如何判断NSDictionary是否包含某个键

<code style="margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; font-family: Consolas, Menlo, Monaco, 'Lucida Console', 'Liberation Mono', 'DejaVu sans Mono', 'Bitstream Vera sans Mono', 'Courier new', monospace, serif;"><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent; color: rgb(0, 0, 139);">if</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;"> </span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">([[</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">dictionary allKeys</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">]</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;"> containsObject</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">:</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">key</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">)</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;"> </span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">{</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">
 <wbr>  <wbr> </wbr></wbr></span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent; color: gray;">// contains key</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">
</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">}</span></code>

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<code style="margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; font-family: Consolas, Menlo, Monaco, 'Lucida Console', 'Liberation Mono', 'DejaVu sans Mono', 'Bitstream Vera sans Mono', 'Courier new', monospace, serif;"><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;"></span></code><pre name="code" style="white-space: pre-wrap; color: rgb(50, 62, 50); font-size: 14px; word-wrap: break-word; line-height: 26px; margin-top: 0px; margin-bottom: 10px; padding: 5px; border-width: 0px; vertical-align: baseline; background-color: rgb(238, 238, 238); font-family: Consolas, Menlo, Monaco, 'Lucida Console', 'Liberation Mono', 'DejaVu sans Mono', 'Bitstream Vera sans Mono', 'Courier new', monospace, serif; overflow: auto; width: auto; max-height: 600px;"><code style="margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; font-family: Consolas, Menlo, Monaco, 'Lucida Console', 'Liberation Mono', 'DejaVu sans Mono', 'Bitstream Vera sans Mono', 'Courier new', monospace, serif;"><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent; color: rgb(0, 0, 139);">if</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;"> </span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">([</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">dictionary objectForKey</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">:</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">key</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">])</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;"> </span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">{  //<span style="word-wrap: normal; word-break: normal; line-height: 18px; border-collapse: collapse; font-family: Arial, 'Liberation sans', 'DejaVu sans', sans-serif;">objectForKey will return nil if a key doesn't exists.</span>
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 <wbr>  <wbr> </wbr></wbr></span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent; color: gray;">// contains key</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">
</span><span style="word-wrap: normal; word-break: normal; line-height: 21px; margin: 0px; padding: 0px; border-width: 0px; vertical-align: baseline; background-color: transparent;">}</span></code>