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【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> Since the dawn of modern web applications, push notifications have gained significant traction in industry. Instead of pooling data from server it has now become common that server should notify client. Ajax is so pervasive these days it is almost seen on every web page we visit. This is all good if you want to refresh a part of page every few seconds. But things get ugly if you want tens or hundreds of requests a second. Pooling webserver is too expensive in such cases. With that in mind HTML5 has introduced a nifty cool feature “Server-Sent Events” . 1.

错误 [NSNull length]:unrecognized selector sent to instance 这个错误是从字典中取出值后，没有判断取出的字符串是否为空，就对字符串进行测长度的 操作. 正确做法是取出字符串后，先进行判断是否为空，再测长度或其他操作 判断是否为空的方法要选正确 开始走入了一个误区，一直用if(runTitle == nil)和[runTitle isEqualsString@""]等一些判断是否为空，结果都报错。 后来使用[runtitle isKindOfClass:[NSNull class]]来判断解决了这个问题。 来源： oschina 链接： https://my.oschina.net/u/2361492/blog/651238