iOS 笔记

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UIView动画

使用[UIView animateWithDuration:…]执行动画改变view期间不宜重新加载view中的数据，尤其是有滑动操作的时候。（比如UITableView、UICollectionView等，最好在动画执行之前重载数据。）

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- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    //最好在这里执行reloadData
    [self.tableView reloadData];
    [UIView animateWithDuration:0.4 animations:^{
        self.tableView.contentOffset = CGPointZero;
        self.tableView.frame = CGRectMake(0, 100, 414, 500);
    }];

    //在这里执行reloadData容易crash
    [self.tableView reloadData];
}

runtime理解

runtime是一套可以进行一些非常底层的用OC无法实现的操作的纯C语言的API

Objective-C类由一个指向objc_class结构体的指针来表示:

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//An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;

objc/runtime.h中objc_class结构体的定义如下：

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struct objc_class {
    Class isa  OBJC_ISA_AVAILABILITY;

#if !__OBJC2__
    Class super_class                       OBJC2_UNAVAILABLE;
    const char *name                        OBJC2_UNAVAILABLE;//类名
    long version                            OBJC2_UNAVAILABLE;
    long info                               OBJC2_UNAVAILABLE;
    long instance_size                      OBJC2_UNAVAILABLE;
    struct objc_ivar_list *ivars            OBJC2_UNAVAILABLE;//该类的成员变量列表
    struct objc_method_list **methodLists   OBJC2_UNAVAILABLE;
    struct objc_cache *cache                OBJC2_UNAVAILABLE;//缓存最近使用的方法
    struct objc_protocol_list *protocols    OBJC2_UNAVAILABLE;
#endif

} OBJC2_UNAVAILABLE;

一个类的实例由结构体objc_object来表示:

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//Represents an instance of a class.
struct objc_object {
    Class isa  OBJC_ISA_AVAILABILITY;
};

//A pointer to an instance of a class.
typedef struct objc_object *id;

当我们向一个Objective-C对象发送消息时，运行时库会根据实例对象的isa指针找到这个实例对象所属的类。Runtime库会在类或其父类的方法列表中查询并执行与消息对应的selector指向的方法。

元类(MetaClass)是一个类对象的类

所有的类自身也是一个对象，我们可以向这个对象发送消息—调用类方法。

meta-class也是一个类，也可以向它发送一个消息。Objective-C的设计者让所有的meta-class的isa指向基类的meta-class，以此作为它们的所属类。即，任何NSObject继承体系下的meta-class都使用NSObject的meta-class作为自己的所属类，而基类的meta-class的isa指针是指向它自己。这样就形成了一个完美的闭环。

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UIView *testView = [[UIView alloc] init];

NSLog(@"%p", self);
NSLog(@"%s", class_getName([testView class] ));
NSLog(@"class is %s, superclass is %s", class_getName([self class]),
      class_getName([self superclass]));

Class currentClass = [self class];
for (int i = 0; i < 3; i++) {
    NSLog(@"Following the isa pointer %d times gives %p", i, currentClass);
    currentClass = objc_getClass((__bridge void *)currentClass);
}

控制台输出：

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0x7fa410c1f990
UIView
class is ViewController, superclass is UIViewController
Following the isa pointer 0 times gives 0x101c4e6c0
Following the isa pointer 1 times gives 0x0
Following the isa pointer 2 times gives 0x0

日历(NSCalendar)

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//日历
static const NSCalendarUnit CalendarUnitYMD = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay;
NSCalendar *calendar = [NSCalendar currentCalendar];
//计算当月第一天
NSDateComponents *component = [calendar components:CalendarUnitYMD fromDate:[NSDate date]];
[component setTimeZone:[NSTimeZone timeZoneWithName:@"UTC"]];
component.day = 1;
NSDate *currentDateFirstMonthDay = [calendar dateFromComponents:component];
NSLog(@"%@", currentDateFirstMonthDay);

//计算当月最后一天
component.month++;
component.day = 0;
NSLog(@"%@", [calendar dateFromComponents:component]);

//计算相对日期
NSDateComponents *offsetComponent = [[NSDateComponents alloc] init];
offsetComponent.day = 50;
NSLog(@"%@", [calendar dateByAddingComponents:offsetComponent toDate:currentDateFirstMonthDay options:0]);

//日期对应的星期
NSArray *weekDaySymbols = [[[NSDateFormatter alloc] init] shortWeekdaySymbols];
NSDateComponents *dateComponents = [calendar components:NSCalendarUnitWeekday fromDate:currentDateFirstMonthDay];
NSLog(@"%@", weekDaySymbols[dateComponents.weekday - 1]);

//每个月的周数，iOS8以下使用NSWeekCalendarUnit代替NSCalendarUnitWeekOfMonth
NSRange rangeOfweeks = [calendar rangeOfUnit:NSCalendarUnitWeekOfMonth inUnit:NSCalendarUnitMonth forDate:currentDateFirstMonthDay];
NSLog(@"%ld", (long)rangeOfweeks.length);

//计算日期间的天数，通过修改NSCalendarUnit可以间隔的月、年等等
NSLog(@"%ld", (long)[calendar components:NSCalendarUnitDay fromDate:currentDateFirstMonthDay toDate:[NSDate date] options:0].day);

控制台输出: 

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2016-01-26 23:11:27.038 Test123[6502:707948] 2016-01-01 00:00:00 +0000
2016-01-26 23:11:27.038 Test123[6502:707948] 2016-01-31 00:00:00 +0000
2016-01-26 23:11:27.038 Test123[6502:707948] 2016-02-20 00:00:00 +0000
2016-01-26 23:11:27.039 Test123[6502:707948] 周五
2016-01-26 23:11:27.039 Test123[6502:707948] 6
2016-01-26 23:11:27.040 Test123[6502:707948] 25