[Android]RecyView通过ItemTouchHelper实现左右滑动来选中或者取消选中的功能
需求
需要为RecyView 添加一个多选的功能，这里我想通过ItemTouchHelper 实现
步骤
新建一个类，继承自
ItemTouchHelper.Callback(关于ItemTouchHelper如何使用这里不讲）。完成之后，发现当你滑动时，这个Viewholder真的不见了，这不是我想要的，我希望它能够滑动，但是不能够被移除。
开始解决问题。-
重写
getSwipeThreshold方法。/** * Returns the fraction that the user should move the View to be considered as swiped. * The fraction is calculated with respect to RecyclerView's bounds. * <p> * Default value is .5f, which means, to swipe a View, user must move the View at least * half of RecyclerView's width or height, depending on the swipe direction. * * @param viewHolder The ViewHolder that is being dragged. * @return A float value that denotes the fraction of the View size. Default value * is .5f . */ @SuppressWarnings("WeakerAccess") public float getSwipeThreshold(@NonNull ViewHolder viewHolder) { return .5f; }说的是当用户移动多少时认为是用户进行了移除行为。这个
fraction是的小数的意思，也就是说相当于RecycleView的宽高多少长度。这是我想要的东西。@Override public float getSwipeThreshold(@NonNull RecyclerView.ViewHolder viewHolder) { return 10; }像这样返回
1000%,就成了。 -
但是，又出现问题了，移动过快也会导致移除事件。需要
getSwipeEscapeVelocity。/** * Defines the minimum velocity which will be considered as a swipe action by the user. * <p> * You can increase this value to make it harder to swipe or decrease it to make it easier. * Keep in mind that ItemTouchHelper also checks the perpendicular velocity and makes sure * current direction velocity is larger then the perpendicular one. Otherwise, user's * movement is ambiguous. You can change the threshold by overriding * {@link #getSwipeVelocityThreshold(float)}. * <p> * The velocity is calculated in pixels per second. * <p> * The default framework value is passed as a parameter so that you can modify it with a * multiplier. * * @param defaultValue The default value (in pixels per second) used by the * ItemTouchHelper. * @return The minimum swipe velocity. The default implementation returns the * <code>defaultValue</code> parameter. * @see #getSwipeVelocityThreshold(float) * @see #getSwipeThreshold(ViewHolder) */ @SuppressWarnings("WeakerAccess") public float getSwipeEscapeVelocity(float defaultValue) { return defaultValue; }说的是定义一个最小速度用来检测当前的滑动是否当成移除操作。然后我们设置一个很大的值就行了。注释里还说了
getSwipeVelocityThreshold这个函数，理解起来应该是说怕我SwipeEscapeVelocity设置的过小，导致错把上下滑动当成了左右滑动吧，与我无关，不管了。@Override public float getSwipeEscapeVelocity(float defaultValue) { return 1000000; } -
这样用户不管怎么滑都不会移除了。但是不能移除，
onSwiped就不会被调用了，这该如何是好？可以重写clearView函数，当移除取消时，这个函数就会被调用。之前也想过在前面直接给它拦截了，但是不行，全部没有权限，只有这个是可以使用的。
通过getSwipeThreshold可以不断的找到调用者为checkHorizontalSwipe，swipeIfNecessary，select，找到select就够了，
部分代码：super.onAnimationEnd(animation); if (this.mOverridden) { return; } if (swipeDir <= 0) { // this is a drag or failed swipe. recover immediately mCallback.clearView(mRecyclerView, prevSelected); // full cleanup will happen on onDrawOver } else { // wait until remove animation is complete. mPendingCleanup.add(prevSelected.itemView); mIsPendingCleanup = true; if (swipeDir > 0) { // Animation might be ended by other animators during a layout. // We defer callback to avoid editing adapter during a layout. postDispatchSwipe(this, swipeDir); } }我们要的就是
this is a drag or failed swipe. recover immediately
这段话的意思是在执行拖拽或者滑动失败时，用来恢复的。所以，如果你还有拖拽的功能就比较麻烦了。
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最后全部完成了，但是，如果能够限制用户左右滑动的长度就更好了，只可惜当前还没有找到办法。
