Probe into block implementation in iOS

Last Update:2015-06-24 Source: Internet

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probe into block implementation in iOS [0. Brief introduction of block]Block is an extension of the C language introduced by ios4.0+ and Mac OS X 10.6+ to implement the features of anonymous Functions.In the words of wikipedia, block is an attribute added by Apple Inc. for c, C + +, and objective-c, allowing these languages to create closures using the syntax of a class lambda expression .In the Apple documentation, A block is a anonymous inline collection of code, and sometimes also called A "closure".As for closures, I think Nanyi's explanation is concise: closures are functions that can read other functions ' internal variables. This explanation is also appropriate for block: a function defines a block that can access the internal variables of the Function.A simple block example is as Follows: [cpp] View Plaincopy<object id=" zeroclipboardmovie_1 " width=" height= " +" align="middle" bgcolor="# FFFFFF " data=" http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf " type=" application/ X-shockwave-flash "><param name=" src " value=" http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf "> <param name="loop" value="loop"><param name="menu" value="false"><param name="quality" value="best"><param Name="allowscriptaccess" value="always"><param name="allowfullscreen" value="false"><param name="pluginspage" value=" Http://www.macromedia.com/go/getflashplayer "><param name=" flashvars " value=" id=1&width=18&height=18 ">< param name= "wmode" value= "transparent" > </object> <ol class="dp-cpp" start="1"> <ol class="dp-cpp" start="1"> <li class="alt">int (^maxblock) (int, int ) = ^ (int x, int Y) { return x > Y x:y;}; </li> </ol> </ol>If you write with a Python lambda expression, it can be written as Follows: [python] View Plaincopy<object id=" zeroclipboardmovie_2 " width=" height= " +" align="middle" bgcolor="# FFFFFF " data=" http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf " type=" application/ X-shockwave-flash "><param name=" src " value=" http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf "> <param name="loop" value="loop"><param name="menu" value="false"><param name="quality" value="best"><param Name="allowscriptaccess" value="always"><param name="allowfullscreen" value="false"><param name="pluginspage" value=" Http://www.macromedia.com/go/getflashplayer "><param name=" flashvars " value=" id=2&width=18&height=18 ">< param name= "wmode" value= "transparent" > </object> <ol class="dp-py" start="1"> <ol class="dp-py" start="1"> <li class="alt">f = Lambda x, y:x if x > y else y </li> </ol> </ol>however, because of the language characteristics of Python itself, in the function body defined by def, it is natural to use DEF statements to define inline functions, because these functions are essentially objects.If you use BNF to represent the context-independent grammar of a block, it is roughly the following: [cpp] View Plaincopy<object id=" zeroclipboardmovie_3 " width=" height= " +" align="middle" bgcolor="# FFFFFF " data=" http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf " type=" application/ X-shockwave-flash "><param name=" src " value=" http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf "> <param name="loop" value="loop"><param name="menu" value="false"><param name="quality" value="best"><param Name="allowscriptaccess" value="always"><param name="allowfullscreen" value="false"><param name="pluginspage" value=" Http://www.macromedia.com/go/getflashplayer "><param name=" flashvars " value=" id=3&width=18&height=18 ">< param name= "wmode" value= "transparent" > </object> <ol class="dp-cpp" start="1"> <ol class="dp-cpp" start="1"> <li class="alt">block_expression:: = ^ Block_declare block_statement</li> <li>block_declare:: = Block_return_type block_argument_list</li> <li class="alt">block_return_type:: = Return_type | Empty</li> <li>block_argument_list:: = Argument_list | Empty</li> </ol> </ol>[1. Why block]In addition to being able to define parameter lists, return types, blocks can also obtain state (such as local variables) within the lexical scope that is defined, and can modify these states under certain conditions, such as using the __block variable. In addition, these modifiable states are shared among multiple blocks within the same lexical scope, and can continue to share or modify these states, even if the lexical scope (such as stack expansion, out of Scope) is Out.In general, blocks are encapsulated in short snippets of code that are used as work units, often for concurrent tasks, traversal, and Callbacks.For example, we can do something when we traverse the nsarray: [cpp] View Plaincopy<object id=" zeroclipboardmovie_4 " width=" height= " +" align="middle" bgcolor="# FFFFFF " data=" http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf " type=" application/ X-shockwave-flash "><param name=" src " value=" http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf "> <param name="loop" value="loop"><param