工廠方法模式：定義創(chuàng)建對象的接口，讓子類決定實列化哪一個類，工廠方法使得一個類的實例化延遲到其子類。

• 工廠方法也稱虛構造器，當一個類無法預期需要生成哪個類的對象，想讓其子類來指定所生成的對象，這時就會用到工廠模式，可能這樣說你還不理解，沒事，會在后面附上代碼，一步步帶你理解何謂工廠方法

何時使用工廠方法

• 編譯時無法確定預期要創(chuàng)建的對象的類型
• 類想讓其子類決定在運行期間創(chuàng)建哪種類型的對象
• 類有若干個輔助類為其子類，而你想將返回哪個子類這一信息局部化

代碼演示及講解

這里我們用一個最簡單創(chuàng)建不同類型的畫布的例子來演示工廠模式的實現(xiàn)方式

• 首先定義其父類，我們把他命名為CanvasView（意思為畫布）,你可以在里面實現(xiàn)畫布的默認樣式，比如我們想畫布默認為黑色

//.h里面可以定義一些屬性和方法，這個取決于具體需求
#import <UIKit/UIKit.h>

@interface CanvasView : UIView
{}
...
@end


//.m文件就實現(xiàn)畫布的默認樣式
#import "CanvasView.h"
@implementation CanvasView
-(instancetype)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
       self.backgroundColor = [UIColor blackColor];
    }
        return self;
}
@end

*第二步，定義生成器，用來生成畫布對象,這個生成器就生成了一張默認為黑色的畫布

#import <Foundation/Foundation.h>
#import "CanvasView.h"

@interface CanvasViewGenerator : NSObject

-(CanvasView *)canvasViewWithFrame:(CGRect)frame;

@end


#import "CanvasViewGenerator.h"

@implementation CanvasViewGenerator

-(CanvasView *)canvasViewWithFrame:(CGRect)frame
{
    return [[CanvasView alloc] initWithFrame:frame];
}

@end

*下面我們開始定義不同類型的畫布對象及其生成器，我們把他們定義為紅色和黃色兩種類型，分別命名為RedCanvasView，RedCanvasGenerator和YellowCanvasView，YellowCanvasGenerator,其中RedCanvasView和YellowCanvasView繼承于CanvasView，RedCanvasGenerator和YellowCanvasGenerator繼承于CanvasGenerator

//--------------------------紅色畫布代碼--------------------------
#import "CanvasView.h"
@interface RedCanvasView : CanvasView
@end

@implementation RedCanvasView
-(instancetype)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
        self.backgroundColor = [UIColor redColor];
    }
    return self;
}
@end

#import <Foundation/Foundation.h>
#import "CanvasViewGenerator.h"
#import "RedCanvasView.h"
@interface RedCanvasViewGenerrator :CanvasViewGenerator
@end

#import "RedCanvasViewGenerrator.h"
@implementation RedCanvasViewGenerrator
-(CanvasView *)canvasViewWithFrame:(CGRect)frame
{
    return [[RedCanvasView alloc] initWithFrame:frame];
}
@end

//--------------------------黃色畫布代碼--------------------------
#import "CanvasView.h"
@interface YellowCanvasView : CanvasView
@end

#import "YellowCanvasView.h"
@implementation YellowCanvasView
-(instancetype)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
        self.backgroundColor = [UIColor YellowColor];
    }
    return self;
}
@end


#import <Foundation/Foundation.h>
#import "CanvasViewGenerator.h"
#import "YellowCanvasView.h"
@interface YellowCanvasViewGenerrator : CanvasViewGenerator
@end


#import "YellowCanvasViewGenerrator.h"
@implementation YellowCanvasViewGenerrator
-(CanvasView *)canvasViewWithFrame:(CGRect)frame
{
    return [[YellowCanvasView alloc] initWithFrame:frame];
}
@end

*畫布使用

#import "ViewController.h"
#import "RedCanvasViewGenerrator.h"
#import "YellowCanvasViewGenerrator.h"

@interface ViewController ()
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    RedCanvasViewGenerrator  *defaultGenerator = [[RedCanvasViewGenerrator alloc] init];
    [self loadCanvasViewWithGenerator:defaultGenerator];
}



-(void)loadCanvasViewWithGenerator:(CanvasViewGenerator *)generator
{
    [_canvasView removeFromSuperview];
    CGRect frame = CGRectMake(0, 0, self.view.bounds.size.width, self.view.bounds.size.height);
    CanvasView *canvasView = [generator canvasViewWithFrame:frame];
    self.canvasView = canvasView;
    [self.view addSubview:self.canvasView];
    
}

在控制器中，我們使用RedCanvasViewGenerrator *defaultGenerator = [[RedCanvasViewGenerrator alloc] init];和
[self loadCanvasViewWithGenerator:defaultGenerator];這兩個方法來創(chuàng)建了一個紅色畫布。

在下一篇中，我們將繼續(xù)討論抽象工廠模式，人們經(jīng)常容易將這兩個模式混淆