现在找到办法了，onChildDraw函数。/** * Called by ItemTouchHelper on RecyclerView's onDraw callback. * <p> * If you would like to customize how your View's respond to user interactions, this is * a good place to override. * <p> * Default implementation translates the child by the given <code>dX</code>, * <code>dY</code>. * ItemTouchHelper also takes care of drawing the child after other children if it is being * dragged. This is done using child re-ordering mechanism. On platforms prior to L, this * is * achieved via {@link android.view.ViewGroup#getChildDrawingOrder(int, int)} and on L * and after, it changes View's elevation value to be greater than all other children.) * * @param c The canvas which RecyclerView is drawing its children * @param recyclerView The RecyclerView to which ItemTouchHelper is attached to * @param viewHolder The ViewHolder which is being interacted by the User or it was * interacted and simply animating to its original position * @param dX The amount of horizontal displacement caused by user's action * @param dY The amount of vertical displacement caused by user's action * @param actionState The type of interaction on the View. Is either {@link * #ACTION_STATE_DRAG} or {@link #ACTION_STATE_SWIPE}. * @param isCurrentlyActive True if this view is currently being controlled by the user or * false it is simply animating back to its original state. * @see #onChildDrawOver(Canvas, RecyclerView, ViewHolder, float, float, int, * boolean) */ public void onChildDraw(@NonNull Canvas c, @NonNull RecyclerView recyclerView, @NonNull ViewHolder viewHolder, float dX, float dY, int actionState, boolean isCurrentlyActive) { ItemTouchUIUtilImpl.INSTANCE.onDraw(c, recyclerView, viewHolder.itemView, dX, dY, actionState, isCurrentlyActive); }If you would like to customize how your View's respond to user interactions, this isa good place to override.
翻译过来就是
如果你想要自定义
View响应用户操作的，可以在这里重写。在使用时，可以通过
actionState来筛选移除和移动操作。重写时，我们需要的是操作完全不变，只有能够移动的位置不同。方法有两种。
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在
dX大于某一个值时，不在使用super调用父类的方法，这样程序什么都不会做。@Override public void onChildDraw(@NonNull Canvas c, @NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState, boolean isCurrentlyActive) { if (actionState == ItemTouchHelper.ACTION_STATE_SWIPE) { if (dX < 100) { super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive); } } } -
不管
dX值有多大，都除以某一个值，比如5，这样就好带有阻尼效果一样，虽然拉的很远，带来的影响却很小。还可以根据自己的需要，在dX不同值时选择不同的阻尼。@Override public void onChildDraw(@NonNull Canvas c, @NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState, boolean isCurrentlyActive) { if (actionState == ItemTouchHelper.ACTION_STATE_SWIPE) { super.onChildDraw(c, recyclerView, viewHolder, dX / 5, dY, actionState, isCurrentlyActive); } }
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更新
上面说的还不够，后续使用时，发现有时候事件没有正常接收到。但是在解决上一个问题时发现了一个onChildDraw函数，可以通过这个函数来处理滑动事件而不是clearView。if (actionState == ItemTouchHelper.ACTION_STATE_SWIPE) { int first_line = 200; if (dX < first_line) { if (swipeEvent) {//这里的`真(true)`是上次触发了事件,用户滑动了回来（在这里可能也可以设置一个取消事件，不过效果不是很好），或者用户第二次滑动，重置为false，为的是用户滑动超过300时能够触发事件 swipeEvent=false; } super.onChildDraw(c, recyclerView, viewHolder, dX / 2, dY, actionState, isCurrentlyActive); } else { int second_line = first_line + 100; if (dX < second_line) { int first_max = first_line / 2; super.onChildDraw(c, recyclerView, viewHolder, first_max + (dX - first_line) / 4, dY, actionState, isCurrentlyActive); } else if (dX >= second_line) { if (!swipeEvent) {//如果没有触发过事件，触发一次，并且重置为true，保证下次不会触发 swipeViewHolder(viewHolder); swipeEvent=true; } } } }swipeViewHolder就是需要自己实现的事件调用部分。
滑动的距离小于200时，绘制的长度减半，大于200，小于300时绘制的长度在原来的基础上缓慢增加，大于300时就认为触发了事件，并且保证了事件不会被多次触发。
