I want to dynamically add code to a block variable, or merge or concatenate a block with another block. Is this possible?
One way of doing it is creating a block that calls the block to be "expanded" before performing its own functions.
For example, consider the example below that adds logging functionality to an arbitrary block passed into it:
typedef void (^MyBlock)(int);
-(MyBlock) expand:(MyBlock)nested {
return ^(int x) {
nested(x);
NSLog("The value of x = %d", x);
};
}
The cumulative effect of calling the block produced by expand: is that of invoking the original block, followed by an operation from the expanded block. You can take it further, to create an appendBlock method:
-(MyBlock) appendBlock:(MyBlock)second toBlock:(MyBlock)first {
return ^(int x) {
first(x);
second(x);
};
}
Is this possible?
No, but you can create a collection of blocks and execute them sequentially.
Sure - just create a new block, which makes use of the original in whatever compositional way you'd like. If you've got block1 and block2, you might create:
someCodeBefore = ^myBlockType(block1) {
someCode()
thatIWantBefore();
block1();
}
someCodeAfter = ^myBlockType(block1) {
block1();
someCode()
thatIWantAfterBlock1();
}
composedBlocks = ^myBlockType(block1, block2) {
block1();
block2();
}
Just make sure you're copying the blocks correctly.
Related
Q (tldr;): How do I use the JavaScanner in android-lint to check if a particular function call with a specific string as a parameter has been surrounded by a try/catch block.
Details: I have completed the android-lint tutorials on the official site and have gone through the source of the existing lint-checks. However, I can't seem to grasp the workflow for this AST-based parsing of JavaScanner. What I'm trying to achieve is to catch a function that sets a specific property and surround it with a try/catch block. For example:
MyPropertySettings.set("SOME_PROPERTY", "SOME_VAL");
should not trigger the lint rule but:
MyPropertySettings.set("SOME_SENSITIVE_PROPERTY", "SOME_VAL");
should because it's not surrounded by a try/catch block with SetPropertyException. I don't want to introduce the try/catch to the function itself because it only throws the exception in extremely rare cases (and the internals of the function are based on some reflection mojo).
For this question, even a workflow/hint would be fine. If I can get the first few steps, I might be able to grasp it better.
Update:
After some more study, I have found that I need to set the set function above in getApplicableMethodNames() and then, somehow read the property of that function to decide if the check applies. That part should be easy.
Surrounding try/catch would be more difficult and I gather I would need to do some "flow analysis". How is the question now.
Well, along with the getApplicableMethodNames() method, you need to override the visitMethod() function. You will get the MethodInvocationNode. Just fetch the arguments passed in the invocation using the node.astArguments() function. This returns a list of arguments that you can iterate through using a StrictListAccessor. Check the arguments passed and if it matches your criterion, run a loop and keep calculating the parent node of the invocation node till a try node is found. If it is a try node, then you can get a list of catches using node.astCatches(). Scan the list and find the appropriate exception. If not found, then report.
You can code like this:
check if it is surrounded by try/catch:
#Override
public void visitMethod(JavaContext context, AstVisitor visitor, MethodInvocation node) {
// check the specified class that invoke the method
JavaParser.ResolvedMethod method = (JavaParser.ResolvedMethod) context.resolve(node);
JavaParser.ResolvedClass clzz = method.getContainingClass();
boolean isSubClass = false;
// sSupportSuperType = {"class name"};
for (int i = 0; i < sSupportSuperType.length; i++) {
if (clzz.isSubclassOf(sSupportSuperType[i], false)) {
isSubClass = true;
break;
}
}
if (!isSubClass) return;
// check if surrounded by try/catch
Node parent = node;
while (true) {
Try tryCatch = context.getParentOfType(parent, Try.class);
if (tryCatch == null) {
break;
} else {
for (Catch aCatch : tryCatch.astCatches()) {
TypeReference catchType = aCatch.astExceptionDeclaration().astTypeReference();
}
parent = tryCatch;
}
}
// get the arguments string
String str = node.astArguments().first().toString();
if (!str.startsWith("\"SOME_PROPERTY\"")) {
context.report(ISSUE, node, context.getLocation(node), "message");
}
}
before this you have to define the specific method by override:
#Override
public List<String> getApplicableMethodNames() {
return Collections.singletonList("set");
}
In the Apple block documentation is an example of code not to write:
void dontDoThisEither() {
void (^block) (void);
int i = random();
if (i > 1000) {
block = ^{printf("got i at: %d\n", i); };
}
// ...
}
The comments for the code say the block literal scope is the "then" clause.
I don't understand what they mean by that, there is no then clause, which is presumably why they have put it in quotes. But why have they put it in quotes and what is the relationship to the block's scope?
Think of an if statement as:
if this then that else this other thing
The {... block = ...} is in the then that part of the if statement. That is, it is *a sub-scope of the scope of the dontDoThisEither() function.
