需要帮助了解NSDictionary对象

Question

im trying to wrap my head around the NSDictionary class, to make my TableView functional. Is anyone able to break down and explain whats happening in the following code for me? cheers

if (!self.tableDataSource)
    {
        self.tableDataSource = [NSArray arrayWithContentsOfFile: [[NSBundle mainBundle] pathForResource: @"MenuData" ofType: @"plist"]];
        self.navigationItem.title = @"Menu";

    } 

NSDictionary *dictionary = [[[self.tableDataSource objectAtIndex: indexPath.section] objectForKey: @"Children"] objectAtIndex: indexPath.row];

Answer 1

You should set the DataSource and Delegate to point to the tableView's owner and then implement the UITableViewDelegate functions to correctly set the content of the table.

Edit: If you're planning on implementing a table view with sections, I advise you to use a MxN datastructure of: NSArray of NSArray, that way you will be able to access vey easily your cell data with:

cellData = [[datasource objectAtIndex:indexPath.section] objectAtIndex:indexPath.row];

Answer 2

I think the code itself is fairly clear if you just unpack all the method calls to see the implied nesting of various objects. Comments:

// if the current object's tableDataSource property has not been defined yet
if (!self.tableDataSource) {
    // then define it.

    // Set it to be an NSArray initialized by reading ...
    self.tableDataSource = [NSArray arrayWithContentsOfFile: 
    // ... the plist file MenuData.plist found within the application's bundle
                      [[NSBundle mainBundle] pathForResource: @"MenuData" ofType: @"plist"]];

    // and also set the current object's property nagitationItem.title to "Menu"
    self.navigationItem.title = @"Menu";

} 

// Now create a dictionary

// Get this dictionary by ...

// going to the element numbered indexPath.section in the tableDataSource
NSDictionary *dictionary = [[[self.tableDataSource objectAtIndex: indexPath.section] 
                  // (assume that element is an NSDictionary)
                  // get that element's value stored witht the key "Children"
                  objectForKey: @"Children"] 
                 // (assume that retrieved value is an NSArray)
                 // then get the element numbered indexPath.row from that array
                 objectAtIndex: indexPath.row];
                             // expect that value to be an NSDictionary

I think the Apple docs on TableDataSource will probably give you the larger context on what this code is doing. In particular, the nesting of a dictionary, in an array, in a dictionary, in an array, which is what's going on in the last line, may be a standard way of encoding all the info for rows, columns, etc., for data driving a UITableView.

Answer 3

NSDictionary objects store other objects referenced by keys. A key is a unique string used to reference the object in the dictionary. An object can be any object. An NSMutableDictionary is the same thing but allows editing (adding/removing/changing) objects for keys after the dictionary has been created.

It is incorrect to set an NSDictionary (or any class that does not support the UITableViewDataSource protocol) to the dataSource property of a table view. Most commonly, your view controller is a UITableViewController subclass or it is a UIViewController subclass that supports the UITableViewDelegate or UITableViewDataSource property.

That's the direct answer to your question. For a fuller understanding you should go back and read Apple's docs to make sure you understand my answer. The Table View Controller Programming Guide will show you how to implement a table views in a view controller. For other things you made need info on Objective-C and other iOS objects.