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Promise机制

时间:2015-03-16 14:24:23      阅读:196      评论:0      收藏:0      [点我收藏+]

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 Promise的诞生与Javascript中异步编程息息相关,js中异步编程主要指的是setTimout/setInterval、DOM事件机制、ajax,通过传入回调函数实现控制反转。异步编程为js带来强大灵活性的同时,也带来了嵌套回调的问题。详细来说主要有两点,第一嵌套太深代码可读性太差,第二并行逻辑必须串行执行。

request = function(url, cb, eb) {
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
        if (xhr.readyState === 4) {
            if ((xhr.status >=200 && xhr.status < 300) || xhr.status === 304) {
                cb(xhr.responseText);
            } else {
                eb(new Error({
                    message: xhr.status
                }));
            }
        }
    };
    xhr.open(‘get‘, url, true);
    xhr.send(null);
}

  

  这个例子中程序要依次处理data1、data2、data3,嵌套太多可读性太差

request = function(url) {
    var def = new Deferred();

    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
        if (xhr.readyState === 4) {
            if ((xhr.status >=200 && xhr.status < 300) || xhr.status === 304) {
                def.resolve(xhr.responseText)
            } else {//简化ajax,没有提供错误回调
                def.reject(new Error({
                    message: xhr.status
                }));
            }
        }
    };
    xhr.open(‘get‘, url, true);
    xhr.send(null);

    return def.promise;
}

request(‘data1.json‘).then(function(data1) {
    console.log(data1);//处理data1
    return request(‘data2.json‘);
}).then(function(data2) {
    console.log(data2);//处理data2
    return request(‘data3.json‘);
}, function(err) {
    console.error(err);
}).then(function(data3) {
    console.log(data3);
    alert(‘success‘);
}, function(err) {
    console.error(err);
});

  

  这个例子中程序需要请求data1、data2、data3数据,得到三个数据后才进行下一步处理。数据并不需要串行请求,但我们的代码却需要串行执行,增加了等待时间。

//并行逻辑串行执行
request(‘data1‘, function(data1) {
    request(‘data2‘, function(data2) {
        request(‘data3‘, function(data3) {
            console.log(data1, data2, data3);//处理全部数据

            alert(‘success‘);
        }, function(err) {
            console.error(err);
        });
    }, function(err) {
        console.error(err);
    });
}, function(err) {
    console.error(err);
});

  

 

Promise机制

  Promise机制便是上述问题的一种解决方案。与他相关的规范有PromiseAPromiseA+,PromiseA中对Promise进行了整体描述,PromiseA+对A进行了补充,在then函数的行为方面进行了更加详尽的阐述。

PromiseA+规范
promise represents the eventual result of an asynchronous operation. 
一个promise代表了一个异步操作的最终结果
The primary way of interacting with a promise is through its then method, which registers callbacks to receive either a promise’s eventual value or the reason why the promise cannot be fulfilled.跟promise交互的主要方式是通过他的then方法来注册回调函数去接收promise的最终结果值或者是promise不能完成的原因。

  我们可以简单总结一下规范。每个promise都有三个状态:pending(默认)、fulfilled(完成)、rejected(失败);默认状态可以转变为完成态或失败态,完成态与失败态之间无法相互转换,转变的过程是不可逆的,转变一旦完成promise对象就不能被修改。通过promise提供的then函数注册onFulfill(成功回调)、onReject(失败回调)、onProgres(进度回调)来与promise交互。Then函数返回一个promise对象(称为promise2,前者成为promise1),promise2受promise1状态的影响,具体请查看A+规范。

  上两个规范中并没有说明promise的状态如何改变,大部分前端框架中使用Deferred来改变promise的状态(resolve()、reject())。二者关系请看下图。

技术分享

  这里根据规范,我们实现一下promise

Promise = function() {
    this.queue = [];
    this.value = null;
    this.status = ‘pending‘;// pending fulfilled rejected
};

