iOS develop note

1) All animation should run on main thread.

2) When animation by changing UINSLayoutConstraint, [[animatingView parentView] layoutIfNeeded] should be called before animation and in the animation block.

3) To call the inner method of UIButton's UIImageView to make animation( such as image transition), you have to make UIImageView not NULL at first.

4) iCloud and iTunes do not back up the contents of the following directories:

<Application_Home>/AppName.app

<Application_Data>/Library/Caches

<Application_Data>/tmp

5)

When a user downloads an app update, iTunes installs the update in a new app directory. It then moves the user’s data files from the old installation over to the new app directory before deleting the old installation. Files in the following directories are guaranteed to be preserved during the update process:

• <Application_Data>/Documents

• <Application_Data>/Library

 6)

[UIImage imageNamed:@"imageName"] will cache the image

[UIImage imageWithContentOfFile:@fullPath"] simply load the image and no cache.

7) Optimize codes: using

[myView.layer setShadowPath:[[UIBezierPath 
    bezierPathWithRect:myView.bounds] CGPath]];

rather than

[myView.layer setShadowOpacity:0.5]

to draw shadow.

refer to https://markpospesel.wordpress.com/2012/04/03/on-the-importance-of-setting-shadowpath/

8)

Offscreen drawing on the other hand refers to the process of generating bitmap graphics in the background using the CPU before handing them off to the GPU for onscreen rendering. In iOS, offscreen drawing occurs automatically in any of the following cases:

• Core Graphics (any class prefixed with CG*)

• The drawRect() method, even with an empty implementation.

• CALayers with a shouldRasterize property set to YES.

• CALayers using masks (setMasksToBounds) and dynamic shadows (setShadow*).

• Any text displayed on screen, including Core Text.
• Group opacity (UIViewGroupOpacity).

refer to https://robots.thoughtbot.com/designing-for-ios-graphics-performance

9) When you call endUpdate the number returned from tableView:numberOfRowsInSection: must be the same as the number at beginUpdate minus the number of rows deleted.

10) In tableView beginUpdates block,  it defers any insertions of rows or sections until after it has handled the deletions of rows or sections.

11) About "performSelector may cause a leak because its selector is unknown"

Solution
The compiler is warning about this for a reason. It's very rare that this warning should simply be ignored, and it's easy to work around. Here's how:

if (!_controller) { return; }
SEL selector = NSSelectorFromString(@"someMethod");
IMP imp = [_controller methodForSelector:selector];
void (*func)(id, SEL) = (void *)imp;
func(_controller, selector);
Or more tersely (though hard to read & without the guard):

SEL selector = NSSelectorFromString(@"someMethod");
((void (*)(id, SEL))[_controller methodForSelector:selector])(_controller, selector);
Explanation
What's going on here is you're asking the controller for the C function pointer for the method corresponding to the controller. All NSObjects respond to methodForSelector:, but you can also use class_getMethodImplementation in the Objective-C runtime (useful if you only have a protocol reference, like id<SomeProto>). These function pointers are called IMPs, and are simple typedefed function pointers (id (*IMP)(id, SEL, ...))1. This may be close to the actual method signature of the method, but will not always match exactly.

Once you have the IMP, you need to cast it to a function pointer that includes all of the details that ARC needs (including the two implicit hidden arguments self and _cmd of every Objective-C method call). This is handled in the third line (the (void *) on the right hand side simply tells the compiler that you know what you're doing and not to generate a warning since the pointer types don't match).

Finally, you call the function pointer2.

Complex Example
When the selector takes arguments or returns a value, you'll have to change things a bit:

SEL selector = NSSelectorFromString(@"processRegion:ofView:");
IMP imp = [_controller methodForSelector:selector];
CGRect (*func)(id, SEL, CGRect, UIView *) = (void *)imp;
CGRect result = _controller ?
  func(_controller, selector, someRect, someView) : CGRectZero;
Reasoning for Warning
The reason for this warning is that with ARC, the runtime needs to know what to do with the result of the method you're calling. The result could be anything: void, int, char, NSString *, id, etc. ARC normally gets this information from the header of the object type you're working with.3

There are really only 4 things that ARC would consider for the return value:4

Ignore non-object types (void, int, etc)
Retain object value, then release when it is no longer used (standard assumption)
Release new object values when no longer used (methods in the init/ copy family or attributed with ns_returns_retained)
Do nothing & assume returned object value will be valid in local scope (until inner most release pool is drained, attributed with ns_returns_autoreleased)
The call to methodForSelector: assumes that the return value of the method it's calling is an object, but does not retain/release it. So you could end up creating a leak if your object is supposed to be released as in #3 above (that is, the method you're calling returns a new object).