Because blocks are created on the stack and are only valid within the scope of their declaration, that means that the block assignment in that example is only valid within the then that scope of the if statement.
I.e. Consider:
void dontDoThisEither() {
void (^block) (void);
int i = random();
if (i > 1000) {
block = ^{printf("got i at: %d\n", i); };
} else {
block = ^{printf("your number is weak and small. ignored.\n");};
}
block();
}
At the time block(); is executed the block that it is pointing to is in a scope that is no longer valid and the behavior will be undefined (and likely crashy in a real world example).
Sup guys,
I'm trying to do a function that calls itself but by putting everything on one block,
As you can see, the following function is intended to be called an indefinite amount of times (until arcrandom returns a number lower than 50) and you should expect as an output a variable number of "RUNNING" messages, depending on chance.
void (^_test_closure)(void) = ^ {
NSLog(#"RUNNING");
if(arc4random() % 100 > 50) {
_test_closure();
}
};
_test_closure();
However, when running it, I get an EXC_BAD_ACCESS error and the reason I've found is that when the code tries to calls _test_closure inside of the closure it basically points to nowhere.
Does anyone know how to make the above code work?
You have to declare your block itself as a block variable:
__block void (^_test_closure)();
_test_closure = ^{
NSLog(#"Running...");
if ((arc4random() % 100) > 50) {
_test_closure();
}
}
_test_closure();
Recursion and blocks is tricky. Because a block captures all variables passed in, the variable _test_closure is not initialized yet (and clang should give you a warning:
Block pointer variable '_test_closure' is uninitialized when captured by block
).
There are several ways you can get around this, but the most obvious & simplest is to just make the block itself a __block variable (what #H2CO3 said). This allows the block to be weak-linked almost, so that when you call it again, it is properly initialized.
Another option you have is making the block a global or static, like this:
// outside of 'main', thus being a global variable
void (^blockRecurse)(int) = ^(int level) {
if (level < 0)
return;
NSLog(#"Level: %i", level);
blockRecurse(--level);
};
int main()
{
#autoreleasepool {
blockRecurse(10);
}
}
This means it's not being captured by the block, but instead it's referencing the global / static variable, which can be changed by all code equally.
It works with XCode 5 - no warnings, no retain cycles:
typedef void(^blockT)();
blockT block1;
blockT __block block1recursive;
block1recursive = block1 = ^(){
block1recursive();
};
block1();
I was surprised to see in a objective-c project, the following line codes
- (void)methodName
{
... some code...
{
... some code
}
{
... some code
}
}
What does the inner brackets stand for ? They seems not be preceded by any statement.
thanks
The brackets create a new scope. Variables defined within the scope will not persist after the end of the scope. I personally use this to separate out bits of logic to make things easier to read.
Example 1
This example demonstrates the lack of access to variables instantiated inside of a more narrowly defined scope.
-(void)blockTestA {
int j = 25;
{
int k = 5;
// You can access both variables 'j' and 'k' inside this block.
}
// You can only access the variable 'j' here.
}
Example 2
This example demonstrates how creating a new block scope allows us to have different variables with the same name. You can read more about scope here.
-(void)blockTestB {
int j = 25;
{
int j = 5;
NSLog(#"j inside block is: %i", j); // Prints '5'
}
NSLog(#"j outside of block is: %i", j); // Prints '25'
}
The inner brackets limit the scope of variables declared inside of them.
They create a block scope. Declared variables inside those blocks will not be available outside the blocks.
- (void)methodName
{
... some code...
{
int i;//the scope of i is within this block only
... some code
}
{
int i;//the scope of i is within this block only
... some code
}
}
I think it will be helpful to you.
I have a controller which serves as a delegate to two scrollviews which are placed in view managed by aforementioned view controller.
To distinguish between two scroll views I'm trying to use switch statement (instead of simple pointer comparison with if statement). I have tagged both scroll views as 0 and 1 like this
NSUInteger const kFirstScrollView = 0;
NSUInteger const kSecondScrollView = 1;
When I try to use these constants in a switch statement, the compiler says that case statements are not constants.
switch (scrollView.tag) {
case kFirstScrollView: {
// do stuff
}
case kSecondScrollView: {
// do stuff
}
}
What am I doing wrong?
This can be solved through the use of an anonymous (though not necessarily so) enum type:
enum {
kFirstScrollView = 0,
kSecondScrollView = 1
};
switch (scrollView.tag) {
case kFirstScrollView: {
// do stuff
}
case kSecondScrollView: {
// do stuff
}
}
This will compile without errors.
This is because case statement requires constant expression. Now in C and thus in Obj-C making a variable const does not create a true constant. Thus you are getting this error. But if you use C++ or Obj-C++ then this will work.
Some more hint is available here and here.