name="menu" value="false"><param name="quality" value="best"><param Name="allowscriptaccess" value="always"><param name="allowfullscreen" value="false"><param name="pluginspage" value=" Http://www.macromedia.com/go/getflashplayer "><param name=" flashvars " value=" id=4&width=18&height=18 ">< param name= "wmode" value= "transparent" > </object> <ol class="dp-cpp" start="1"> <ol class="dp-cpp" start="1"> <li class="alt">-(void) enumerateobjectsusingblock: (void (^) (id obj, nsuinteger idx, BOOL *stop)) block; </li> </ol> </ol>If stop is set to yes, it jumps out of the loop and does not continue to Traverse.In many frameworks, block is increasingly used as a callback function instead of the traditional callback Method. <ul class="ul1"> <ul class="ul1"> <li class="li1">Using block as a callback function allows programmers to write code more smoothly, without running halfway to another place to write a callback function, and sometimes to consider where the callback function is Appropriate. With block, you can write the subsequent processing code directly when the function is called, passing it as a parameter for the callback at the end of its task Execution.</li> <li class="li1">Another benefit is that block is used as a callback to directly access local Variables. For example, I want to modify the name of a user in a batch of users, and then update the corresponding User's cell UI with a callback after the modification is Complete. At this time I need to know the corresponding user cell index, if the use of traditional callback, you need to bring the index to the past, callback and return it back; the index of the current action cell is recorded by an external scope (this limits the name of only one user at a Time) ; traverse to find the corresponding user. With block, you can access the Cell's index directly.</li> </ul> </ul>This document mentions several applications of Block: <ul class="ul1"> <ul class="ul1"> <li class="li1">Callback processing at task completion</li> <li class="li1">Message Listener Callback Processing</li> <li class="li1">Error callback Handling</li> <li class="li1">Enumeration callback</li> <li class="li1">View animations, Transformations</li> <li class="li1">Sort</li> </ul> </ul>[2. about __block_impl]Clang provides an option for intermediate code presentation to further understand the principle of block.Take a very simple code example:Compile using THE-REWRITE-OBJC option:To get a copy of the Block0.cpp file, you can see the following code snippet in this document:It can be seen from the naming that this is the implementation of block and that block is implemented in the front-end of the clang compiler to generate C intermediate Code. Many languages can implement only the compiler front end, generate C intermediate code, and then take advantage of many of the existing C-compiler backend. As you can see from the members of the struct, Flags and reserved can be skipped first, and theISA pointer indicates that the block can be a nsobject, and the funcptr pointer is clearly the corresponding function pointer of the BLOCK.thus, The mystery of block was Unveiled.But where is the block-related variable? The above mentioned block can capture the state of the lexical scope (or the outer context, scope), even if it is out of the range, you can still modify these States. How is this done?[3. Implementation of a simple block]Let's look at a block that only outputs one sentence.Generate the intermediate code to get the fragment as Follows:The first appearance of the structure is __main_block_impl_0, it can be seen according to the function (main Function) and the occurrence of a sequence (No. 0) named. If it is a global block, it is named according to the variable name and the occurrence sequence. __MAIN_BLOCK_IMPL_0 contains two member variables and a constructor, member variables are __block_impl struct and descriptive information desc, followed by initialization of block type information and function pointers in the Constructor. The next occurrence is the __main_block_func_0 function, which is the block corresponding function body. The function accepts a __cself parameter, which is the corresponding block itself.The following is the __main_block_desc_0 structure, where the more valuable information is the block Size.finally, the creation and invocation of the block in the main function shows that the block is called a function that takes the block itself as a parameter, which corresponds to the Block's Execution. here, The block type is represented by _nsconcretestackblock , indicating that the block is in the STACK. In the same vein, there are _nsconcretemallocblock and _nsconcreteglobalblock.Since block is also a nsobject, we can do retain operations on it. however, when the block is passed as a callback function to the underlying framework, the underlying framework needs to copy it. For example, If the callback block is used as a property, you cannot use retain, but copy. We usually write blocks in the stack, and when we need callbacks, the callback block is not already in the stack, and using the copy property, you can put the block into the heap. or use block_copy() and block_release().