为什么在 iOS 8+ 中无法在文档文件夹中保存和加载 UIImages？

Question

我一直在尝试将 UIImages 保存并加载到我的应用程序的 Documents 文件夹或最好的临时文件夹中（因此每次我的应用程序关闭时，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，我想知道正确的方法是什么？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

在读取文件之前，请确保它存在于那里。如果确实存在，则问题出在[NSData dataWithContentsOfURL:url] 中，否则图像未转换为数据。

我建议将您的代码分成多个语句，而不是编写单个冗长的语句，以便更好地了解它的失败之处。

Answer 2

我创建了class，它允许保存在文件夹中

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];
    }
}

为了获取使用这个

[[FileUtility basePath] stringByAppendingPathComponent

编辑

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