A brief analysis on the communication and thread safety between threads in IOS application development _ios

Communication between threads

Simple description
Inter-thread communication: In 1 processes, threads are often not isolated, and multiple threads need to communicate frequently

The embodiment of communication between threads
1 Threads passing data to another 1 threads
After performing a specific task in 1 threads, go to another 1 threads to continue the task

Common methods for communication between threads

 code as follows:

-(void) Performselectoronmainthread: (SEL) Aselector withobject: (ID) arg waituntildone: (BOOL) wait;
-(void) Performselector: (SEL) aselector onthread: (Nsthread *) THR Withobject: (ID) arg waituntildone: (BOOL) wait;

Sample communication between threads-Picture download

code as follows:

//
Yyviewcontroller.m
06-nsthread04-Communication between threads
//
Created by Apple on 14-6-23.
Copyright (c) 2014 itcase. All rights reserved.
//

#import "YYViewController.h"
@interface Yyviewcontroller ()
@property (Weak, nonatomic) Iboutlet Uiimageview *iconview;
@end

code as follows:

@implementation Yyviewcontroller

-(void) viewdidload
{
[Super Viewdidload];
}

-(void) Touchesbegan: (Nsset *) touches withevent: (Uievent *) event
{

Calling the download method in a child thread downloads a picture
[Self performselectorinbackground: @selector (download) Withobject:nil];
}

-(void) Download
{
1. Download the picture according to the URL
Downloading pictures from the network
Nsurl *urlstr=[nsurl urlwithstring:@ "FDSF"];

Convert a picture into binary data
NSData *data=[nsdata datawithcontentsofurl:urlstr];//This line of operations can be time-consuming

Convert the data into pictures
UIImage *image=[uiimage Imagewithdata:data];

2. Go back to the main thread to set the picture
[Self Performselectoronmainthread: @selector (settingimage:) withobject:image Waituntildone:no];
}

Set Display picture
-(void) Settingimage: (UIImage *) image
{
Self.iconview.image=image;
}

@end

Code 2:

 code as follows:

//
Yyviewcontroller.m
06-nsthread04-Communication between threads
//
Created by Apple on 14-6-23.
Copyright (c) 2014 itcase. All rights reserved.
//

#import "YYViewController.h"
#import <NSData.h>

@interface Yyviewcontroller ()
@property (Weak, nonatomic) Iboutlet Uiimageview *iconview;
@end

code as follows:

@implementation Yyviewcontroller

-(void) viewdidload
{
[Super Viewdidload];
}

-(void) Touchesbegan: (Nsset *) touches withevent: (Uievent *) event
{
Calling the download method in a child thread downloads a picture

[Self performselectorinbackground: @selector (download) Withobject:nil];
}

-(void) Download
{

1. Download the picture according to the URL
Downloading pictures from the network
Nsurl *urlstr=[nsurl urlwithstring:@ "FDSF"];

Convert a picture into binary data
NSData *data=[nsdata datawithcontentsofurl:urlstr];//This line of operations can be time-consuming

Convert the data into pictures
UIImage *image=[uiimage Imagewithdata:data];

2. Go back to the main thread to set the picture
The first way
[Self Performselectoronmainthread: @selector (settingimage:) withobject:image Waituntildone:no];

The second way
[Self.imageview performselector: @selector (setimage:) onthread:[nsthread Mainthread] Withobject:image Waituntildone:no];

The Third Way
[Self.iconview performselectoronmainthread: @selector (setimage:) withobject:image Waituntildone:no];
}

Set Display picture
-(void) Settingimage: (UIImage *) image
//{
Self.iconview.image=image;
//}

@end

Thread Safety

One, the security hidden trouble of multithreading
resource Sharing
1 of resources may be shared by multiple threads, that is, multiple threads may access the same resource
For example, multiple threads accessing the same object, the same variable, the same file
Data confusion and data security issues can easily be triggered when multiple threads access the same resource
Example one:

Example two:

Problem code:

code as follows:

//
Yyviewcontroller.m
05-Thread Safety
//
Created by Apple on 14-6-23.
Copyright (c) 2014 itcase. All rights reserved.
//

#import "YYViewController.h"

@interface Yyviewcontroller ()
Number of votes remaining

@property (nonatomic,assign) int leftticketscount;
@property (Nonatomic,strong) Nsthread *thread1;
@property (Nonatomic,strong) Nsthread *thread2;
@property (Nonatomic,strong) Nsthread *thread3;

@end

 code as follows:

@implementation Yyviewcontroller

-(void) viewdidload
{
[Super Viewdidload];

By default there are 20 tickets

self.leftticketscount=10;

