Posts in Technology (67)

Debugging retain cycles in Objective-C: four likely culprits

Your iPhone app seems to be working fine. But suddenly, it starts to run slow and crashes! You suspect it's memory-related. Here's how to track down and fix issues with retain cycles, which can cause memory not to be released properly.

I'm assuming you are using ARC and iOS5.0+. While ARC simplifies a lot of memory management, it won't spot all retain cycles for you!

First, check you're genuinely dealing with a memory condition. Try to reproduce the crash on a device, and look for crash reports using Xcode > Organiser > Device Logs.

Low memory crashes don't look like normal crash reports, you won't see a stack trace! Instead you'll probably just see Process = Unknown, and the crash report will contain a list of processes which were running at the time of the crash.

Screen Shot 2013-04-16 at 14.01.59

In this example, you can see that Biblegram was using over 25000 pages of memory. 1 page of memory is 4KB, so that's 100MB of memory, which seems much too high.

Next, fire up Instruments via Product > Profile and select the "Allocations" template. Playing around with the app, you should easily be able to locate places where the memory allocation keeps going up and up.

Screen Shot 2013-04-16 at 14.12.13

This "staircase" pattern is a giveaway. I'm repeatedly pushing and popping one view controller, but the memory goes up and up.

Try putting a breakpoint in the dealloc method of the problematic view controller. Most likely, this will never get hit, showing that the view controller is never released.

Screen Shot 2013-04-16 at 14.30.22

This is a strong indication that we have a retain cycle. When the view controller is dismissed there are still some strong references to it, so it doesn't get dealloced.

As one last verification, enter the name of the class into the search box in the top right of Instruments, and check the "# living" column. This is showing 3, when we'd expect it to show 1.

Screen Shot 2013-04-16 at 14.15.38

You can drill down into the instances of the view controller by tapping the small arrow to the right of the class name, and drill down further to see every place that the view controller is retained and released.

This view can be rather overwhelming: the system frameworks do a lot of retaining and releasing on your behalf! For example, instantiating a NIB can easily increase the retain count to 40 temporarily.

So: here are four common errors to look out for that may cause your retain count to be higher than expected.

1. NSTimer

If you create an NSTimer object on a view controller, be sure that invalidate is called on it when dismissing the view controller, otherwise it will retain self.

2. Observers/NSNotificationCenter

If you add an observer to NSNotificationCenter, make sure you remove all observers when dismissing the view controller, otherwise they will retain self.

3. Blocks

You should not call [self doSomething] from inside a block, this can easily lead to the block capturing a reference to self. Instead, make a weak reference to self:

BAD:

  1.  
  2. dispatch_async(queue, ^{
  3. [self doSomething];
  4. });
  5.  

GOOD

  1.  
  2. __weak MyViewController *safeSelf = self;
  3. dispatch_async(queue, ^{
  4. [safeSelf doSomething];
  5. });
  6.  

4. Delegates

if you use

  1.  
  2. someObj.delegate = self;
  3.  

inside the view controller, check the delegate property on someObj is weak.

  1.  
  2. @property (nonatomic, weak) id delegate;
  3.  

Once you've made your fixes, check that dealloc is getting hit and the allocations no longer increase endlessly.

Controlling a 10-bit Digital Potentiometer via SPI with Arduino

1. A brief introduction to the Serial Peripheral Interface (SPI)

The Serial Peripheral Interface a.k.a SPI or four-wire serial interface is a full-duplex serial data protocol used by microcontrollers for communicating with one or more peripheral devices. Devices communicate in master/slave mode and there are four logic signals:

  • - MOSI: Master Out Slave In, is the master line to send data to the peripheral.
  • - MISO: Master In Slave Out, is the slave line to send data to the master.
  • - SCK: Serial Clock, the clock pulses which synchronize data transmission generated by the master.
  • - SS: Slave Select, the pin on each device that the master can use to enable and disable specific devices.

For more on this topic, visit Wikipedia's page on SPI.

2. Wiring the Master - Slave

For this tutorial we will use the Non-volatile memory, 1024 Position (10-bit) Digital Potentiometer AD5131 from Analog Devices. Wiring the IC to Arduino should be pretty straight-forward by looking at the pin configuration from the Data Sheet.

You might encounter different naming conventions for the lines on the SPI interface; the most commonly used are:

  • - The Motorola terms: MOSI and MISO for 'Master Out Slave In' and 'Master In Slave Out'.
  • - The PIC terms: SDI and SDO for 'SPI Data In' and 'SPI Data Out'.

MOSI always connects to MOSI or SDI (slave) and MISO always connects MISO or SDO (slave). CLK or SCK both refer to the serial clock and SS (slave select) or CS (chip select) are the same line.

