斯坦福iOS7 2013-2014秋Assignment 6的一种答案 #4

这篇文章是针对斯坦福iOS7 2013-1014的公开课Assignment 6 Top Regions所进行的解答的第四部分。
The fact that the UIManagedDocument opens/creates asynchronously has ramifications for your entire application design since its NSManagedObjectContext may not be ready the instant your application’s UI appears. Design
your code as if it might take 10 seconds or more to open/create the document (meanwhile your UI is up, but empty, which is fine). It never will actually take that long, but your code should work if it did.
本节我们先只完成UIManagedDocument的打开和创建。
首先创建一个DocumentHelper的类，在.h中定义一个public API

+ (void)useDocumentWithOperation:(void (^)(UIManagedDocument *document, BOOL success))operation;

使用单例模式，确保只有一个对象，这也为后面的Extra Task  6做准备，防止由于UIManagedDocument被多线程访问导致程序崩溃

+ (DocumentHelper *)sharedDocumentHelper
{
static dispatch_once_t pred = 0;
__strong static DocumentHelper *_sharedDocumentHelper = nil;
dispatch_once(&pred, ^{
_sharedDocumentHelper = [[self alloc] init];
});
return _sharedDocumentHelper;
}

实现pulic API的类方法，需要传入公用文件的handler

+ (void)useDocumentWithOperation:(void (^)(UIManagedDocument *document, BOOL success))operation
{
DocumentHelper *dh = [DocumentHelper sharedDocumentHelper];
[dh useDocumentWithOperation:operation];
}

回忆一下课上的知识，UIManagedDocument是autoSave，所以我们不关心它是否会被保存，但我们必须确保只有一个进程在使用这个文件，并进行正常的确认documentState

#define DOCUMENT_NOT_READY_RETRY_TIMEOUT 1.0
- (void)useDocumentWithOperation:(void (^)(UIManagedDocument *document, BOOL success))operation
{
UIManagedDocument *document = self.document;
if ([self checkAndSetPreparingDocument]) {
dispatch_async(dispatch_get_main_queue(), ^{
[self performSelector:@selector(useDocumentWithOperation:)
withObject:operation afterDelay:DOCUMENT_NOT_READY_RETRY_TIMEOUT];
});
} else {
if (![[NSFileManager defaultManager] fileExistsAtPath:[document.fileURL path]]) {
[document saveToURL:document.fileURL
forSaveOperation:UIDocumentSaveForCreating
completionHandler:^(BOOL success) {
operation(document, success);
self.preparingDocument = NO;
}];
} else if (document.documentState == UIDocumentStateClosed) {
[document openWithCompletionHandler:^(BOOL success) {
operation(document, success);
self.preparingDocument = NO;
}];
} else {
BOOL success = YES;
operation(document, success);
self.preparingDocument = NO;
}
}
}

下面我们lasily instantiate这个documen属性
@property (nonatomic, strong) UIManagedDocument *document;

<pre name="code" class="objc">#define DATABASE_DOCUMENT_NAME @"TopRegions"
- (UIManagedDocument *)document
{
if (!_document) {
NSURL *url = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory
inDomains:NSUserDomainMask] lastObject];
url = [url URLByAppendingPathComponent:DATABASE_DOCUMENT_NAME];
_document = [[UIManagedDocument alloc] initWithFileURL:url];
}
return _document;
}

使用一个属性来确认进程是否处于活动状态，然后确保只有一个进程调用这个属性
@property (nonatomic) BOOL preparingDocument;

- (BOOL)checkAndSetPreparingDocument
{
static dispatch_queue_t queue;
if (!queue) {
queue = dispatch_queue_create("Flickr Helper Queue", NULL);
}
__block BOOL result = NO;
dispatch_sync(queue, ^{
if (!_preparingDocument) {
_preparingDocument = YES;
} else {
result = YES;
}
});
return result;
}

这一节没法运行程序，我们只是准备了文件……