访问其他类/对象中的方法和变量（对象在哪里）？

Question

Ok, I am new to Objective-C/iPhone programming, so there are some questions arising when I try to do things that would be quite easy in C++.

I´m building a tab bar based iPhone application with three views, one for each tab bar button. In the first view the user builds a NSdictionary, which the second view shall display as a graph. To access this dictionary, I save it to a .plist in the first view controller, then bulding a new dictionary from this .plist in the second.

To make the graph view, i use s7graphview, which is initialized etc in SecondViewController, but has its own .h and .m files, which I import. The method to load values into the graph (which is from the created dictionary), is implemented in the GraphInfoList.m file, which means I have to make another dictionary from the .plist to access the data. How can I access the already created dictionary?

While doing this, I also made a method "dataFilePath", which returns the file path of the .plist, which I use to load the data into a dictionary. I have found no other way of implementing this method than to copy/paste it to every .m file that uses it! There´s got to be another way?

An while I´m at it: where are the objects in iPhone programming? The .m files is classes, aren´t they? I never create a new object using the new operator, and I thought I may would be able to access the methods if I had any object to call (like [FirstViewController dataFilePath] ).

I really don´t know how this is handled in Obj-C/Cocoa, and I don´t know what to search for to find the answers. Help would be really appreciated.

Answer 1

An object is an instance of a class (a .m file). You alloc/init a new object like you do in java with the new operator. you can import the .h of a class you want to use/have access to in and then use something like

MyClass *objectInstance = [[MyClass alloc] init]; 

to create an instance.

As far as the dataFilePath method... you can just have it in your app's delegate (probably something like MyAppNameAppDelegate.m) which is a singleton (one instance for the entire app). you can then get the file path using:

myFilePath = [[[UIApplication sharedApplication] delegate] dataFilePath];

I would suggest looking into a beginning objective-c book for more information. I'd suggest Learn Objective-C on the Mac . Also, I would take a look at a basic introduction to Object Oriented Programming, as it seems this is what is tripping you up more than anything. Good luck.

Answer 2

I had the same problem using s7graphview in a different view controller. I solved it by adding an NSDictionary as a property in the AppDelegate and when the viewwilldisappear method fires i added this code:

[((MyAppDelegate*) [[UIApplication sharedApplication] delegate]) setDictionary:self.dictionary];

I think it isn't the only way to do it but you will remove the .plist code who slows you app

Answer 3

Your app delegate is a good place to share things. you can keep the data file path function in your app delegate and access it

path = [[[UIApplication sharedApplication] delegate] dataFilePath];

Dont forget to #import "YourAppDelegate.h" file.

.m files are like .cpp files or .c files. Basically, implementation files.

Answer 4

Using a NSDictionary is a bad replacement for proper domain objects, you get problems when juggling your keys. And often objects are not the best way to model your data, for example numbers as NSNumber is just cumbersome.

Instead introduce a proper domain class. There is not problem in letting both via controllers in your tab bar have access to the same object, you can easily use the viewWillAppear: method to update states from one view to the other.

There might even be a time when you want a singleton. If there can be only one logical instance of any object, the singleton is the way to go. Don't fear them. Usually a lazy patters just as Apple use in their frameworks is what you want. With for example this interface:

@interface MyDomainManager : NSObject { /* ivars here */ }

+(MyDomainManager*)sharedManager;

// More proper tie and method here    

@end

And then an implementation like this:

@implementation MyDomainManager

+(MyDomainManager*)sharedManager;
{
    static MyDomainManager* manager = nil;
    if (manager == nil) {
        manager = [[self alloc] init];
    }
    return manager;
}

// More implementation cruft…

@end

Starting out simple as this is a good start, and then build from there.