On the Arduino Uno or Duemilanove the SPI lines are available from the digital pins 11 (MOSI), 12 (MISO) and 13 (SCK). Also, MOSI, MISO and SCK are available in a consistent physical location on the ICSP header.

If you are using a different Arduino board, the SPI lines might be located on different pins numbers. To find this information, please go to the SPI Library reference on the Arduino website.  Please note that when using the SPI feature on the Arduino, the digital pins 11, 12 and 13 can't be used as digital I/O.

Now, let's take a look at our wiring schematic.

Note that, as in this example we are not using all the features from the AD5231.  We left some of the pins unconnected (O1, O2 and RDY). Also, the pins marked with the optional features Write Protect (WP) and Hardware Override (PR) should be connected to VDD if they are not used.

3. Basic Operation

The AD5231 Digital Potentiometer is designed to operate as a true variable resistor replacement device for analog signals. The digital potentiometer wiper position is determined by the RDAC register contents (Address 0x0). The RDAC register acts as a scratchpad register, allowing as many values changes as necessary to place the potentiometer wiper in the correct position.

The basic mode of setting the variable resistor wiper position (programming the scratchpad register) is accomplished by loading the serial data input register with the instruction: 0xB, Address 0x0, and the desired wiper position data.

4. Serial Data Interface

The AD5231 uses a 24-bit serial data-word loaded Most Significant Bit first (MSBFIRST). The CS (chip select) pin must be held LOW until the complete data-word is loaded into the SDI pin. When CS returns to HIGH, the serial data-word is decoded. The command bits (Cx) control the operation of the digital potentiometer. The address bits (Ax) determine which register is activated. The data bits (Dx) are the values that are loaded into the decoded register.

 

MSB | Command Byte 0
Data Byte 1
Data Byte 0 | LSB
C3 C2 C1 C0 Ax Ax Ax Ax
X X X X X X D9 D8
D7 D6 D5 D4 D3 D2 D1 D0

5. Programing the Interface with Arduino

The Arduino SPI library allows us to transfer [SPI.transfer(byte)] only 8-bits (1 byte) at a time. In order to transfer a complete instruction (24-bits) to the digital potentiometer we will have to send the instruction in 3 parts of 8-bits each.

The first byte or 'Command Byte' is composed by the Command + Address which allows us to write data into the RDAC register:

Command = 0xB = 1011
Address = 0x0 = 0000
then:
Command + Address = 10110000

The following 2 bytes, the Data Bytes, contain the 10-bits (from 0 to 1023) wiper position. The Data Byte 1 only contains 2 bits from this value and 6 bits that are not used (X). In order to achieve this operation we will use the bitwise operands Logical Right Shift '>>' and AND '&'.

If you are not familiar with bitwise operations, I suggest reading  the 'Absolute Beginner's Guide to Bit Shifting' post from StackOverflow.

For this example, we will assume that we want to send the value 620 to the digital potentiometer to move the Wiper W to approximately 60%  full-scale position, then:

620 = 0000001001101100
620 >> 8 = 00000010 //Shifting 8 bits to the right
620 & 11111111 = 01101100  //Truncate to 8-bits on the LSB side

And finally, the Arduino code.

#include <SPI.h> 

const int csPin = 10;

void setup() {

 SPI.begin();
 SPI.setBitOrder(MSBFIRST); //We know this from the Data Sheet

 pinMode(csPin,OUTPUT);
 digitalWrite(csPin, HIGH);
}

void loop() {
 for(int i=0; i<1023; i++) { 
  digitalPotWrite(0,i);
  delay(10);
 } 
}

void digitalPotWrite(int address, int value) {
 digitalWrite(csPin, LOW); //select slave
 byte command = 0xB0; //0xB0 = 10110000 
 command += address; 
 SPI.transfer(command); 
 byte byte1 = (value >> 8);
 byte byte0 = (value & 0xFF); //0xFF = B11111111
 SPI.transfer(byte1);
 SPI.transfer(byte0);
 digitalWrite(csPin, HIGH); //de-select slave
}

There are much more features available in the AD5231 such as memory storing and restoring, increment/decrement, ±6 dB/step log taper adjustment, wiper setting read-back and extra EEMEM  for user-defined information, that are not covered in this post.

ReignDesign’s most popular blog posts of 2012

ReignDesign had a fun and exciting year! We released some apps, visited conferences and  contemplated new technology! Here's a Top Ten List of our biggest moments.

