解读Jasmine的Spy机制
众所周知,Angular所用的单元测试框架是Karma+Jasmine,最近在写Angular的Unit Test的时候,在Given“创建测试条件”部分会在很多地方用到Spy去模拟和监测函数调用,而jasmine为我们提供的关于Spy的函数有很多种,比如createSpyObj,createSpy,SpyOn等等,而这些方法命名相似但是用法却不相同,常常让人容易混淆而产生很多错误,下面就通过研读Jasmine关于Spy的源码来弄清楚这些Spy函数到底是干什么的,在什么场合下使用它们。
先从createSpyObj开始研究:
j$.createSpyObj = function(baseName, methodNames) { var baseNameIsCollection = j$.isObject_(baseName) || j$.isArray_(baseName); if (baseNameIsCollection && j$.util.isUndefined(methodNames)) { methodNames = baseName; baseName = 'unknown'; } var obj = {}; var spiesWereSet = false; //如果参数2是method的数组,则调用createSpy(base.method) if (j$.isArray_(methodNames)) { for (var i = 0; i < methodNames.length; i++) { obj[methodNames[i]] = j$.createSpy(baseName + '.' + methodNames[i]); spiesWereSet = true; } } //如果参数2是method:returnValue的键值对组成的对象,则除了调用createSpy(base.method),还用“and.returnValue”来定义了方法的返回值 else if (j$.isObject_(methodNames)) { for (var key in methodNames) { if (methodNames.hasOwnProperty(key)) { obj[key] = j$.createSpy(baseName + '.' + key); obj[key].and.returnValue(methodNames[key]); spiesWereSet = true; } } } if (!spiesWereSet) { throw 'createSpyObj requires a non-empty array or object of method names to create spies for'; } return obj; }; };
再来看SpyOn:
this.spyOn = function(obj, methodName) { //开始是一连串的错误处理,这些错误是在写UT的时候经常出现的错误,可以对号入座 if (j$.util.isUndefined(obj) || obj === null) { throw new Error(getErrorMsg('could not find an object to spy upon for ' + methodName + '()')); } if (j$.util.isUndefined(methodName) || methodName === null) { throw new Error(getErrorMsg('No method name supplied')); } if (j$.util.isUndefined(obj[methodName])) { throw new Error(getErrorMsg(methodName + '() method does not exist')); } if (obj[methodName] && j$.isSpy(obj[methodName]) ) { if ( !!this.respy ){ return obj[methodName]; }else { throw new Error(getErrorMsg(methodName + ' has already been spied upon')); } } var descriptor; try { descriptor = Object.getOwnPropertyDescriptor(obj, methodName); } catch(e) { // IE 8 doesn't support `definePropery` on non-DOM nodes } if (descriptor && !(descriptor.writable || descriptor.set)) { throw new Error(getErrorMsg(methodName + ' is not declared writable or has no setter')); } var originalMethod = obj[methodName], //这里调用了createSpy,createSpy的param1是这个Spy的名字,意义不大;param2是要去Spy的函数 spiedMethod = j$.createSpy(methodName, originalMethod), restoreStrategy; if (Object.prototype.hasOwnProperty.call(obj, methodName)) { restoreStrategy = function() { obj[methodName] = originalMethod; }; } else { restoreStrategy = function() { if (!delete obj[methodName]) { obj[methodName] = originalMethod; } }; } currentSpies().push({ restoreObjectToOriginalState: restoreStrategy }); obj[methodName] = spiedMethod; return spiedMethod; };
再来看一下createSpyObj和spyOn共同用到的方法createSpy(),也可以单独调用:
j$.createSpy = function(name, originalFn) { return j$.Spy(name, originalFn); };
很简单,就是调用了j$.Spy这个方法,
继续看最底层的Spy():
function Spy(name, originalFn) { var numArgs = (typeof originalFn === 'function' ? originalFn.length : 0), //做了一个包装函数,作为虚拟调用 wrapper = makeFunc(numArgs, function () { return spy.apply(this, Array.prototype.slice.call(arguments)); }), //Spy策略:处理Spy的and属性:callThrough执行调用, returnValue指定返回值, callFake执行指定函数,throwError抛出异常,stub原始状态 spyStrategy = new j$.SpyStrategy({ name: name, fn: originalFn, getSpy: function () { return wrapper; } }), callTracker = new j$.CallTracker(), spy = function () { /** * @name Spy.callData * @property {object} object - `this` context for the invocation. * @property {number} invocationOrder - Order of the invocation. * @property {Array} args - The arguments passed for this invocation. */ var callData = { object: this, invocationOrder: nextOrder(), args: Array.prototype.slice.apply(arguments) }; callTracker.track(callData); var returnValue = spyStrategy.exec.apply(this, arguments); callData.returnValue = returnValue; return returnValue; }; function makeFunc(length, fn) { switch (length) { case 1 : return function (a) { return fn.apply(this, arguments); }; case 2 : return function (a,b) { return fn.apply(this, arguments); }; case 3 : return function (a,b,c) { return fn.apply(this, arguments); }; case 4 : return function (a,b,c,d) { return fn.apply(this, arguments); }; case 5 : return function (a,b,c,d,e) { return fn.apply(this, arguments); }; case 6 : return function (a,b,c,d,e,f) { return fn.apply(this, arguments); }; case 7 : return function (a,b,c,d,e,f,g) { return fn.apply(this, arguments); }; case 8 : return function (a,b,c,d,e,f,g,h) { return fn.apply(this, arguments); }; case 9 : return function (a,b,c,d,e,f,g,h,i) { return fn.apply(this, arguments); }; default : return function () { return fn.apply(this, arguments); }; } } for (var prop in originalFn) { if (prop === 'and' || prop === 'calls') { throw new Error('Jasmine spies would overwrite the \'and\' and \'calls\' properties on the object being spied upon'); } wrapper[prop] = originalFn[prop]; } wrapper.and = spyStrategy; wrapper.calls = callTracker; return wrapper; } return Spy; };
由此可以得到,createSpyObj、createSpy、SpyOn、Spy这几个方法的调用关系:
它们适用的场合如图所示:
解释:
createSpyObj:原本没有对象,无中生有地去创建一个对象,并且在对象上创建方法,然后去spy上面的方法
spyOn:原本有对象,对象上也有方法,只是纯粹地在方法上加个spy
createSpy:原本有对象,但是没有相应的方法,虚拟地创建一个方法(虚线),在虚拟的方法上去spy。如果对象上原来有方法,也可以用createSpy去spy,也就是无论有没有这个方法,createSpy都会去spy你指定的方法。
常见的出错信息:
基本上出错的信息都是在spyOn函数上,摘录出来以备查找原因:
- 'could not find an object to spy upon for ' + methodName + '()'
spy的对象为null或undefined
- 'No method name supplied’
spy的方法为null或undefined
- methodName + '() method does not exist'
spy的方法不存在对象上(spyOn必须要在存在的方法上去spy)
- methodName + ' has already been spied upon'
已经有一个spy在这个方法上了,看看有没有地方已经spy了它