直接贴代码

        //获取collapsing_tool_bar_layout的collapsingTextHelper，由于不是public的，因此使用java的反射来获取
        val field = collapsing_tool_bar_layout.javaClass.getDeclaredField("collapsingTextHelper")
        field.isAccessible = true
        val collapsingTextHelper = field.get(collapsing_tool_bar_layout) as CollapsingTextHelper
        //同理，获取collapsingTextHelper中的currentBounds
        val fieldCurrentBounds = collapsingTextHelper.javaClass.getDeclaredField("currentBounds")
        fieldCurrentBounds.isAccessible = true
        val currentRectF = fieldCurrentBounds.get(collapsingTextHelper) as RectF

        //自定义折叠Toolbar，使TextView随着AppBarLayout高度的变化而改变位置
        //AppBarStateChangeListener是自定义继承自AppBarLayout.OnOffsetChangedListener的抽象类
        app_bar_layout.addOnOffsetChangedListener(object : AppBarStateChangeListener() {
            override fun onExpandPercentChange(state: State, expandPercent: Float) {
                Log.d("CUSTOM_COLLAPSED_TITLE", "currentRectF：${JSON.toJSONString(currentRectF)}")

                val layoutParams = LinearLayout.LayoutParams(text_view_symbol.layoutParams)
                layoutParams.setMargins(
                    currentRectF.left.toInt(), layoutParams.topMargin, layoutParams.rightMargin,
                    (DensityUtils.dp2px(15f) * expandPercent).toInt() +
                            //因为textview大小为18sp，与折叠ToolbarLayout的20sp不一致，为了在折叠后尽量保持在一条水平先上
                            (DensityUtils.dp2px(2.5f) * (1 - expandPercent)).toInt()
                )
//                        text_view_symbol.setTextSize(20-((20-18)*expandPercent))
                text_view_symbol.layoutParams = layoutParams

                text_view_creator_info.alpha = expandPercent
                if (expandPercent > 0.8) {
                    fab_add_record.show()
                } else {
                    fab_add_record.hide()
                }
            }

            override fun onStateChanged(appBarLayout: AppBarLayout?, state: State?, i: Int) {
                when (state) {
                    State.COLLAPSED -> {
                        XLog.d("RecordListActivity， State.COLLAPSED")
//                        fab_add_record.hide()
                    }
                    State.EXPANDED -> {
                        XLog.d("RecordListActivity， State.EXPANDED")
                        collapsing_tool_bar_layout?.title = null
                        text_view_symbol.visibility = View.VISIBLE
                        text_view_title.visibility = View.VISIBLE
                    }
                    State.SCROLLING -> {
                        collapsing_tool_bar_layout?.title = "     " + viewModel.recordType.title
                        text_view_title.visibility = View.INVISIBLE
                    }
                    else -> {
                        //ignore
                    }
                }
            }
        })
​

附 AppBarStateChangeListener 代码

abstract class AppBarStateChangeListener : AppBarLayout.OnOffsetChangedListener {

    enum class State {
        EXPANDED, COLLAPSED, SCROLLING, IDLE
    }

    private var mCurrentState = State.IDLE

    override fun onOffsetChanged(appBarLayout: AppBarLayout?, i: Int) {
        val expandPercent = 1f + (i / (appBarLayout?.totalScrollRange ?: -i).toFloat())
        when {
            i == 0 -> {
                if (mCurrentState != State.EXPANDED) {
                    mCurrentState = State.EXPANDED
                    onStateChanged(appBarLayout, mCurrentState, i)
                }
            }
            abs(i) >= appBarLayout!!.totalScrollRange -> {
                if (mCurrentState != State.COLLAPSED) {
                    mCurrentState = State.COLLAPSED
                    onStateChanged(appBarLayout, mCurrentState, i)
                }
            }
            else -> {
                mCurrentState = State.SCROLLING
                onStateChanged(appBarLayout, State.SCROLLING, i)
            }
        }
        Log.d("AppBarLayout.onOffsetChanged"，"$i/${appBarLayout?.totalScrollRange}，${mCurrentState}，${expandPercent}")
        onExpandPercentChange(mCurrentState, expandPercent)
    }

    abstract fun onExpandPercentChange(state: State, expandPercent: Float)

    abstract fun onStateChanged(appBarLayout: AppBarLayout?, state: State?, i: Int)

}
​

问题解决过程

1. 查看 CollapsingToolbarLayout 的源码，发现 CollapsingTextHelper，利用反射获取到 collapsingTextHelper
2. 查看 CollapsingTextHelper 的源码，发现 currentBounds，再利用反射获取到 currentBounds
3. 监听 AppBarLayout 高度变化，然后获取 currentBounds.left 等信息，然后实时更改 TextView 的位置