1. Oooohh, look at the pretty pictures, iPad Retina vs. Non-retina. We loved the clarity of the retina display. How did we ever live without it?
2.  Git and SCP: this tutorial is perfect if you can't find your USB stick or if you don't feel like walking across the room to find one.
3.  Ever wondered about the controls for touchscreen games? Watch this talk that founder, Matt Mayer, gave in Kiev.
4.  In iOS5, Apple introduced native Twitter integration via the "Tweet Sheet" (aka TWTweetComposeViewController), allowing you to easily implement Twitter in your iOS app without worrying about all the heavy lifting. Continue reading this tutorial for more details.
5.  Look At Teh Pritee Kittehs! Our app, Cat Scan, was fun to make, and we enjoyed turning all of our phone contacts into cute kittens.
6.  Adding labels and re-assigning Github issues: Two ways to automate this common task!
7. Making Flockwork was a great learning experience filled with a few technical hurdles. We wrote a series of blog posts explaining the challenges in cocos2d.
8. Controls for games are important....like, really important.
9. Almost a year ago, we wondered how to best design an app for Windows 8  How accurate were we?
10. Remember the trend of making videos and lists about Sh*t People Say/Do, etc? We made one for iPhone Developers. You're welcome.
ReignDesign is excited about 2013, and we are looking forward to creating and updating.
Happy New Year!

How to launch Google Maps or Apple Maps from an iOS app

Today Google announced their much-anticipated Google Maps app for iOS. They also plan to make available a Google Maps SDK for iOS allowing iOS developers to integrate Google Maps in their apps, however at present API keys are only available for selected developers.

If you want to provide your app's users with the ability to open a location in Google Maps, there is an option which works today: Google have added a comgooglemaps URL scheme to their app so third-party apps can launch the Google Maps app.

Assuming you'd like to give your users a choice of opening a location in both Apple or Google maps, you can set up some code like this in a new view controller:

  1. #import "ViewController.h"
  2. #import <MapKit/MapKit.h>
  3.  
  4. @implementation ViewController
  5.  
  6. - (void)viewDidLoad
  7. {
  8. [super viewDidLoad];
  9. // Add a button to pop open an action sheet
  10. UIButton *btn = [UIButton buttonWithType:UIButtonTypeRoundedRect];
  11. btn.frame = CGRectMake(80,100,160,50);
  12. [btn setTitle:@"Open placemark" forState:UIControlStateNormal];
  13. [btn addTarget:self action:@selector(openActionSheet:) forControlEvents:UIControlEventTouchUpInside];
  14. [self.view addSubview:btn];
  15.  
  16. }
  17. -(void)openActionSheet:(id)sender {
  18. //give the user a choice of Apple or Google Maps
  19. UIActionSheet *sheet = [[UIActionSheet alloc] initWithTitle:@"Open in Maps" delegate:self cancelButtonTitle:nil destructiveButtonTitle:nil otherButtonTitles:@"Apple Maps",@"Google Maps", nil];
  20. [sheet showInView:self.view];
  21. }
  22. -(void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex {
  23. //coordinates for the place we want to display
  24. CLLocationCoordinate2D rdOfficeLocation = CLLocationCoordinate2DMake(31.20691,121.477847);
  25. if (buttonIndex==0) {
  26. //Apple Maps, using the MKMapItem class
  27. MKPlacemark *placemark = [[MKPlacemark alloc] initWithCoordinate:rdOfficeLocation addressDictionary:nil];
  28. MKMapItem *item = [[MKMapItem alloc] initWithPlacemark:placemark];
  29. item.name = @"ReignDesign Office";
  30. [item openInMapsWithLaunchOptions:nil];
  31. } else if (buttonIndex==1) {
  32. //Google Maps
  33. //construct a URL using the comgooglemaps schema
  34. NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"comgooglemaps://?center=%f,%f",rdOfficeLocation.latitude,rdOfficeLocation.longitude]];
  35. if (![[UIApplication sharedApplication] canOpenURL:url]) {
  36. NSLog(@"Google Maps app is not installed");
  37. //left as an exercise for the reader: open the Google Maps mobile website instead!
  38. } else {
  39. [[UIApplication sharedApplication] openURL:url];
  40. }
  41. }
  42. }
  43. @end

Here's the app in action, example code is on Github.

    

For more details, see the Google Maps URL Scheme documentation.

Sharing images to Weixin/WeChat from an iOS app

Weixin (微信) is Tencent's mobile messaging product. It's highly popular in China, with hundreds of millions of users, as well as many international users who may know it by its English name WeChat. It includes both a chat feature, similar to WhatsApp, and a timeline feature similar to Path.

Tencent have a developer site in English, but the documentation is still quite sparse.

For a recent project, we needed to integrate sharing of images to Weixin.

The workflow for the user will be:

1. Create an image in the app
2. Tap a button to share to Weixin
3. The Weixin app will launch, and the user confirms they want to add the image to their "Moments".
4. The user taps a button in the Weixin app to return to our app.