Promise.prototype.getQueue = function() {
    return this.queue;
};
Promise.prototype.getStatus = function() {
    return this.status;
};
Promise.prototype.setStatus = function(s, value) {
    if (s === ‘fulfilled‘ || s === ‘rejected‘) {
        this.status = s;
        this.value = value || null;
        this.queue = [];
        var freezeObject = Object.freeze || function(){};
        freezeObject(this);// promise的状态是不可逆的
    } else {
        throw new Error({
            message: "doesn‘t support status: " + s
        });
    }
};
Promise.prototype.isFulfilled = function() {
    return this.status === ‘fulfilled‘;
};
Promise.prototype.isRejected = function() {
    return this.status === ‘rejected‘;
}
Promise.prototype.isPending = function() {
    return this.status === ‘pending‘;
}
Promise.prototype.then = function(onFulfilled, onRejected) {
    var handler = {
        ‘fulfilled‘: onFulfilled,
        ‘rejected‘: onRejected
    };
    handler.deferred = new Deferred();

    if (!this.isPending()) {//这里允许先改变promise状态后添加回调
        utils.procedure(this.status, handler, this.value);
    } else {
        this.queue.push(handler);//then may be called multiple times on the same promise;规范2.2.6
    }
    return handler.deferred.promise;//then must return a promise;规范2.2.7
};

var utils = (function(){
    var makeSignaler = function(deferred, type) {
        return function(result) {
            transition(deferred, type, result);
        }
    };

    var procedure = function(type, handler, result) {
        var func = handler[type];
        var def = handler.deferred;

        if (func) {
            try {
                var newResult = func(result);
                if (newResult && typeof newResult.then === ‘function‘) {//thenable
                    // 此种写法存在闭包容易造成内存泄露,我们通过高阶函数解决
                    // newResult.then(function(data) {
                    //     def.resolve(data);
                    // }, function(err) {
                    //     def.reject(err);
                    // });
                    //PromiseA+规范,x代表newResult,promise代表def.promise
                    //If x is a promise, adopt its state [3.4]:
                    //If x is pending, promise must remain pending until x is fulfilled or rejected.
                    //If/when x is fulfilled, fulfill promise with the same value.
                    //If/when x is rejected, reject promise with the same reason.
                    newResult.then(makeSignaler(def, ‘fulfilled‘), makeSignaler(def, ‘rejected‘));//此处的本质是利用了异步闭包
                } else {
                    transition(def, type, newResult);
                }
            } catch(err) {
                transition(def, ‘rejected‘, err);
            }
        } else {
            transition(def, type, result);
        }
    };

    var transition = function(deferred, type, result) {
        if (type === ‘fulfilled‘) {
            deferred.resolve(result);
        } else if (type === ‘rejected‘) {
            deferred.reject(result);
        } else if (type !== ‘pending‘) {
            throw new Error({
                ‘message‘: "doesn‘t support type: " + type
            });
        }
    };

    return {
        ‘procedure‘: procedure
    }
})();

Deferred = function() {
    this.promise = new Promise();
};

Deferred.prototype.resolve = function(result) {
    if (!this.promise.isPending()) {
        return;
    }

    var queue = this.promise.getQueue();
    for (var i = 0, len = queue.length; i < len; i++) {
        utils.procedure(‘fulfilled‘, queue[i], result);
    }
    this.promise.setStatus(‘fulfilled‘, result);
};

Deferred.prototype.reject = function(err) {
    if (!this.promise.isPending()) {
        return;
    }

    var queue = this.promise.getQueue();
    for (var i = 0, len = queue.length; i < len; i++) {
        utils.procedure(‘rejected‘, queue[i], err);
    }
    this.promise.setStatus(‘rejected‘, err);
}

  通过Promise机制我们的编程方式可以变成这样:

request = function(url) {
    var def = new Deferred();

    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
        if (xhr.readyState === 4) {
            if ((xhr.status >=200 && xhr.status < 300) || xhr.status === 304) {
                def.resolve(xhr.responseText)
            } else {//简化ajax,没有提供错误回调
                def.reject(new Error({
                    message: xhr.status
                }));
            }
        }
    };
    xhr.open(‘get‘, url, true);
    xhr.send(null);

    return def.promise;
}

request(‘data1.json‘).then(function(data1) {
    console.log(data1);//处理data1
    return request(‘data2.json‘);
}).then(function(data2) {
    console.log(data2);//处理data2
    return request(‘data3.json‘);
}, function(err) {
    console.error(err);
}).then(function(data3) {
    console.log(data3);
    alert(‘success‘);
}, function(err) {
    console.error(err);
});

  

  

