无论做什么，衡量标准是有没有给自己长本事
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pannel (noun)
piece of something ,a flat piece of wood with straight sides,which forms part of a door
a stained glass panel
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设置tabbar样式的时候，设置的是tabbar controllers子控制器的tabbarItem属性，可以是普通VC，也可以是navigation controller
每一个普通view controller初始化的时候都会初始化一个 tabbar item对象，但是属性是空的
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NSObject initialize
initialize the class before it receives its first message
+ (void)initialize;
- the runtime sends initialize to each class in a program just before the class is sent its first message from within the program,super class receive this message before their subclasses.
- initialize is invoked only once per class.if you want to perform independent initialization for the class,you should implement LOAD method.
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设置导航栏的返回按钮
用外观类方法把bar对象拿出来，设置bar的外观，bar有一个backindicator属性，两个image都设置以后，就可以把导航栏的返回按钮初始化好
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用masonry平分布局的思想
view1的上左下等于容器，view2的上下，宽度等于view1，view2的左边等于view1的右边，以此类推，lastView的右边等于容器的左边，这样，masonry就会自动平分view的宽度
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为什么block里面不建议使用下划线变量 _property
因为 _property 等于 self -> _property,隐式表达了self，容易造成循环引用
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设置tabbar 文字的颜色在当前控制器的tabbarItem上进行设置
[viewcontroller.tabBarItem setTitleTextAttributes:@{NSForegroundColorAttributeName:[UIColor whiteColor]} forState:UIControlStateNormal];