For selectors you're trying to call that return void or other non-objects, you could enable compiler features to ignore the warning, but it may be dangerous. I've seen Clang go through a few iterations of how it handles return values that aren't assigned to local variables. There's no reason that with ARC enabled that it can't retain and release the object value that's returned from methodForSelector: even though you don't want to use it. From the compiler's perspective, it is an object after all. That means that if the method you're calling, someMethod, is returning a non object (including void), you could end up with a garbage pointer value being retained/released and crash.

Additional Arguments
One consideration is that this is the same warning will occur with performSelector:withObject: and you could run into similar problems with not declaring how that method consumes parameters. ARC allows for declaring consumed parameters, and if the method consumes the parameter, you'll probably eventually send a message to a zombie and crash. There are ways to work around this with bridged casting, but really it'd be better to simply use the IMP and function pointer methodology above. Since consumed parameters are rarely an issue, this isn't likely to come up.

Static Selectors
Interestingly, the compiler will not complain about selectors declared statically:

[_controller performSelector:@selector(someMethod)];
The reason for this is because the compiler actually is able to record all of the information about the selector and the object during compilation. It doesn't need to make any assumptions about anything. (I checked this a year a so ago by looking at the source, but don't have a reference right now.)

Suppression
In trying to think of a situation where suppression of this warning would be necessary and good code design, I'm coming up blank. Someone please share if they have had an experience where silencing this warning was necessary (and the above doesn't handle things properly).

More
It's possible to build up an NSMethodInvocation to handle this as well, but doing so requires a lot more typing and is also slower, so there's little reason to do it.

History
When the performSelector: family of methods was first added to Objective-C, ARC did not exist. While creating ARC, Apple decided that a warning should be generated for these methods as a way of guiding developers toward using other means to explicitly define how memory should be handled when sending arbitrary messages via a named selector. In Objective-C, developers are able to do this by using C style casts on raw function pointers.

With the introduction of Swift, Apple has documented the performSelector: family of methods as "inherently unsafe" and they are not available to Swift.

Over time, we have seen this progression:

Early versions of Objective-C allow performSelector: (manual memory management)
Objective-C with ARC warns for use of performSelector:
Swift does not have access to performSelector: and documents these methods as "inherently unsafe"
The idea of sending messages based on a named selector is not, however, an "inherently unsafe" feature. This idea has been used successfully for a long time in Objective-C as well as many other programming languages.

1 All Objective-C methods have two hidden arguments, self and _cmd that are implicitly added when you call a method.

2 Calling a NULL function is not safe in C. The guard used to check for the presence of the controller ensures that we have an object. We therefore know we'll get an IMP from methodForSelector: (though it may be _objc_msgForward, entry into the message forwarding system). Basically, with the guard in place, we know we have a function to call.

3 Actually, it's possible for it to get the wrong info if declare you objects as id and you're not importing all headers. You could end up with crashes in code that the compiler thinks is fine. This is very rare, but could happen. Usually you'll just get a warning that it doesn't know which of two method signatures to choose from.

4 See the ARC reference on retained return values and unretained return values for more details.

#cited from http://stackoverflow.com/questions/7017281/performselector-may-cause-a-leak-because-its-selector-is-unknown

12) In your code, replace occurrences of id as a return value with instancetype where appropriate. This is typically the case for init methods and class factory methods. Even though the compiler automatically converts methods that begin with “alloc,” “init,” or “new” and have a return type of id to return instancetype, it doesn’t convert other methods. Objective-C convention is to write instancetype explicitly for all methods.

from https://developer.apple.com/library/ios/releasenotes/ObjectiveC/ModernizationObjC/AdoptingModernObjective-C/AdoptingModernObjective-C.html

 

13) UIButton of custom type can't set tint color. The porperties imageEdgeInsets and  titleEdgeInsets can be used to set the position of image and title, respectively.

 

14) When hide navigation bar, the controller's view will adjust its frame, which leads to all its subviews adjust their frame respectively. To avoid that, the subviews can be added to the controller's view in viewwillappear after hide the navigation bar. ( ios 9, 6 plus)

According to 

http://stackoverflow.com/questions/20380676/view-changes-its-own-frame-when-status-bar-gets-hidden-or-shown

overlap a new window above navigation bar is also a solution, which however is not recommended because it is risky and dirty.