Open multiple threads, simulate ticket booking

Self.thread1=[[nsthread alloc]initwithtarget:self selector: @selector (selltickets) Object:nil];

self.thread1.name=@ "Conductor a";

Self.thread2=[[nsthread alloc]initwithtarget:self selector: @selector (selltickets) Object:nil];

self.thread2.name=@ "Conductor B";

Self.thread3=[[nsthread alloc]initwithtarget:self selector: @selector (selltickets) Object:nil];
self.thread3.name=@ "Conductor C";
}

&NBSP
-(void) selltickets
{
    while (1) {
        //1. Check the votes first
        int count=self.leftticketscount;
         if (count>0) {
            //Pause for a period of time
            [nsthread SLEEPFORTIMEINTERVAL:0.002];

2. Number of votes-1
Self.leftticketscount= count-1;

Get current thread
Nsthread *current=[nsthread CurrentThread];
NSLog (@ "%@--sold a ticket, still left%d tickets", current,self.leftticketscount);
}else
{
Exit thread
[Nsthread exit];
}
}
}

-(void) Touchesbegan: (Nsset *) touches withevent: (Uievent *) event
{
Open Thread

[Self.thread1 start];
[Self.thread2 start];
[Self.thread3 start];

}

@end

Print results:

Analysis on hidden danger of safety

Third, how to solve

Mutex use format
@synchronized (Lock Object) {//Lock code required}
Note: Lock 1 copies of code with only 1 locks, which are not valid with multiple locks

code example:

code as follows:

//
Yyviewcontroller.m
05-Thread Safety
//
Created by Apple on 14-6-23.
Copyright (c) 2014 itcase. All rights reserved.
//

#import "YYViewController.h"

@interface Yyviewcontroller ()

Number of votes remaining
@property (nonatomic,assign) int leftticketscount;
@property (Nonatomic,strong) Nsthread *thread1;
@property (Nonatomic,strong) Nsthread *thread2;
@property (Nonatomic,strong) Nsthread *thread3;
@end

@implementation Yyviewcontroller

-(void) viewdidload
{
[Super Viewdidload];
By default there are 20 tickets
self.leftticketscount=10;
Open multiple threads, simulate ticket booking

Self.thread1=[[nsthread alloc]initwithtarget:self selector: @selector (selltickets) Object:nil];

self.thread1.name=@ "Conductor a";

Self.thread2=[[nsthread alloc]initwithtarget:self selector: @selector (selltickets) Object:nil];

self.thread2.name=@ "Conductor B";

Self.thread3=[[nsthread alloc]initwithtarget:self selector: @selector (selltickets) Object:nil];

self.thread3.name=@ "Conductor C";
}

-(void) selltickets
{
while (1) {
@synchronized (self) {//can only add one lock
1. Check the number of votes first

int count=self.leftticketscount;
if (count>0) {
Suspend for some time
[Nsthread sleepfortimeinterval:0.002];
2. Number of votes-1

Self.leftticketscount= count-1;
Get current thread
Nsthread *current=[nsthread CurrentThread];
NSLog (@ "%@--sold a ticket, still left%d tickets", current,self.leftticketscount);

}else
{
Exit thread
[Nsthread exit];
}
}
}
}

-(void) Touchesbegan: (Nsset *) touches withevent: (Uievent *) event
{

Open Thread
[Self.thread1 start];
[Self.thread2 start];
[Self.thread3 start];
}

@end

Execution Effect Chart

Advantages and disadvantages of mutual exclusion locks
Advantages: It can effectively prevent the data security problem caused by multithread grabbing resources
Disadvantage: Need to consume a lot of CPU resources

The use of mutexes: Multiple threads rob the same piece of resources
Related jargon: Thread synchronization, multiple threads performing tasks sequentially
Mutex, which is the use of thread synchronization technology

Four: Atomic and non-atomic properties

OC has nonatomic and atomic two choices when defining attributes
Atomic: Atomic properties, lock for setter methods (default is Atomic)
Nonatomic: Non-atomic property, no lock for setter method

Atomic principle of adding locks

 code as follows:

@property (assign, atomic) int age;

-(void) Setage: (int) Age
{

@synchronized (self) {
_age = age;
}
}

Selection of atomic and non-atomic properties
Comparison of Nonatomic and atomic

• Atomic: Thread-safe, requires a lot of resources to consume
• Nonatomic: Non-thread safe, suitable for small memory mobile devices

Recommendations for iOS development

• All attributes are declared as Nonatomic
• Try to avoid multithreading to rob the same piece of resources
• As far as possible, lock, resource-grabbing business logic to server-side processing, reduce the pressure of mobile clients