为什么在iOS 8+中无法正常保存和加载UIImages文档文件夹？

Question

我一直试图保存和加载UIImages在我的应用程序的文档文件夹或最好是临时文件夹（所以每次我的应用程序被关闭的图像将被删除），但他们不能正常工作。我已经尝试了三种方法，但只有第三种方法有效，我想知道什么是正确的方法？我非常感谢任何帮助。

//First Approach, to save and load later, Doesn't work. 
//Saving the image 
NSURL *documentsDirectoryURL = [[[NSFileManager defaultManager] 
            URLsForDirectory:NSDocumentDirectory 
            inDomains:NSUserDomainMask] 
            lastObject]; 
NSString*fileDirectoryStr = [[documentsDirectoryURL path] stringByAppendingPathComponent:@"E1BG.png"]; 
NSURL *url = [NSURL URLWithString:fileDirectoryStr]; 
//Saving either as Jpeg 
[UIImagePNGRepresentation(finalFG) writeToFile:fileDirectoryStr atomically:YES]; 
//Or as PNG, because I need the transparent part to be kept transparent but non of them work. 
[UIImageJPEGRepresentation(finalFG, 1.0) writeToFile:fileDirectoryStr atomically:YES]; 
//Loading image 
UIImage *loadedBGImg = [UIImage imageWithData: [NSData dataWithContentsOfURL:url]]; 
//Returns nil 

//Second approach. 
//Saving the image 
NSArray*pathArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES); 
NSString * documentsPath = [pathArray objectAtIndex:0]; 
NSString* fileDirectoryStr = [documentsPath stringByAppendingPathComponent:@"E1FG.png"]; 
//Again saving either as Jpeg 
[UIImagePNGRepresentation(finalFG) writeToFile:fileDirectoryStr atomically:YES]; 
//Or as PNG, because I need the transparent part to be kept transparent but non of them work. 
[UIImageJPEGRepresentation(finalFG, 1.0) writeToFile:fileDirectoryStr atomically:YES]; 
NSURL *url1 = [NSURL URLWithString:fileDirectoryStr]; 
//Loading image 
UIImage *loadedImg = [UIImage imageWithData: [NSData dataWithContentsOfURL:url1]]; 

//Third approach, Works but doesnt seem to be very efficient. 
NSString *tmpDirectory = NSTemporaryDirectory(); 
//Save 
NSString *tmpFileStrFG = [tmpDirectory stringByAppendingPathComponent:@"E1FG.png"]; 
NSURL *urlFG = [NSURL URLWithString:tmpFileStrFG]; 
NSData *imgDataFG = UIImagePNGRepresentation(finalFG); 
[imgDataFG writeToURL:urlFG atomically:YES]; 
//Load 
UIImage *ImggFG = [UIImage imageWithData: imgDataFG]; 

Answer 1

我创建class允许在文档文件夹中保存

这里是.h文件中

@interface FileUtility : NSObject { 

} 

// Folder methods 
+ (NSString*)basePath; 
+ (void)createFile:(NSString *)filename; 
+ (void)createFile:(NSString *)filePath withData: (NSData *) imgData; 

@end 

和.m文件

#import "FileUtility.h" 

@implementation FileUtility 

// ---------------------------------------------------------------------------- 

#pragma mark - 
#pragma mark Path methods 

// ----------------------------------------------------------------------------- 

+ (NSString*)basePath { 
    NSArray* paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES); 
    NSAssert([paths count] == 1, @""); 
    return [paths objectAtIndex:0];  
} 

// ----------------------------------------------------------------------------- 




    + (void)createFile:(NSString *)filename { 
    NSFileManager* manager = [NSFileManager defaultManager]; 
    NSString * filePath = [NSString stringWithFormat:@"%@/%@", [FileUtility basePath],filename]; 
    DLOG(@"filePath = %@", filePath); 
    if (![FileUtility fileExists:filePath]){ 
     [manager createFileAtPath:filePath contents:nil attributes:nil]; 
    } 

    } 

// ----------------------------------------------------------------------------- 

+ (void)createFile:(NSString *)filePath withData: (NSData *) imgData { 
    NSFileManager* manager = [NSFileManager defaultManager]; 
    DLOG(@"filePath = %@", filePath); 
    if (![FileUtility fileExists:filePath]){ 
     [manager createFileAtPath:filePath contents:imgData attributes:nil]; 
    } 
} 

为了获取使用此

[[FileUtility basePath] stringByAppendingPathComponent

编辑

+ (BOOL)fileExists:(NSString *)filename { 
    NSFileManager* manager = [NSFileManager defaultManager]; 
    return [manager fileExistsAtPath:filename]; 
} 

Answer 2

在读取文件之前，请确保它在那里存在。如果确实存在，则问题是[NSData dataWithContentsOfURL:url]其他图像未被转换为数据。

我会推荐你​​将代码分解成几条语句，而不是写一条冗长的语句，以便更好地理解它失败的地方。