 

15) tableview status bar left a blank bar above the view, many one use magic number 20 to adjust the frame to solve this, however the solution is not suggested. Below links offer some hints for the proper solution, record that for the future use.

http://stackoverflow.com/questions/18900428/ios-7-uitableview-shows-under-status-bar

http://stackoverflow.com/questions/17074365/status-bar-and-navigation-bar-appear-over-my-views-bounds-in-ios-7

 

16) iOS capture packet

1.Connect your iOS device to your Mac via USB.

2.Get the UDID for the connected device from iTunes or organiser.

3.Open terminal in your Mac
4.type the following commands in the terminal

$ ifconfig -l // First get the current list of interfaces.

$ rvictl -s <udid> // Then run the tool with the UDID of the device.

                   // This adds a new virtual network interface rvi0.
$ ifconfig -l 
                   // Get the list of interfaces again, and you can see the new
virtual network interface, rvi0, added by the previous command.

$ sudo tcpdump -i rvi0 -w ./output.pcap // Get the traced packets and save it to a file

Note : output.pacp is the packet traced file and will be located in the systems root directory 
When you're done you can stop the process with the following command.

$ rvictl -x <udid>

open the .pacp using wireshark and do your default procedures. Thats it !!!!!!!!

-- cited from https://ask.wireshark.org/questions/17559/packet-capturing-application-for-the-iphone

 

17)

Override methods for subclass of UIView.

Methods to Override

Initialization

initWithFrame: It is recommended that you implement this method. You can also implement custom initialization methods in addition to, or instead of, this method.
initWithCoder: Implement this method if you load your view from an Interface Builder nib file and your view requires custom initialization.
layerClass Implement this method only if you want your view to use a different Core Animation layer for its backing store. For example, if you are using OpenGL ES to do your drawing, you would want to override this method and return the CAEAGLLayer class.
Drawing and printing

drawRect: Implement this method if your view draws custom content. If your view does not do any custom drawing, avoid overriding this method.
drawRect:forViewPrintFormatter: Implement this method only if you want to draw your view’s content differently during printing.
Constraints

requiresConstraintBasedLayout Implement this class method if your view class requires constraints to work properly.
updateConstraints Implement this method if your view needs to create custom constraints between your subviews.
alignmentRectForFrame:, frameForAlignmentRect: Implement these methods to override how your views are aligned to other views.
Layout

sizeThatFits: Implement this method if you want your view to have a different default size than it normally would during resizing operations. For example, you might use this method to prevent your view from shrinking to the point where subviews cannot be displayed correctly.
layoutSubviews Implement this method if you need more precise control over the layout of your subviews than either the constraint or autoresizing behaviors provide.
didAddSubview:, willRemoveSubview: Implement these methods as needed to track the additions and removals of subviews.
willMoveToSuperview:, didMoveToSuperview Implement these methods as needed to track the movement of the current view in your view hierarchy.
willMoveToWindow:, didMoveToWindow Implement these methods as needed to track the movement of your view to a different window.
Event Handling:

touchesBegan:withEvent:, touchesMoved:withEvent:, touchesEnded:withEvent:, touchesCancelled:withEvent: Implement these methods if you need to handle touch events directly. (For gesture-based input, use gesture recognizers.)
gestureRecognizerShouldBegin: Implement this method if your view handles touch events directly and might want to prevent attached gesture recognizers from triggering additional actions.

-- cited from http://stackoverflow.com/questions/15978370/proper-practice-for-subclassing-uiview

 18)

Make UIImagePickerController full screen

  

      CGSize screenSize = [UIScreen mainScreen].bounds.size;

        float cameraAspectRatio = 4.0 / 3.0;

        float imageHeight = floorf(screenSize.width * cameraAspectRatio);

        float scale = ceilf((screenSize.height / imageHeight) * 10.0) / 10.0;

        _imagePicker.cameraViewTransform = CGAffineTransformMakeTranslation(0, (screenSize.height - imageHeight) / 2);

        _imagePicker.cameraViewTransform = CGAffineTransformScale(_imagePicker.cameraViewTransform,scale, scale);

 19)

Coordinate Systems
Since an image is a 2D map of pixels, the origin needs specification. Usually it’s the top-left corner of the image, with the y-axis pointing downwards, or at the bottom left, with the y-axis pointing upwards.
There’s no “correct” coordinate system, and Apple uses both in different places.
Currently, UIImage and UIView use the top-left corner as the origin and Core Image and Core Graphics use the bottom-left. This is important to remember so you know where to find the bug when Core Image returns an “upside down” image.