  对于并行逻辑串行执行问题我们可以这样解决

//所有异步操作都完成时,进入完成态,
//其中一项异步操作失败则进入失败态
all = function(requestArray) {
    // var some = Array.prototype.some;
    var def = new Deferred();
    var results = [];
    var total = 0;
    requestArray.some(function(r, idx) {
        //为数组中每一项注册回调函数
        r.then(function(data) {
            if (def.promise.isPending()) {
                total++;
                results[idx] = data;

                if (total === requestArray.length) {
                    def.resolve(results);
                }
            }
        },  function(err) {
            def.reject(err);
        });
        //如果不是等待状态则停止,比如requestArray[0]失败的话,剩下数组则不用继续注册
        return !def.promise.isPending();
    });

    return def.promise;
}

all(
    [request(‘data1.json‘),
    request(‘data2.json‘),
    request(‘data3.json‘)]
    ).then(
        function(results){
            console.log(results);// 处理data1,data2,data3
            alert(‘success‘);
    }, function(err) {
        console.error(err);
    });

  

  以下是几个测试案例

//链式调用
var p1 = new Deferred();
p1.promise.then(function(result) {
    console.log(‘resolve: ‘, result);
    return result;
}, function(err) {
    console.log(‘reject: ‘, err);
    return err;
}).then(function(result) {
    console.log(‘resolve2: ‘, result);
    return result;
}, function(err) {
    console.log(‘reject2: ‘, err);
    return err;
}).then(function(result) {
    console.log(‘resolve3: ‘, result);
    return result;
}, function(err) {
    console.log(‘reject3: ‘, err);
    return err;
});
p1.resolve(‘success‘);
//p1.reject(‘failed‘);
p1.promise.then(function(result) {
    console.log(‘after resolve: ‘, result);
    return result;
}, function(err) {
    console.log(‘after reject: ‘, err);
    return err;
}).then(function(result) {
    console.log(‘after resolve2: ‘, result);
    return result;
}, function(err) {
    console.log(‘after reject2: ‘, err);
    return err;
}).then(function(result) {
    console.log(‘after resolve2: ‘, result);
    return result;
}, function(err) {
    console.log(‘after reject2: ‘, err);
    return err;
});

//串行异步
var p2 = new Deferred();
p2.promise.then(function(result) {
    var def = new Deferred();
    setTimeout(function(){
        console.log(‘resolve: ‘, result);
        def.resolve(result);
    })
    return def.promise;
}, function(err) {
    console.log(‘reject: ‘, err);
    return err;
}).then(function(result) {
    var def = new Deferred();
    setTimeout(function(){
        console.log(‘resolve2: ‘, result);
        def.reject(result);
    })
    return def.promise;
}, function(err) {
    console.log(‘reject2: ‘, err);
    return err;
}).then(function(result) {
    console.log(‘resolve3: ‘, result);
    return result;
}, function(err) {
    console.log(‘reject3: ‘, err);
    return err;
});
p2.resolve(‘success‘);

//并行异步
var p1 = function(){
    var def = new Deferred();
    setTimeout(function() {
        console.log(‘p1 success‘);
        def.resolve(‘p1 success‘);
    }, 20);

    return def.promise;
}
var p2 = function(){
    var def = new Deferred();
    setTimeout(function() {
        console.log(‘p2 failed‘);
        def.reject(‘p2 failed‘);
    }, 10);

    return def.promise;
}

var p3 = function(){
    var def = new Deferred();
    setTimeout(function() {
        console.log(‘p3 success‘);
        def.resolve(‘p3 success‘);
    }, 15);

    return def.promise;
}

all([p1(), p2(), p3()]).then(function(results) {
    console.log(results);
}, function(err) {
    console.error(err);
});

  

Promise优点

对比使用Promise前后我们可以发现,传统异步编程通过嵌套回调函数的方式,等待异步操作结束后再执行下一步操作。过多的嵌套导致意大利面条式的代码,可读性差、耦合度高、扩展性低。通过Promise机制,扁平化的代码机构,大大提高了代码可读性;用同步编程的方式来编写异步代码,保存线性的代码逻辑,极大的降低了代码耦合性而提高了程序的可扩展性。 

参考文章:

javascript 异步编程

jQuery 2.0.3 源码分析 Deferred(最细的实现剖析,带图)

JavaScript异步编程原理

Promise机制

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原文地址:http://www.cnblogs.com/zhuyang/p/4341685.html

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