Android Compose Column列表不自动刷新问题

来源：jb51　　时间：2023/2/1 9:35:30　　对本文有异议

1. 背景

我们都知道，Compose可以使用mutableStateOf和UI进行绑定，改变值之后，就可以改变UI。

var value by remember { mutableStateOf(0) }
var imageVisible by remember { mutableStateOf(true) }
Column {
    Text(text = "现在的值是:$value")
    Button(onClick = {
        value++ //修改值，自动改变UI
    }) {
        Text(text = "Add Value")
    }
    AnimatedVisibility(visible = imageVisible) {
        Image(
            painter = painterResource(id = R.mipmap.photot1),
            contentDescription = "",
            Modifier.width(260.dp)
        )
    }
    Button(onClick = {
        imageVisible = !imageVisible //修改值，自动显示/隐藏UI
    }) {
        Text(text = "Show/Hide")
    }
}

效果如下

但是如果是使用Column/Row/LazyColumn/LazyRow列表的时候，无论怎么更新数据，界面都不会刷新

val list = ArrayList<String>()
for (i in 0..10) {
    list.add(i.toString())
}
var stateList by remember { mutableStateOf(list) }
Button(onClick = {
    stateList.add("添加的值:${Random.nextInt()}")
}, modifier = Modifier.fillMaxWidth()) {
    Text(text = "添加值")
}
Button(onClick = {
    stateList.removeAt(stateList.size - 1)
}, modifier = Modifier.fillMaxWidth()) {
    Text(text = "删除值")
}
LazyColumn {
    items(stateList.size) { index ->
        Text(
            text = "${stateList.get(index)}",
            textAlign = TextAlign.Center,
            modifier = Modifier
                .height(24.dp)
                .fillMaxWidth()
        )
    }
}

可以看到，点击了按钮后，列表完全没有刷新

这是为什么了 ?

2. 解决方案

当时很不解，为啥其他类型都是可以的，使用List就不行了呢 ?

查阅了好久，终于找到了解决方案

把mutableStateOf改用mutableStateListOf就可以了

var stateList = remember { mutableStateListOf<String>() }
for (i in 0..10) {
    stateList.add(i.toString())
}
Button(onClick = {
    stateList.add("添加的值:${Random.nextInt()}")
}, modifier = Modifier.fillMaxWidth()) {
    Text(text = "添加值")
}
Button(onClick = {
    stateList.removeAt(stateList.size - 1)
}, modifier = Modifier.fillMaxWidth()) {
    Text(text = "删除值")
}
LazyColumn {
    items(stateList.size) { index ->
        Text(
            text = "${stateList.get(index)}",
            textAlign = TextAlign.Center,
            modifier = Modifier
                .height(24.dp)
                .fillMaxWidth()
        )
    }
}

3. 原因

解决方案很简单，但是这是为什么呢 ?

3.1 mutableStateOf为什么可以更新UI

我们以mutableStateOf()这个为例

var value by mutableStateOf(0)

首先，我们要明白，mutableStateOf()返回的是一个MutableState对象，MutableState中有一个var value: T属性

interface MutableState<T> : State<T> {
    override var value: T
    operator fun component1(): T
    operator fun component2(): (T) -> Unit
}
interface State<out T> {
    val value: T
}

查看mutableStateOf源码，可以发现，mutableStateOf()返回的是继承自MutableState的SnapshotMutableState对象，路径mutableStateOf()-> createSnapshotMutableState() -> ParcelableSnapshotMutableState-> SnapshotMutableStateImpl，可以看到有这样一段代码

override var value: T
    get() = next.readable(this).value
    set(value) = next.withCurrent {
        if (!policy.equivalent(it.value, value)) {
            next.overwritable(this, it) { this.value = value }
        }
    }
private var next: StateStateRecord<T> = StateStateRecord(value)