 

20)

NSDateFormatter

a: AM/PM
A: 0~86399999 (Millisecond of Day)

c/cc: 1~7 (Day of Week)
ccc: Sun/Mon/Tue/Wed/Thu/Fri/Sat
cccc: Sunday/Monday/Tuesday/Wednesday/Thursday/Friday/Saturday

d: 1~31 (0 padded Day of Month)
D: 1~366 (0 padded Day of Year)

e: 1~7 (0 padded Day of Week)
E~EEE: Sun/Mon/Tue/Wed/Thu/Fri/Sat
EEEE: Sunday/Monday/Tuesday/Wednesday/Thursday/Friday/Saturday

F: 1~5 (0 padded Week of Month, first day of week = Monday)

g: Julian Day Number (number of days since 4713 BC January 1)
G~GGG: BC/AD (Era Designator Abbreviated)
GGGG: Before Christ/Anno Domini

h: 1~12 (0 padded Hour (12hr))
H: 0~23 (0 padded Hour (24hr))

k: 1~24 (0 padded Hour (24hr)
K: 0~11 (0 padded Hour (12hr))

L/LL: 1~12 (0 padded Month)
LLL: Jan/Feb/Mar/Apr/May/Jun/Jul/Aug/Sep/Oct/Nov/Dec
LLLL: January/February/March/April/May/June/July/August/September/October/November/December

m: 0~59 (0 padded Minute)
M/MM: 1~12 (0 padded Month)
MMM: Jan/Feb/Mar/Apr/May/Jun/Jul/Aug/Sep/Oct/Nov/Dec
MMMM: January/February/March/April/May/June/July/August/September/October/November/December

q/qq: 1~4 (0 padded Quarter)
qqq: Q1/Q2/Q3/Q4
qqqq: 1st quarter/2nd quarter/3rd quarter/4th quarter
Q/QQ: 1~4 (0 padded Quarter)
QQQ: Q1/Q2/Q3/Q4
QQQQ: 1st quarter/2nd quarter/3rd quarter/4th quarter

s: 0~59 (0 padded Second)
S: (rounded Sub-Second)

u: (0 padded Year)

v~vvv: (General GMT Timezone Abbreviation)
vvvv: (General GMT Timezone Name)

w: 1~53 (0 padded Week of Year, 1st day of week = Sunday, NB: 1st week of year starts from the last Sunday of last year)
W: 1~5 (0 padded Week of Month, 1st day of week = Sunday)

y/yyyy: (Full Year)
yy/yyy: (2 Digits Year)
Y/YYYY: (Full Year, starting from the Sunday of the 1st week of year)
YY/YYY: (2 Digits Year, starting from the Sunday of the 1st week of year)

z~zzz: (Specific GMT Timezone Abbreviation)
zzzz: (Specific GMT Timezone Name)
Z: +0000 (RFC 822 Timezone)

21)

Judge the iphone version

#define IS_IPAD (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
#define IS_IPHONE (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone)
#define IS_RETINA ([[UIScreen mainScreen] scale] >= 2.0)

#define SCREEN_WIDTH ([[UIScreen mainScreen] bounds].size.width)
#define SCREEN_HEIGHT ([[UIScreen mainScreen] bounds].size.height)
#define SCREEN_MAX_LENGTH (MAX(SCREEN_WIDTH, SCREEN_HEIGHT))
#define SCREEN_MIN_LENGTH (MIN(SCREEN_WIDTH, SCREEN_HEIGHT))

#define IS_IPHONE_4_OR_LESS (IS_IPHONE && SCREEN_MAX_LENGTH < 568.0)
#define IS_IPHONE_5 (IS_IPHONE && SCREEN_MAX_LENGTH == 568.0)
#define IS_IPHONE_6 (IS_IPHONE && SCREEN_MAX_LENGTH == 667.0)
#define IS_IPHONE_6P (IS_IPHONE && SCREEN_MAX_LENGTH == 736.0)

iPhone 6 Plus   736x414 points  2208x1242 pixels    3x scale    1920x1080 physical pixels   401 physical ppi    5.5"
iPhone 6        667x375 points  1334x750 pixels     2x scale    1334x750 physical pixels    326 physical ppi    4.7"
iPhone 5        568x320 points  1136x640 pixels     2x scale    1136x640 physical pixels    326 physical ppi    4.0"
iPhone 4        480x320 points  960x640 pixels      2x scale    960x640 physical pixels     326 physical ppi    3.5"
iPhone 3GS      480x320 points  480x320 pixels      1x scale    480x320 physical pixels     163 physical ppi    3.5" 

22)

 UITextField requires the view height larger than the font size 3 or more pixels that it can scroll when input reaches the end of the box. Otherwise the cursor just freeze over the end of the box, not the text, while in fact it on the end of the text if you print the property selectedRange.