Page({
    data: {
        products: [
            {name: 'A', quantity: 0},
            {name: 'B', quantity: 0},
        ],


        total: 0,
    },


    plus({currentTarget: {id}}) {
        let {products, total} = this.data
        let index = Number(id)
        let {quantity} = products[index]
        quantity += 1
        total += 1
        let key = `products[${index}].quantity`
        console.log({key, quantity})
        this.setData(
            {
                key: quantity, 
                total
            },
            () => {
                console.log('quantity=', this.data.products[index].quantity)
            }
        )
    }
})

点击按钮，total 正常，而商品数量始终为0。