这里就是重点，SnapshotMutableStateImpl的value属性重写了get()和set()方法

当value被读的时候，不光把值返回，还会记录一下在哪被读的
当value被写的时候，不止把这个值给改了，还会去查找在哪里被读过，然后通知这些被读过的地方，通知UI进行刷新

4. 结论

因为我们操作String、Int等基础类型的时候，都是通过get、set()来获取、设置数据的，所以这操作会被SnapshotMutableStateImpl记录下来，而List、Map这种集合，我们是通过add、remove来更新数据的，所以不会触发SnapshotMutableStateImpl value属性的set。

4.1 解决方案一

使用mutableStateListOf替代mutableStateOf，mutableStateListOf内部对add、remove方法也进行了重写

4.2 解决方案二

新创建一个List，然后赋值给原来的list，这样就会触发set了

var stateList by remember { mutableStateOf(list) }
val tempList = ArrayList<String>()
for (value in stateList) {
    tempList.add(value)
}
tempList.add("添加的值:${Random.nextInt()}")
stateList = tempList //赋值的时候会触发刷新UI

5.自己实现一个mutableStateOf()

我们也可以自己来实现一个mutableStateOf，伪代码如下

class Test {
    interface State<out T> {
        val value: T
    }
    interface MutableState<T> : State<T> {
        override var value: T
        /*operator fun component1(): T
        operator fun component2(): (T) -> Unit*/
    }
    inline operator fun <T> State<T>.getValue(thisObj: Any?, property: KProperty<*>): T = value
    inline operator fun <T> MutableState<T>.setValue(
        thisObj: Any?,
        property: KProperty<*>,
        value: T
    ) {
        this.value = value
    }
    interface SnapshotMutableState<T> : MutableState<T> {
        val policy: SnapshotMutationPolicy<T>
    }
    interface SnapshotMutationPolicy<T> {
        fun equivalent(a: T, b: T): Boolean
        fun merge(previous: T, current: T, applied: T): T? = null
    }
    internal open class SnapshotMutableStateImpl<T>(
        val _value: T,
        override val policy: SnapshotMutationPolicy<T>
    ) : /*StateObject, */SnapshotMutableState<T> {
        private var next : T = 52 as T
        @Suppress("UNCHECKED_CAST")
        override var value: T
            get() = next
            /*get() {
                Log.i(TAGs.TAG, "getValue:$field")
                return "" as T
            }*/
            set(value) {
                Log.i(TAGs.TAG, "setValue")
                this.value = value
            }
        /*override fun component1(): T {
            //TODO("Not yet implemented")
        }
        override fun component2(): (T) -> Unit {
            //TODO("Not yet implemented")
        }*/
    }
    internal class ParcelableSnapshotMutableState<T>(
        value: T,
        policy: SnapshotMutationPolicy<T>
    ) : SnapshotMutableStateImpl<T>(value, policy)/*, Parcelable*/ {
    }
    fun <T> mutableStateOf(
        value: T,
        policy: SnapshotMutationPolicy<T> = structuralEqualityPolicy()
    ): MutableState<T> = createSnapshotMutableState(value, policy)
    fun <T> structuralEqualityPolicy(): SnapshotMutationPolicy<T> =
        StructuralEqualityPolicy as SnapshotMutationPolicy<T>
    private object StructuralEqualityPolicy : SnapshotMutationPolicy<Any?> {
        override fun equivalent(a: Any?, b: Any?) = a == b
        override fun toString() = "StructuralEqualityPolicy"
    }
    fun <T> createSnapshotMutableState(
        value: T,
        policy: SnapshotMutationPolicy<T>
    ): SnapshotMutableState<T> = ParcelableSnapshotMutableState(value, policy)
    fun main() {
        var sizeUpdate by mutableStateOf(48)
        Log.i(TAGs.TAG, "sizeUpdate:$sizeUpdate")
        sizeUpdate = 64
        Log.i(TAGs.TAG, "sizeUpdate>>$sizeUpdate")
    }
}