeIfNecessary(ViewHolder viewHolder) {
	        if (mActionState == ACTION_STATE_DRAG) {
	            return 0;
	        }
	        final int originalMovementFlags = mCallback.getMovementFlags(mRecyclerView, viewHolder);
	        final int absoluteMovementFlags = mCallback.convertToAbsoluteDirection(
	                originalMovementFlags,
	                ViewCompat.getLayoutDirection(mRecyclerView));
	        final int flags = (absoluteMovementFlags
	                & ACTION_MODE_SWIPE_MASK) >> (ACTION_STATE_SWIPE * DIRECTION_FLAG_COUNT);
	        if (flags == 0) {
	            return 0;
	        }
	        final int originalFlags = (originalMovementFlags
	                & ACTION_MODE_SWIPE_MASK) >> (ACTION_STATE_SWIPE * DIRECTION_FLAG_COUNT);
	        int swipeDir;
	        if (Math.abs(mDx) > Math.abs(mDy)) {
	            if ((swipeDir = checkHorizontalSwipe(viewHolder, flags)) > 0) {
	                // if swipe dir is not in original flags, it should be the relative direction
	                if ((originalFlags & swipeDir) == 0) {
	                    // convert to relative
	                    return Callback.convertToRelativeDirection(swipeDir,
	                            ViewCompat.getLayoutDirection(mRecyclerView));
	                }
	                return swipeDir;
	            }
	            if ((swipeDir = checkVerticalSwipe(viewHolder, flags)) > 0) {
	                return swipeDir;
	            }
	        } else {
	            if ((swipeDir = checkVerticalSwipe(viewHolder, flags)) > 0) {
	                return swipeDir;
	            }
	            if ((swipeDir = checkHorizontalSwipe(viewHolder, flags)) > 0) {
	                // if swipe dir is not in original flags, it should be the relative direction
	                if ((originalFlags & swipeDir) == 0) {
	                    // convert to relative
	                    return Callback.convertToRelativeDirection(swipeDir,
	                            ViewCompat.getLayoutDirection(mRecyclerView));
	                }
	                return swipeDir;
	            }
	        }
	        return 0;
	    }
	
	    private int checkHorizontalSwipe(ViewHolder viewHolder, int flags) {
	        if ((flags & (LEFT | RIGHT)) != 0) {
	            final int dirFlag = mDx > 0 ? RIGHT : LEFT;
	            if (mVelocityTracker != null && mActivePointerId > -1) {
	                mVelocityTracker.computeCurrentVelocity(PIXELS_PER_SECOND,
	                        mCallback.getSwipeVelocityThreshold(mMaxSwipeVelocity));
	                final float xVelocity = mVelocityTracker.getXVelocity(mActivePointerId);
	                final float yVelocity = mVelocityTracker.getYVelocity(mActivePointerId);
	                final int velDirFlag = xVelocity > 0f ? RIGHT : LEFT;
	                final float absXVelocity = Math.abs(xVelocity);
	                if ((velDirFlag & flags) != 0 && dirFlag == velDirFlag
	                        && absXVelocity >= mCallback.getSwipeEscapeVelocity(mSwipeEscapeVelocity)
	                        && absXVelocity > Math.abs(yVelocity)) {
	                    return velDirFlag;
	                }
	            }
	
	            final float threshold = mRecyclerView.getWidth() * mCallback
	                    .getSwipeThreshold(viewHolder);
	
	            if ((flags & dirFlag) != 0 && Math.abs(mDx) > threshold) {
	                return dirFlag;
	            }
	        }
	        return 0;
	    }
	
	    private int checkVerticalSwipe(ViewHolder viewHolder, int flags) {
	        if ((flags & (UP | DOWN)) != 0) {
	            final int dirFlag = mDy > 0 ? DOWN : UP;
	            if (mVelocityTracker != null && mActivePointerId > -1) {
	                mVelocityTracker.computeCurrentVelocity(PIXELS_PER_SECOND,
	                        mCallback.getSwipeVelocityThreshold(mMaxSwipeVelocity));
	                final float xVelocity = mVelocityTracker.getXVelocity(mActivePointerId);
	                final float yVelocity = mVelocityTracker.getYVelocity(mActivePointerId);
	                final int velDirFlag = yVelocity > 0f ? DOWN : UP;
	                final float absYVelocity = Math.abs(yVelocity);
	                if ((velDirFlag & flags) != 0 && velDirFlag == dirFlag
	                        && absYVelocity >= mCallback.getSwipeEscapeVelocity(mSwipeEscapeVelocity)
	                        && absYVelocity > Math.abs(xVelocity)) {
	                    return velDirFlag;
	                }
	            }
	
	            final float threshold = mRecyclerView.getHeight() * mCallback
	                    .getSwipeThreshold(viewHolder);
	            if ((flags & dirFlag) != 0 && Math.abs(mDy) > threshold) {
	                return dirFlag;
	            }
	        }
	        return 0;
	    }
	
	    private void addChildDrawingOrderCallback() {
	        if (Build.VERSION.SDK_INT >= 21) {
	            return; // we use elevation on Lollipop
	        }
	        if (mChildDrawingOrderCallback == null) {
	            mChildDrawingOrderCallback = new RecyclerView.ChildDrawingOrderCallback() {
	                @Override
	                public int onGetChildDrawingOrder(int childCount, int i) {
	                    if (mOverdrawChild == null) {
	                        return i;
	                    }
	                    int childPosition = mOverdrawChildPosition;
	                    if (childPosition == -1) {
	                        childPosition = mRecyclerView.indexOfChild(mOverdrawChild);
	                        mOverdrawChildPosition = childPosition;
	                    }
	                    if (i == childCount - 1) {
	                        return childPosition;
	                    }
	                    return i