iOS的初始化

自定义类定义接口时应该定义一个全能初始化方法，所用到的传入参数用只读属性，默认的init初始化可以通过调用全能初始化设置默认参数或抛出异常处理。
父类:

#import "TestRectangle.h"

@interface TestRectangle : NSObject
@property (nonatomic, assign, readonly) CGFloat width;
@property (nonatomic, assign, readonly) CGFloat height;

- (instancetype)initWithWidth:(CGFloat)width height:(CGFloat)height;


@end

@implementation TestRectangle

- (instancetype)initWithWidth:(CGFloat)width height:(CGFloat)height {
    if (self = [super init]) {
        _width = width;
        _height = height;
    }
    return self;
}

//- (instancetype)init {
  //  @throw [NSException exceptionWithName:NSInternalInconsistencyException reason:@"Must use xxx instead" userInfo:nil];
//}
- (instancetype)init {
       return  [self initWithWidth:5.0 height:10.0];
}

@end

子类:

#import "TestSquare.h"

@interface TestSquare : TestRectangle
- (instancetype)initWithDimension:(CGFloat)dimension;

@end

@implementation TestSquare
- (instancetype)initWithDimension:(CGFloat)dimension {
    return [super initWithWidth:dimension height:dimension];
}

- (instancetype)initWithWidth:(CGFloat)width height:(CGFloat)height {
    CGFloat dimension = MAX(width, height);
    return [self initWithDimension:dimension];
}
@end

这里要注意子类的复写，子类通过复写父类的全能初始化方法，可以覆盖到默认的init方法。想想子类实例的调用过程：init ->父类init->子类的initWithWidth:height:方法->initWithDimension:方法。
所以，子类复写时一定要复写父类的全能初始化方法。