iOS：已经显示邮件撰写视图时，如何在后台创建/准备自定义电子邮件附件？

Question

I'd love to share custom content of my iOS 6.0 app via email.

But since my content takes some time to be prepared before I can share it, I can't manage to show the mail compose view immediately when the user presses the 'share' button. Unfortunately the MFMailComposeViewController needs all attachments right from the start, so I have to wait for the attachment before I can init and show the MFMailComposeViewController (like it's suggested here: UIActivityView attach file to Email )

-- Edit after reading Sapan's answer: What I'm looking for is a behavior like UIActivityViewController when sharing eg a video file that yet has to be created: the user presses the share button, and immediately the UIActivityController shows up. The attachment is created in the background and creation does not interrupt the user's experience. --

I tried UIActivityViewController with it's nice asynchronous UIActivityItemProvider feature, but unfortunately this only seems to work for attachments with public mime types like images and video. My custom NSData 'item' simply gets ignored not only for sharing on facebook (which makes perfect sense) but also for email sharing. Or am I doing something wrong here:

MyUIActivityItemProvider *myCustomDocProvider = [[MyUIActivityItemProvider alloc] initWithPlaceholderItem:[[NSData alloc]init] andCustomInfoToCreateNSDataFrom:customAppInfo];
UIActivityViewController *activityVC = [[UIActivityViewController alloc]initWithActivityItems:@[myTextString, myCustomDocProvider] applicationActivities:nil];
[self presentViewController:activityVC animated:YES completion:nil];

I'm already thinking about creating my own custom email composer view, so that I can create the attachment in the background while/after the user is typing. But I don't want to. I like the idea of 'consistency of UI elements' I read about in the HIG.

What can I do?

Answer 1

You can create a method as follows and then invoke it using [self performSelectorInBackground:@selector(sendMail)];

-(void) sendMail
{
    // Prepare the data

    // Attach the data
    MFMailComposeViewController *picker = [[MFMailComposeViewController alloc] init];
    [picker addAttachmentData:myData mimeType:@"<YOUR-MIME-TYPE>" fileName:@"Filename"];

    // Show compose view controller on the main thread
    dispatch_async(dispatch_get_main_queue(), ^(void) {
        // Fill out the email body text
        NSString *emailBody = @"Body";
        [picker setMessageBody:emailBody isHTML:NO];
        [self presentModalViewController:picker animated:YES];
    });
}

Answer 2

What you can do is set your mail body to HTML with images or media referenced from a remote server. Then the user will see a broken image in the mail compose view (perhaps you can hide this using some clever HTML) but at the time the mail is received the upload should be done.

Not optimal because it can fail after sending the email but probably your best bet.

Answer 3

This can be done by using a boolean in your myCustomDocProvider to hold the return of the item until the building of the file is complete. If you want to show progress or have an activity indicator show on your main view, you can do this by calling it from the main thread. Here is a code snippet of this in action:

Set the Activity View Controller in your provider with this function in order to load a progress view.

self.parentViewController = parentVc;

Here is the item function with blocking and feedback view load.

self.wait = true;

[self prepareFile:^(){
    [self performSelectorOnMainThread:@selector(dismissProgressView) 
                              withObject:nil waitUntilDone:NO];
}];

[self performSelectorOnMainThread:@selector(loadProgressView) 
                              withObject:nil waitUntilDone:NO];

while (self.wait) {
    [self performSelectorOnMainThread:@selector(updateProgressView) 
                                     withObject:nil waitUntilDone:NO];
}

return self.completedUrl;

You can then create the three functions that are called on main thread to show a progress view, update the progress view while waiting for completion of the file. In the implementation of the dismissProgressView function, make sure to set the wait boolean to false when dismissViewController is complete.