为什么在iOS 8+中保存和加载文档文件夹中的UIImages无法正常工作？

Question

我一直在尝试在我的应用程序的Documents文件夹或最好是临时文件夹中保存和加载UIImages（所以每次我的应用程序关闭图像都会被删除）但它们无法正常工作。我尝试了三种方法，但只有第三种方法有效，我想知道什么是正确的方法呢？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

在阅读文件之前，请确保它存在。如果它确实存在，则问题出在[NSData dataWithContentsOfURL:url]，否则图像不会转换为数据。

我建议，将代码分成几个语句，而不是编写单个冗长的语句，以便更好地理解它的失败位置。

Answer 2

我创建了class，允许保存在文档文件夹

中

这是.h文件

@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];
    }
}

对于fetch，请使用此

[[FileUtility basePath] stringByAppendingPathComponent

修改

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