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<div><span style="font-size: 24px;"><b><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);">模塊</span></span></b></span></div>
<div><span style="font-size: 16px;"><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);"><b>YYCache</b> </span></span></span></div>
<div><span style="font-size: 16px;"><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);">這個類是底層YYMemoryCache和YYDiskCache的對外的統一接口。里面的方法不具體執行，只是調用了</span></span></span><span style="font-size: 16px;"><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);">YYMemoryCache和</span></span></span><span style="font-size: 16px;"><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);">YYDiskCache的方法。</span></span></span></div>
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<div><b><span style="font-size: 16px;"><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);">YYMemoryCache</span></span></span></b></div>
<div><span style="font-size: 16px;"><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);">這是一個內存緩存類，里面實現雙向鏈表和淘汰算法。為了防止多線程操作，使用了</span></span></span><span style="font-size: 16px;"><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);">OSSpinLock</span></span></span><span style="box-sizing: border-box; padding: 0px; border: 0px; outline: 0px; font-size: 16px; vertical-align: baseline; background: transparent;">自旋鎖，</span></div>
<div><span style="font-size: 16px;"><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);">OSSpinLock 不需要進入內核，只是簡單的重復檢測鎖是否釋放。如果此任務需要執行很久的話，效率會非常低。但是由于它節省了系統調用和上下文切換的時間，所以當任務確實很快的時候，效率很好。</span></span></span><span style="color: rgb(29, 29, 29);"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;"><span style="box-sizing: border-box; padding: 0px; border: 0px; outline: 0px; font-size: 16px; vertical-align: baseline; background: transparent;">對于內存緩存的存取來說，它非常合適。</span></span></span></div>
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<div><b><span style="font-size: 16px;"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;"><span style="color: rgb(29, 29, 29);">YYDiskCache</span></span></span></b></div>
<div><span style="font-size: 16px;"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;"><span style="color: rgb(29, 29, 29);">它是以YYKVStorageItem類的前端，它和YYCache一樣，都是不執行具體代碼，只是調用</span></span></span><span style="font-size: 16px;"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;"><span style="color: rgb(29, 29, 29);">YYKVStorageItem的方法來具體執行，它使用了</span></span></span></div>
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<div style="box-sizing: border-box; padding: 0px 0px 0.6em; border: 0px; outline: 0px; vertical-align: baseline; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 1; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); background-position: initial initial; background-repeat: initial initial;">
<div><span style="color: rgb(29, 29, 29);"><span style="font-size: 16px;"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;"><span style="box-sizing: border-box; padding: 0px; border: 0px; outline: 0px; font-size: 16px; vertical-align: baseline; background: transparent;">dispatch_semaphore 是信號量，但當信號總量設為 </span><span style="box-sizing: border-box; padding: 0px; border: 0px; outline: 0px; font-size: 16px; vertical-align: baseline; background: transparent;">1</span><span style="box-sizing: border-box; padding: 0px; border: 0px; outline: 0px; font-size: 16px; vertical-align: baseline; background: transparent;"> 時也可以當作鎖來。在沒有等待情況出現時，它的性能比 pthread_mutex 還要高，但一旦有等待情況出現時，性能就會下降許多。相對于 OSSpinLock 來說，它的優勢在于等待時不會消耗 CPU 資源。對磁盤緩存來說，它比較合適。</span></span></span></span></div>
<div><span style="color: rgb(29, 29, 29);"><span style="font-size: 16px;"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;">
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<div><b><span style="font-size: 16px;"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;"><span style="color: rgb(29, 29, 29);">YYKVStorageItem</span></span></span></b></div>
<div><span style="font-size: 16px;"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;"><span style="color: rgb(29, 29, 29);">它是以</span></span></span><b><span style="font-size: 16px;"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;"><span style="color: rgb(29, 29, 29);">YYDiskCache</span></span></span></b><span style="font-size: 16px;"><span style="font-family: 'PingFang SC', 'Helvetica Neue', Helvetica, Arial, Verdana, 'Hiragino Sans GB', 'Noto Sans CJK SC', 'Source Han Sans CN', STHeiti, 'Microsoft YaHei', 'WenQuanYi Micro Hei', SimSun, sans-serif;"><span style="color: rgb(29, 29, 29);">類的后端，負責執行存文件或者存在數據庫（小于20K的數據）里面。它也是使用了</span></span></span><span style="font-size: 16px;"><span style="font-family: Lato, 'Microsoft Jhenghei', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif;"><span style="color: rgb(85, 85, 85);">OSSpinLock。</span></span></span></div>
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