STEP 1: Download and add the iOS SDK

1. Download the latest iOS SDK from the WeChat developer site. After unzipping you should have four files:

2. If you're building with the latest Xcode and targeting iPhone 5, you'll probably want to use the version of the library which supports armv7s, so copy WXApi.hWXApiObject.h and libWeChatSDK_armv7_armv7s.a to your project folder. Rename libWeChatSDK_armv7_armv7s.a to libWeChatSDK.a.

3. Now in your XCode project go to File > Add Files and add the three files to your project.

STEP 2: Register for an app id

1. Register as a WeChat developer using the signup form. Once you've confirmed your email address, head to the My Apps page and tap "Register my apps"

2. Fill out the name of your app and other details. You can skip some optional fields like icon for now. Make sure you choose "Mobile app" as the app type, not Web app.

3. At the end of the process you will be given an app id which looks like wx123456789012

STEP 3: Integrate with the SDK

1. In your application:didFinishLaunchingWithOptions: method, add code to register your app. Be sure to replace wx123456789012 with the code you obtained earlier.

  1. if (![WXApi registerApp:@"wx123456789012"]) {
  2. NSLog(@"Failed to register with Weixin");
  3. }

2. After the Weixin app is launched it needs a way to re-launch your app. It does this by trying to open a URL with your app id as the protocol, for example wx123456789012://. We need to ensure we can handle these URLs, so implement these two methods:

  1. - (BOOL)application:(UIApplication *)application handleOpenURL:(NSURL *)url {
  2. return [WXApi handleOpenURL:url delegate:self];
  3. }
  4. - (BOOL)application:(UIApplication *)application openURL:(NSURL *)url sourceApplication:(NSString *)sourceApplication annotation:(id)annotation {
  5. return [WXApi handleOpenURL:url delegate:self];
  6. }

3. We need to ensure that our app delegate implements WXApiDelegate, so first add WXApiDelegate to your AppDelegate.h file:

  1. @interface AppDelegate : UIResponder<WXApiDelegate>

and then add empty implementations of these two callbacks in the .m:

  1. - (void) onReq:(BaseReq*)req {
  2. }
  3. - (void) onResp:(BaseResp*)resp {
  4. }

4. We also need to make a change in the Info.plist to ensure we can handle these kind of URLs. Go to your target, select the Info tab and add a new URL type as follows. The identifier can be "weixin" and the URL scheme should match your app id.

5. Now let's add a method to send an image.

  1. - (void) sendImageContentToWeixin:(UIImage *)image {
  2. //if the Weixin app is not installed, show an error
  3. if (![WXApi isWXAppInstalled]) {
  4. UIAlertView *alert = [[UIAlertView alloc] initWithTitle:nil message:@"The Weixin app is not installed" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles: nil];
  5. [alert show];
  6. return;
  7. }
  8. //create a message object
  9. WXMediaMessage *message = [WXMediaMessage message];
  10. //set the thumbnail image. This MUST be less than 32kb, or sendReq may return NO.
  11. //we'll just use the full image resized to 100x100 pixels for now
  12. [message setThumbImage:[image resizedImage:CGSizeMake(100,100) interpolationQuality:kCGInterpolationDefault]];
  13. //create an image object and set the image data as a JPG representation of our UIImage
  14. WXImageObject *ext = [WXImageObject object];
  15. ext.imageData = UIImageJPEGRepresentation(image, 0.8);
  16. message.mediaObject = ext;
  17. //create a request
  18. SendMessageToWXReq* req = [[SendMessageToWXReq alloc] init];
  19. //this is a multimedia message, not a text message
  20. req.bText = NO;
  21. //set the message
  22. req.message = message;
  23. //set the "scene", WXSceneTimeline is for "moments". WXSceneSession allows the user to send a message to friends
  24. req.scene = WXSceneTimeline;
  25. //try to send the request
  26. if (![WXApi sendReq:req]) {
  27. UIAlertView *alert = [[UIAlertView alloc] initWithTitle:nil message:@"Error" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles: nil];
  28. [alert show];
  29. }
  30. }

6. Add callback code. This will be called once control is returned to our app after returning from Weixin.

  1. - (void) onResp:(BaseResp*)resp {
  2. if([resp isKindOfClass:[SendMessageToWXResp class]]) {
  3. NSString *strMsg = [NSString stringWithFormat:@"Result:%d", resp.errCode];
  4. NSLog(@"Response from Weixin was: %@",strMsg);
  5. }
  6. }

STEP 4: Test!

All being well, you should now be able to share an image from your code, using something like:

  1.  
  2. UIImage *test = [UIImage imageNamed:@"test.png"];
  3. [self sendImageContentToWeixin:test];
  4.  

After sharing the image, you should be returned to your app.

If you found this tutorial useful, or have any suggestions, do leave a comment!