[4. Capture Local variable]Let's look at a block that accesses local Variables.Generate the intermediate code to get the fragment as Follows:It can be seen that the block structure __main_block_impl_0 a member variable i, used to store the local variable i (value 1024), and at this point can see __cself the function of the parameter, similar to this in C + + and the self of Objective-c. If we try to modify the local variable i, we get the following error:The error message is verbose, telling us that the variable is not assignable and reminds us to use the __block type Identifier.Why can't I assign a value to the variable i?Because the local variable i and the function __main_block_func_0 in the main function are not in the same scope, only the value is passed during the Call. of course, in the above code, we can use pointers to implement local variable Modification. This is due to the fact that the main function stack has not been completed while calling __main_block_func_0, and thevariable i is still in the STACK. however, in many cases, the block is passed as a parameter for subsequent callbacks to Execute. Usually in these cases, when the block is executed, the function stack where the definition is defined is already expanded, and the local variable is no longer in the stack (where is the block at this time?). ), and then use the pointer to access it??。 therefore, it is reasonable to disallow block modification for local variables of auto Type.[5. Modify static local variable]We can also infer how static local variables are modified in the block execution Body-through Pointers.Because static local variables exist in the data segment, there is no risk of an illegal fetch after stack Expansion. The difference between the above intermediate code fragment and the previous fragment is mainly that the address of Iis passed in the main function (&i), and the __main_block_impl_ 0 The member I in the struct becomes the pointer type (int *). The value is then modified by the pointer when the block is Executed.of course, global variables, static global variables can be modified in the block execution Body. More precisely, The block can modify the variables within its scope when it is called (this is __main_block_func_0). For example, when a block is a member variable, it can also access other member variables in the same object.[6. Implementation of __block variable]so, How does the __block type variable support the modification?We add the __block indicator to the INT type variable so that the variable I can be modified in the block function body.Then look at the middle code, there will be a lot more information. The first is the corresponding structure of the __block variable:The first member of the __isa Pointer can also know that __block_byref_i_0 can also be nsobject.The second member __forwarding points to himself, why point to Himself? There is no point pointing to yourself, only to say that sometimes you need to point to another __block_byref_i_0 Structure.The last member is the target storage variable I.At this point, the __MAIN_BLOCK_IMPL_0 structure is as follows:The member variable I of __MAIN_BLOCK_IMPL_0 becomes the __block_byref_i_0 * type.The corresponding function __main_block_func_0 is as Follows:The highlight is the __BLOCK_BYREF_I_0 pointer type variable i, which operates another member variable through its member variable __forwarding pointer. :-)and the main function is as Follows:With this seemingly complex change, we can modify the value of the variable I. But the problem also exists: the __BLOCK_BYREF_I_0 type variable i is still on the stack, when the Block is executed by the callback, the stack of the variable i is already expanded, what should I do?At this critical moment, __main_block_desc_0 stood out:At this point, __main_block_desc_0 has two more member Functions: copy and dispose, respectively, pointing to __main_block_copy_0 and __main_block_dispose_ 0. When the block is copied onto the heap from the stack, it calls __main_block_copy_0 to copy the member variable I of the __block type from the stack to the heap, and when the block is released, it calls __main_block_dispose_0 to release _ The member variable I of the _block type.One will be on the stack, one on the heap, then what if the variable is manipulated on the stack and on the heap?At this point, the role of __forwarding is manifested: when a __block variable is copied from the stack onto the heap, the __forwarding pointer in the __BLOCK_BYREF_I_0 struct on the stack also points to the structure on the heap . /* ---------------------------------------------------------------------------------------------------- */Would like to continue to write, the results found that the article is a bit long. Come here First.Original Link: http://blog.csdn.net/jasonblog/article/details/7756763Jason Lee @ HangzhouProbe into block implementation in iOS

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