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如何在Objective-C中实现链式语法? #4

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dabing1022 opened this issue Jan 2, 2015 · 4 comments

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@dabing1022
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commented Jan 2, 2015

在接触到开源项目 Masonry 后,里面的布局约束的链式写法让我颇感兴趣,就像下面这样

UIEdgeInsets padding = UIEdgeInsetsMake(10, 10, 10, 10);

[view1 mas_makeConstraints:^(MASConstraintMaker *make) {
    make.top.equalTo(superview.mas_top).with.offset(padding.top); //with is an optional semantic filler
    make.left.equalTo(superview.mas_left).with.offset(padding.left);
    make.bottom.equalTo(superview.mas_bottom).with.offset(-padding.bottom);
    make.right.equalTo(superview.mas_right).with.offset(-padding.right);
}];

其他语言比如 Lua, 实现链式语法很容易。但在 Objective-C 中,如何实现链式语法呢?

注:这里讨论的链式语法特指的是点链式语法,不同于中括号链式语法,如[[[[someObj method1] method2] method3] method4:someParam]。中括号链式语法相对而言更简单些,每个方法的返回值是下一个方法的发送者即可。

查看 Masonry 源码,起初没看明白,于是搜索了下 Stackoverflow,没有发现类似的问题,便将这个问题发布在了 Stackoverflow 上。这里是地址。

总结了下,贴下代码,做个说明。

//------------------------------------------
@interface ClassB : NSObject
@property(nonatomic, readonly) ClassB *(^ddd)(BOOL enable);

- (id)initWithString:(NSString *)str;

@end

@implementation ClassB

- (ClassB *(^)(BOOL))ddd
{
    return ^(BOOL enable) {
        //code
        if (enable) {
            NSLog(@"ClassB yes");
        } else {
            NSLog(@"ClassB no");
        }
        return self;
    };
}

@end



@class ClassB;
@interface ClassA : NSObject

// 1. 定义一些 block 属性
@property(nonatomic, readonly) ClassA *(^aaa)(BOOL enable);
@property(nonatomic, readonly) ClassA *(^bbb)(NSString* str);
@property(nonatomic, readonly) ClassB *(^ccc)(NSString* str);

@end

@implementation ClassA

// 2. 实现这些 block 方法,block 返回值类型很关键,影响着下一个链式
- (ClassA *(^)(BOOL))aaa
{
    return ^(BOOL enable) {
        //code
        if (enable) {
            NSLog(@"ClassA yes");
        } else {
            NSLog(@"ClassA no");
        }
        return self;
    };
}

- (ClassA *(^)(NSString *))bbb
{
    return ^(NSString *str) {
        //code
        NSLog(@"%@", str);
        return self;
    };
}


// 这里返回了ClassB的一个实例,于是后面就可以继续链式 ClassB 的 block 方法
// 见下面例子 .ccc(@"Objective-C").ddd(NO)
- (ClassB * (^)(NSString *))ccc
{
    return ^(NSString *str) {
        //code
        NSLog(@"%@", str);
        ClassB* b = [[ClassB alloc] initWithString:str];
        return b;
    };
}

@end


// 最后我们可以这样做
id a = [ClassA new];
a.aaa(YES).bbb(@"HelloWorld!").ccc(@"Objective-C").ddd(NO);
@veryitman

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commented Sep 4, 2015

你的代码能编译通过吗?你有没有试过啊?
returning block that lives on the local....

@veryitman

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commented Sep 4, 2015

你的代码的确是编译有问题(少了结束的分号), 但是在 ios 工程里面就没有returning block that lives on the local....这个错误,何解?

@dabing1022

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commented Sep 6, 2015

当时这段代码写的时候没有用xCode,没有编译,但其核心思想是没有问题的。重新在xCode上整理了下,有些分号加上了。注意最后

id a = [ClassA new];
a.aaa(YES).bbb(@"HelloWorld!").ccc(@"Objective-C").ddd(NO);

代码的位置,显然要放在main()函数里面或者自己新建一个类。

@dabing1022 dabing1022 closed this Sep 6, 2015

@dabing1022 dabing1022 reopened this Sep 6, 2015

@veryitman

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commented Oct 10, 2015

ok

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