Tag: alassetslibrary tutorial

#import <UIKit/UIKit.h>;
#import <AssetsLibrary/AssetsLibrary.h>;

@interface MyImagePickerViewController : UIViewController {
 
	IBOutlet UITableView *tableview;
	IBOutlet UIActivityIndicatorView *activity;
 
	NSMutableArray *assets;
}
 
@property (nonatomic, retain) IBOutlet UITableView *tableview;
@property (nonatomic, retain) IBOutlet UIActivityIndicatorView *activity;
 
@end

We are going be using the assets array here to hold the assets that we are going to pull using the AssetsLibrary Framework.

Now open up MyImagePickerViewController.xib and drag in a UITableView and a UIActivityIndicatorView. With these in place connect them to the IBOutlets you created for this class. Also make sure to connect the UITableViewDataSource and UITableViewDelegate to the MyImagePickerViewController as well. With that done we can start to use the AssetsLibrary Framework.

Using the Asset Library

Now we are going to use our AssetsLibrary framework. This will be done within the viewDidLoad method of your MyImagePickerViewController.m. Put the following code in there.

1 - (void)viewDidLoad {
2
3    [super viewDidLoad];
4    [activity startAnimating];
5
6    void (^assetEnumerator)(struct ALAsset *, NSUInteger, BOOL *) = ^(ALAsset *result, NSUInteger index, BOOL *stop) {
7	if(result != NULL) {
8		NSLog(@"See Asset: %@", result);
9		[assets addObject:result];
10
11	}
12    };
13
14    void (^assetGroupEnumerator)(struct ALAssetsGroup *, BOOL *) =  ^(ALAssetsGroup *group, BOOL *stop) {
15	if(group != nil) {
16		[group enumerateAssetsUsingBlock:assetEnumerator];
17	}
18
19
20	[self.tableView reloadData];
21	[self.activity stopAnimating];
22	[self.activity setHidden:YES];
23    };
24
25    assets = [[NSMutableArray alloc] init];
26    library = [[ALAssetsLibrary alloc] init];
27    [library enumerateGroupsWithTypes:ALAssetsGroupAlbum
28					   usingBlock:assetGroupEnumerator
29					 failureBlock: ^(NSError *error) {
30						 NSLog(@"Failure");
31					 }];
32 }

We are going to be going though this code line by line. On line 4, we start animating our activity indicator. I will explain why we have to do that a little bit later. Lines 6 – 23 are the lines that will  declare the two blocks that we will use to fill our tableview with pictures. We are going to declare two blocks to use as parameters to methods within a ALAssetLibrary object and a ALAssetGroup object. The first method call we will make will be on our ALAssetLibrary object. So for the moment lets skip passed the block declarations and look at line 25. On line 25 we instantiate our NSMutableArray to hold the ALAssets that we will pull out from the AssetsLibrary. On line 26 we will create our ALAssetsLibrary object. We will call a single method on this object to loop through all of the assets in a library. The method is:

– enumerateGroupsWithTypes:usingBlock:failureBlock:

This method takes in 3 parameters. An ALAssetGroup type, a block to be performed on each group and a block to be performed on failure. Lets talk about block syntax. A block is a native Objective C object. It is a subclass of NSObject. With that said it is also an Objective C method. An objective C method is composed of three things. A return type, a parameter list and a piece of code to be executed. We are going to pass the assetsGroupEnumerator block object that declare on line 14 into this method. Lets take a look at how the block is declared.

You begin by specifying a return type for the block. Void in this case. Next you enclose a name for the block which is proceeded by a carrot ^ character. This is the special character Apple has decided on to denote blocks. You begin block names with blocks as well as the actual declaration of what a block is. This name is wrapped in parenthesis. This is followed by the parameter list that the block will accept. In this case it is a ALAssetsGroup object and a Boolean indicating whether to stop. Now I did not come up with these parameters by myself. Since I will be passing this block into an ASAssetLibrary the documentation will tell us what will be passed into the block. ALAssetsLibrary objects documentation can be seen here. With that all said, lets take a look at the code that will actually compose this block. Starting on line 15 we will make sure that we are seeing a valid group. If the ALAssetsGroup object that we are passed is valid then we will call another enumeration on the group which will enumerate through assets. This enumeration method also requires a block. We declared this block above. This block is called assetEnumerator. Once again we retrieved the parameters list from the documentation of the object asking for, in this case an ALAssetsGroup.

For this block, which begins on line 6, the block is passed an ALAsset object, an index and a stopping condition boolean. Within the code of the block we will ensure that the returned ALAsset is valid and if it is we will add it to our array which will hold all of our assets. With this block declared we pass it into the call to our ALAssetsLibrary object on line 16.  Once every group has been enumerated and our array is filled with all of our ALAssets we will tell the tableview to reload its data and get rid of our UIActivityIndicatorView.

With all of this done all that remains if filling in the required UITableViewDataSource methods. The tableview will be filled with the assets we collected. ALAsset objects are cool in that they include a method called -thumbnail which returns a CGImageRef to a thumbnail we will use. I won’t explain the development of these methods any further since they are pretty straight forward.

// Customize the number of sections in the table view.
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    return 1;
}
 
// Customize the number of rows in the table view.
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    return [assets count];
}
 
// Customize the appearance of table view cells.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
 
    static NSString *CellIdentifier = @"Cell";
 
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
    }
 
	ALAsset *asset = [assets objectAtIndex:indexPath.row];
	[cell.imageView setImage:[UIImage imageWithCGImage:[asset thumbnail]]];
	[cell.textLabel setText:[NSString stringWithFormat:@"Photo %d", indexPath.row+1]];
 
    return cell;
}