lua版promise实现 - 从异步回调多层嵌套开始

异步回调代码，很容易就写出下面这样的不断嵌套的代码。如果还夹杂着各种逻辑的话，可读性会很差，还容易出错。

AsyncLoadRes("ResA", function(textA)
    print("ResA load finish")
    AsyncLoadRes("ResB", function(textB)
        print("ResB load finish")
        AsyncLoadRes("ResC", function(textC)
            print("ResC load finish")
            print(textA, textB, textC)
        end)
    end)
end)

接下来把上面的多层嵌套优化下

回调内不直接写逻辑，而是调用对象的doNext函数

local obj1 = { value = nil, doNext = nil }
AsyncLoadRes("ResA", function(textA)
    obj1.value = textA
    obj1.doNext(textA)
end)

local obj2 = { value = nil, doNext = nil }
obj1.doNext = function(textA)
    print("ResA load finish")
    AsyncLoadRes("ResB", function(textB)
        obj2.value = textB
        obj2.doNext(textB)
    end)
end

local obj3 = { value = nil, doNext = nil }
obj2.doNext = function(textB)
    print("ResB load finish")
    AsyncLoadRes("ResC", function(textC)
        obj3.value = textC
        obj3.doNext(textC)
    end)
end

obj3.doNext = function(textC)
    print("ResC load finish, all loaded")
end

再把上面的代码调整下

doNext改成了含有run方法的对象，objX.doNext(xxx)改成objX.doNextObj.run(textA)

local obj1 = { value = nil, doNextObj = nil }
AsyncLoadRes("ResA", function(textA)
    obj1.value = textA
    obj1.doNextObj.run(textA)
end)

local obj2 = { value = nil, doNextObj = nil }
obj2.run = function(textA)
    print("ResA load finish")
    AsyncLoadRes("ResB", function(textB)
        obj2.value = textB
        obj2.doNextObj.run(textB)
    end)
end
obj1.doNextObj = obj2

local obj3 = { value = nil, doNextObj = nil }
obj3.run = function(textB)
    print("ResB load finish")
    AsyncLoadRes("ResC", function(textC)
        obj3.value = textC
        obj3.doNextObj.run(textC)
    end)
end
obj2.doNextObj = obj3

local obj4 = { value = nil, doNextObj = nil }
obj4.run = function(textC)
    print("ResC load finish, all loaded")
end
obj3.doNextObj = obj4

 为什么要这么改？

因为这样改之后，obj之间就组成了一个单向链表，为后面还要实现的一些功能增加了可能性。

对上面的代码稍微封装下，就可以得到第1版的promise

---@class PromiseV1
local PromiseV1 = {}
PromiseV1.__cname = "Promise"
PromiseV1.__index = PromiseV1

function PromiseV1.new()
    local inst = {}
    setmetatable(inst, PromiseV1)
    inst:ctor()
    return inst
end

function PromiseV1:ctor()
    self.m_Value = nil

    self.m_DoNextObj = nil
    self.m_Run = nil
end

function PromiseV1:Next(obj, run)
    obj.m_Run = run
    self.m_DoNextObj = obj
 end

function PromiseV1:SetFinished(v)
    if nil == self.m_Value then
        self.m_Value = v
        self.m_DoNextObj.m_Run(v)
    end
end

用法

local obj1 = PromiseV1.new()
AsyncLoadRes("ResA", function(textA)
    obj1:SetFinished(textA)
end)

local obj2 = PromiseV1.new()
local loadResB = function(textA)
    print("ResA load finish")
    AsyncLoadRes("ResB", function(textB)
        obj2:SetFinished(textB)
    end)
end
obj1:Next(obj2, loadResB)

local obj3 = PromiseV1.new()
local loadResC = function(textB)
    print("ResB load finish")
    AsyncLoadRes("ResC", function(textC)
        obj3:SetFinished(textC)
    end)
end
obj2:Next(obj3, loadResC)

local obj4 = PromiseV1.new()
local printContent = function(textC)
    print("ResC load finish, all loaded")
end
obj3:Next(obj4, printContent)