Discovery, location and solution of a BUG

Preface

After the release of iOS 11, a series of adaptation-related issues have emerged, such as the insensitive sliding gesture of UIScrollView when pagingEnabled=YES, the sliding deletion function of UITableView changed, and the click area of ​​the cancel button of UIImagePickerViewController became smaller. This article introduces one of the UIAlertView issues and shares its discovery, location, and solution.

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1. Problem

The problem first occurred shortly after the official release of iOS 11. A tester reported an iOS 11-related bug. When sending a chat message in the live broadcast room, if the user is banned, a "banned" prompt will pop up, the keyboard will retract, and then it will not pop up again.

When the developer received this BUG, ​​he first abstracted the problem into several elements: inside the live broadcast room, keyboard pop-up, pop-up prompt, keyboard retracted, and keyboard unable to pop up.

The pop-up prompt uses the UIAlertView method. When the keyboard appears, the UIAlertView prompt pops up, and the keyboard will be retracted. After the UIAlertView disappears, the keyboard will pop up again, which is a normal performance.

2. Problem reproduction

Make an attempt according to the recurrence path and find that the BUG can be reproduced, confirming that the problem exists;

• Based on experience, I guess the problem may be with the keyboard and UIAlertView, and has nothing to do with the "mute" business.
• Try other non-"mute" scenarios in the live broadcast room. When the keyboard appears, the UIAlertView prompt pops up, which will also cause the subsequent keyboard to fail to pop up.

After trying other main scenes other than the live broadcast room, I found that the problem can be described as:

• On iOS 11 machines, once the UIAlertView pops up, the keyboard cannot be called up through becomeFirstResponder afterwards; you must manually click the input area to trigger the system's keyboard pop-up behavior, or switch to the background and then switch back before the keyboard can pop up normally.
• After clicking on a comment on some pages, a transparent maskView is added and the keyboard pops up. Clicking on the transparent maskView will call resignFirstResponder, and the maskView will be eliminated in the notification of keyboard disappearance. Because the keyboard cannot be popped up (nor can the notification of keyboard disappearance be received, but the maskView is still added normally), subsequent interactions on these pages cannot be performed.

3. Problem Assessment

• After reproducing the problem, you need to assess the severity of the problem and determine the priority of bug fixing.
• Judging from the known performance, the usage impact under iOS 11 is greater (UIAlertView has more prompts).
• I downloaded the external version on a machine running iOS 11 and tested it, but found that the BUG could not be reproduced!
• Although it is strange, the priority of the problem can be lowered and included in the normal bug solving list.

4. Problem Analysis

The external version is compiled by Xcode8, and the local version is compiled by Xcode9 GM. Could it be caused by Xcode 9 compilation?

• Create a new demo with only input box and button, simulate UIAlertView pop-up, and find that the demo is normal;
• Copy the project settings of the app to the demo, and also copy the property settings of the input box to the demo, the demo is still normal;
• Copy the demo code to the app, and assign the rootViewController of the app to the VC in the demo, and it will still work normally;

It can be determined that some code in the app causes the keyboard to not pop up.

Through binary annotation, we quickly (about 4 or 5 times) located that the problem was caused by a Service class in the app.

After carefully arranging the properties of the Service class, I found that there is a property that inherits UIWindow and its level is higher than UIWindowLevelStatusBar.

From here, based on what we’ve learned and Apple’s UIKit documentation, we can backtrack the problem.

5. Problem tracing

The description of the response chain and UIWindow on Apple's official website explains becomeFirstResponder() as follows:

Asks UIKit to make this object the first responder in its window.

For iOS 11 system behavior of UIAlertView, guess:

• When UIAlertView pops up, it will seize the system's keyWindow, so the keyboard will be retracted when UIAlertView is in progress (because keyWindow changes);
• When UIAlertView disappears, it will traverse all Windows and find the z-axis *** as the keyWindow, so the keyboard will pop up after UIAlertView disappears (keyWindow becomes the original one);

By writing code and debugging the app, the above guess was confirmed.

In iOS 11, if there is a UIWindow with a windowLevel greater than UIWindowLevelNormal when UIAlertView pops up, it will trigger a bug that prevents the keyboard from popping up.

6. Problem fixes

• Ensure that there is no permanent UIWindow in the app;
• Fixed the BUG that maskView cannot be eliminated when the keyboard cannot pop up;
• UIAlertView will be replaced in subsequent versions;

Summarize

• The time it took to generate, reproduce, locate, evaluate, and fix this problem was about the same as the time it took to write this article.
• The bug resolution process is different for each bug, so use this to remind yourself that bug resolution should be purposeful and prioritized.