如何把Target/Action放進array裡面呢?

Target-Action相信這個design pattern對於寫ios一陣子的人都不陌生
這是幾個ios fundamental design pattern之一
然而有時候我們會希望把幾個target action放進array或是dictionary之中，或是直接當作member variable
但是SEL並不是一般繼承於NSObject的class，所以不能直接放進Array當中
當然我們可以用
NSStringFromSelector(SEL aSelector)
以及
NSSelectorFromString(NSString *aSelectorName)
讓SEL可以跟NSString*互換
但是怎麼都覺得不是很方便
我就在想有沒有比較漂亮的方法

我找了一下iOS foundation library，當中就屬NSInvocation最合適
這個Class就是把target, action, arguments包在一起，
當我們呼叫-[NSInvocation invoke]
就會呼叫這個target的selector，並且傳進這些arguments...
太棒了，這就是我要的class!!

但是NSInvocation的產生方式實在不怎麼好用，如果我們要把這些東西全部丟進去
那我們需要如下面這段code

NSMethodSignature* sig = 
    [[target class] instanceMethodSignatureForSelector:@selector(mySelector:];
NSInvocation* invocation = [NSInvocation invocationWithMethodSignature:sig];
[invocation setTarget:target];
[invocation setSelector:selector];

看起來很不友善吧...

不好用的話，那我們就來自己動手做Category吧...

NSInvocation+PopcornyLu.h / NSInvocation+PopcornyLu.m

	#import <Foundation/Foundation.h>
	@interface NSInvocation (PopcornyLu)
	+ (NSInvocation*) invocationWithTarget:(id)target
	selector:(SEL)selector;
	+ (NSInvocation*) invocationWithTarget:(id)target
	selector:(SEL)selector
	arguments:(id)firstArg, ... NS_REQUIRES_NIL_TERMINATION;
	@end

view raw NSInvocation+PopcornyLu.h hosted with ❤ by GitHub

	#import "NSInvocation+PopcornyLu.h"
	@implementation NSInvocation (PopcornyLu)
	+ (NSInvocation*) invocationWithTarget:(id)target
	selector:(SEL)selector
	{
	NSMethodSignature* sig = [[target class] instanceMethodSignatureForSelector:selector];
	NSInvocation* invocation = [NSInvocation invocationWithMethodSignature:sig];
	[invocation setTarget:target];
	[invocation setSelector:selector];
	return invocation;
	}
	+ (NSInvocation*) invocationWithTarget:(id)target
	selector:(SEL)selector
	arguments:(id)firstArg, ...
	{
	NSMethodSignature* sig = [[target class] instanceMethodSignatureForSelector:selector];
	NSInvocation* invocation = [NSInvocation invocationWithMethodSignature:sig];
	[invocation setTarget:target];
	[invocation setSelector:selector];
	va_list args;
	id arg;
	va_start(args, firstArg);
	NSUInteger index = 2;
	for(arg = firstArg;
	arg != nil;
	arg = va_arg(args, id))
	{
	[invocation setArgument:&arg atIndex:index++];
	}
	va_end(args);
	return invocation;
	}
	@end

view raw NSInvocation+PopcornyLu.m hosted with ❤ by GitHub

這邊我提供了兩個Class messages來產生NSInvocation
其中第二個可以提供不定量的arguments.
使用上很簡單

// Put invocations in array
NSArray* myActions =
    [NSArray arrayWithObjects:
        [NSInvocation invocationWithTarget:self
                                  selector:@selector(action1)],
        [NSInvocation invocationWithTarget:self
                                  selector:@selector(action2:)
                                 arguments:@"foo", nil],
        [NSInvocation invocationWithTarget:self
                                  selector:@selector(action3WithFoo:andBar:)
                                 arguments:@"foo", @"bar", nil],
        nil];

// invoke all the actions
for(NSInvocation* invocation in myActions)
{
    [invocation invoke];
}

[補充]
當然也可以用block來取代target/action
以上面的例子我們可以用block來實作

NSArray* myActions2 = 
 [NSArray arrayWithObjects:
  
 [[^(void){
  [self action1];
 } copy] autorelease],
  
 [[^(void){
  [self action2:@"foo"];
 } copy] autorelease],         
  
 [[^(void){
  [self action3WithFoo:@"foo" andBar:@"bar"];
 } copy] autorelease],            
  
 nil];        

// invoke all the actions
for(void(^invocation)() in myActions2)
{
    invocation();
}

但是缺點是code比較凌亂一點，然後action1, action2:, action3WithFoo:withBar這些message都要在前面要先定義
稍微麻煩了點
但基本上Target/Action或是Block都有其方便之處
像這邊我希望array creation的地方不要寫太多code，所以我會選擇用target/action
但有時候使用上會用inline block比較方便，那我就用block