php設(shè)計(jì)模式介紹之值對象模式第1/5頁
Jun 13, 2016 pm 12:29 PM
例如：通常用一個對象描述一個日期、一個數(shù)字或者貨幣。日期、整數(shù)或美元的類定義是都是便于使用的、快捷、便于封裝的，并且方便進(jìn)行拷貝，相互比較，甚至是創(chuàng)建。
從表面上看，這些描述簡單的對象很容易被執(zhí)行：它們的語句非常少，在構(gòu)造類時無論是應(yīng)用于Customer還是SKU都沒有什么不同。這個想法似乎是正確的，但是所謂的"似乎正確"很容易產(chǎn)生一些bug。
請看下面的代碼，這是一個關(guān)于以美元給員工發(fā)放工資的對象的定義和執(zhí)行操作。多數(shù)情況下，它的運(yùn)行是沒有問題的。（這個類被命名為BadDollar，因?yàn)樗€存在著bug）?？紤]一下，看你是否能發(fā)現(xiàn)它的bug。
//?PHP5
class?BadDollar?{
protected?$amount;
public?function?__construct($amount=0)?{
$this->amount?=?(float)$amount;
}
public?function?getAmount()?{
return?$this->amount;
}
public?function?add($dollar)?{
$this->amount?+=?$dollar->getAmount();
}
}
class?Work?{
protected?$salary;public?function?__construct()?{
$this->salary?=?new?BadDollar(200);}
public?function?payDay()?{
return?$this->salary;
}
}
class?Person?{
public?$wallet;
}
function?testBadDollarWorking()?{
$job?=?new?Work;
$p1?=?new?Person;
$p2?=?new?Person;
$p1->wallet?=?$job->payDay();
$this->assertEqual(200,?$p1->wallet->getAmount());
$p2->wallet?=?$job->payDay();
$this->assertEqual(200,?$p2->wallet->getAmount());
$p1->wallet->add($job->payDay());
$this->assertEqual(400,?$p1->wallet->getAmount());
//this?is?bad?—?actually?400
$this->assertEqual(200,?$p2->wallet->getAmount());
//this?is?really?bad?—?actually?400
$this->assertEqual(200,?$job->payDay()->getAmount());
}
那么，?bug是什么呢？如果不能上面的代碼例子中直觀地發(fā)現(xiàn)問題，這里有個提示:雇員對象$p1和對象$p2使用著同一個BadDollar對象實(shí)例。
首先，類Work和類Person的實(shí)例已經(jīng)創(chuàng)建。那么，假設(shè)每一個雇員最初有一個空的電子錢包，雇員的電子錢包Person:wallet是通過Work::payDay()函數(shù)返回的對象資源變量賦值的，所以被設(shè)定為一個BadDollar類的對象實(shí)例。
還記得PHP5的對象賦值處理方式嗎？因?yàn)镻HP5的對象賦值的處理方式，所以$job::salary,、$p1::wallet和$p2::wallet這三個看上去不同的對象實(shí)例雖然使用著不同的“標(biāo)識符”，但是事實(shí)上，它們?nèi)慷贾付ǖ酵粋€對象實(shí)例。
因此，接下來的發(fā)放工資的操作（PayDay表示發(fā)放工資的日子，這里表示發(fā)放工資的動作），使用$job->payDay()本來僅僅是想增加$P1的工資，卻出乎意料地次給$P2也發(fā)放了。并且，這個動作還改變了工作的基本工資的額度。因此，最后兩個值的檢測報錯。
Value?Object?PHP5?Unit?Test
1)?Equal?expectation?fails?because?[Integer:?200]?differs?from?[Float:?400]?by?200
in?testBadDollarWorking
in?ValueObjTestCase
2)?Equal?expectation?fails?because?[Integer:?200]?differs?from?[Float:?400]?by?200
in?testBadDollarWorking
in?ValueObjTestCase
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