Android中如何快速的实现RecycleView的拖动重排序功能

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要实现这个拖动重排序功能，主要是用到了RecycleView的ItemTouchHelper类

首先是定义一个接口

interface ItemTouchHelperAdapter {
    fun onItemMove(fromPosition: Int, toPosition: Int)
}

然后我们的Adapter里面要实现这个接口

import androidx.recyclerview.widget.RecyclerView
import java.util.*

class MyAdapter(private val items: MutableList<String>) : RecyclerView.Adapter<MyAdapter.ViewHolder>(), ItemTouchHelperAdapter {

    // ... ViewHolder and other methods ...

    override fun onItemMove(fromPosition: Int, toPosition: Int) {
        if (fromPosition < toPosition) {
            for (i in fromPosition until toPosition) {
                Collections.swap(items, i, i + 1)
            }
        } else {
            for (i in fromPosition downTo toPosition + 1) {
                Collections.swap(items, i, i - 1)
            }
        }
        notifyItemMoved(fromPosition, toPosition)
    }
}

实现ItemTouchHelper的Callback接口方法

class SimpleItemTouchHelperCallback(private val adapter: ItemTouchHelperAdapter) : ItemTouchHelper.Callback() {

    override fun isLongPressDragEnabled() = true

    override fun isItemViewSwipeEnabled() = false

    override fun getMovementFlags(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder): Int {
        //这里是水平拖动
        val dragFlags = ItemTouchHelper.LEFT or ItemTouchHelper.RIGHT
        return makeMovementFlags(dragFlags, 0)
    }

    override fun onMove(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder, target: RecyclerView.ViewHolder): Boolean {
        adapter.onItemMove(viewHolder.adapterPosition, target.adapterPosition)
        return true
    }

    override fun onSwiped(viewHolder: RecyclerView.ViewHolder, direction: Int) {
        // do nothing
    }
}

其中控制拖动方向的主要是在getMovementFlags方法中的dragFlags参数

如果是水平拖动，则用

ItemTouchHelper.LEFT or ItemTouchHelper.RIGHT

如果是竖直方向上拖动，则是用

ItemTouchHelper.UP or ItemTouchHelper.DOWN

最后绑定到我们的RecycleView上

val adapter = MyAdapter(myDataset)
val recyclerView = findViewById<RecyclerView>(R.id.my_recycler_view)
recyclerView.adapter = adapter

val callback = SimpleItemTouchHelperCallback(adapter)
val touchHelper = ItemTouchHelper(callback)
touchHelper.attachToRecyclerView(recyclerView)

完成以上的步骤就瞬间实现了让RecycleView支持拖动重排序的功能

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