NSString * urlStr = @" www.jianshu.com";
NSRange range = NSMakeRange(4,7);
NSString *prefix = [urlStr substringWithRange:range];
NSLog(@"%@",prefix);
(1)[aString autorelease];
(2)NSArray * arr = [NSArray array];
(3)[arr addObject:aString]; //不可变数组没有addObject方法
(4)NSLog(@"string = %lu",[aString retainCount]);
NSTimer *myTimer = [NSTimer timerWithTimeInterval:1.0 target:self selector:@selector(doSomeThing:) userInfo:nil repeats:YES];
[myTimer fire]
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