Migrate mac installer to delegate-driven model

chrome/installer/mac/app/MainDelegate.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_INSTALLER_MAC_APP_MAINDELEGATE_H_
+#define CHROME_INSTALLER_MAC_APP_MAINDELEGATE_H_
+
+#import <Cocoa/Cocoa.h>
+
+@protocol OmahaDelegate
+- (void)talkToOmaha;
+@end

[Sidney San Martín, 2016/07/19 23:19:20]

main.m knows that it's dealing with an instance of MainDelegate, so
`talkToOmaha` can be a regular method. You don't need this protocol.

+
+@protocol DownloadDelegate
+- (void)downloadChromeWithData:(NSData*)data;
+@end

[Sidney San Martín, 2016/07/19 23:19:20]

In an app like this, delegate protocols are meant to help with separation of
concerns. MainDelegate needs to know about OmahaCommunication, but
OmahaCommunication and the other helper classes don't need to know about
MainDelegate (and shouldn't import MainDelegate.h).

Instead of this DownloadDelegate protocol, you might want one like this in
OmahaCommunication.h:

    @protocol OmahaCommunicationDelegate
    - (void)omahaRequestFinishedWithData:(NSData *);
    @end

It makes moving responsibility around easier: if you ever want a different class
to be an OmahaCommunicationDelegate, or you want something other than
downloading Chrome to happen after you get the response from Omaha, you don't
have to change OmahaCommunication.{h,m} at all.

It also makes control flow in MainDelegate clearer: It calls
`-[OmahaCommunication sendRequest]`, then later it gets called back with
`omahaRequestFinishedWithData:`. (You don't have to remember that `sendRequest`
causes `downloadChromeWithData:` to be called).

[Anna Zeng, 2016/07/21 18:06:44]

Let's talk about this in real time too!

+
+@protocol UnpackDelegate
+- (void)unpackDMG;
+@end

[Sidney San Martín, 2016/07/19 23:19:20]

Same thing here.

+
+@interface MainDelegate
+    : NSObject<OmahaDelegate, DownloadDelegate, UnpackDelegate>
+
+@end
+
+#endif  // CHROME_INSTALLER_MAC_APP_MAINDELEGATE_H_