<div dir="ltr"><div class="gmail_default" style="font-family:georgia,serif">Hi everyone,</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">See below codes,</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif"><p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)">//: Playground - noun: a place where people can play</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;min-height:14px"><br></p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)"><span style="color:rgb(186,45,162)">import</span> Foundation</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;min-height:14px"><br></p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(186,45,162)">struct<span style="color:rgb(0,0,0)"> Foo {</span></p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">    <span style="color:rgb(186,45,162)">var</span> bar = <span style="color:rgb(39,42,216)">0</span></p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">}</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;min-height:14px"><br></p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(186,45,162)">class<span style="color:rgb(0,0,0)"> A {</span></p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">    <span style="color:rgb(186,45,162)">var</span> foo = <span style="color:rgb(79,129,135)">Foo</span>() {</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">        <span style="color:rgb(186,45,162)">didSet</span> {</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">            <span style="color:rgb(62,30,129)">print</span>(<span style="color:rgb(209,47,27)">&quot;foo changed&quot;</span>)</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">        }</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">    }</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">}</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;min-height:14px"><br></p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)"><span style="color:rgb(186,45,162)">let</span> a = <span style="color:rgb(79,129,135)">A</span>()</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;min-height:14px"><br></p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)"><span style="color:rgb(186,45,162)">for</span> i <span style="color:rgb(186,45,162)">in</span> <span style="color:rgb(39,42,216)">1</span>...<span style="color:rgb(39,42,216)">5</span> {</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">    <span style="color:rgb(79,129,135)">a</span>.<span style="color:rgb(79,129,135)">foo</span>.<span style="color:rgb(79,129,135)">bar</span> = i</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,0,0)">}</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;min-height:14px"><br></p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)">// prints five times</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)">// foo changed</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)">// foo changed</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)">// foo changed</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)">// foo changed</p>
<p style="margin:0px;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)">// foo changed</p></div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">If I change `struct Foo` to `class Foo`, nothing will happen.</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">My code depends on the class version didSet but I used the struct version. It took me a long time to debug this out. Is this a knowledge that has been well known already?</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">Xcode 9.1, Swift 4.0.2</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">Zhao Xin</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif"><br></div></div>