It is possible in php to ignore some methods by the php interpretor? I need to ignore some methods or functions if the project is, for example, in the release mode, and executed them if the project is in the debug mode.
If you're indeed talking about methods (not functions) then the solution is using Overloading:
class MyClass
{
static public $debugging = true;
public function __call($function, $arguments)
{
if (!self::$debugging)
trigger_error("Cannot call $function in release mode!", E_USER_ERROR);
return call_user_func_array(array($this,'__real_'.$function), $arguments);
}
protected function __real_debug($a,$b,$c)
{
// Do something here
}
}
Then for all not implicitly declared methods of MyClass, the overloaded __call method will be invoked. If you do:
$c = new MyClass();
$c->debug(1,2,3);
Then, if $debugging is true, the protected __real_debug is called.
BTW: the above sample is not restricted to PHP 5.3. It works for any PHP 5.x version.
Related
I am new to Dart and Flutter. While I am going through tutorials, I got that we can make singleton using factory keyword. But after that, I got this code.
class AccountService {
static final _instance = AccountService._internal();
AccountService._internal();
static AccountService getInstance() {
return _instance;
}
}
My questions.
How does the code work?
when getInstance() get called?
is AccountService._internal() a constructor?
static final _instance = AccountService._internal(); - When this get called?
Please help me
Static fields in Dart are all lazy evaluated so they will first get its value the first time you access the field.
So:
When you call getInstance(), it will return the value of the field _instance. If this is the first time the field will be evaluated so AccountService._internal() is called. If it is second time, the value from previous access is reused.
First time you call the method somewhere in your code? If you are never calling the method, the object referenced by _instance will never be created.
Yes, it is a named constructor and because the name starts with "_" it is only available from the library this class is part of. By doing so, it is possible to restrict new objects from this class so only the class itself are allowed to create an instance.
It is called first time _instance is accessed. Since this name also starts with "_" it is only available from the library this class is part of.
The lazy initialization of static fields is described in the Dart specification with the following reasoning:
Static variable declarations with an initializing expression are initializedlazily (8.1).
The lazy semantics are given because we do not want a language where one tends to define expensive initialization computations, causing long application startup times. This is especially crucial for Dart, which must support the coding of client applications.
https://dart.dev/guides/language/specifications/DartLangSpec-v2.2.pdf
Added code example
class AccountService {
static final _instance = AccountService._internal();
AccountService._internal() {
print(':: Calling AccountService._internal constructor');
}
static AccountService getInstance() {
print(':: Calling getInstance()');
return _instance;
}
}
void main() {
print(':: Step 1');
AccountService.getInstance();
print(':: Step 2');
AccountService.getInstance();
print(':: End');
}
Output:
:: Start
:: Step 1
:: Calling getInstance()
:: Calling AccountService._internal constructor
:: Step 2
:: Calling getInstance()
:: End
I am using JNA in my project and my Java JNA Callbacks throw exception in some cases. I want to know from C/C++ code an exception was thrown by last calbback method call. In JNI, one can do it using env->ExceptionCheck() but could not find any equivalent in JNA.
Is there any possibility to achieve this?
The native code calling your callback certainly has no expectation that a Java exception will be raised. There is no guarantee that a JNA callback will be invoked from a containing JVM context. Even if it were, you'd have to establish an out of band channel to pass the exception from the callback to the JVM further up the stack, since you have no guarantees about the calling C code.
Assuming you have Java code -> C code -> callback, I'd recommend you catch all your callback's exceptions, then put them somewhere for the calling Java code to examine after the call.
You could make this happen under the covers with an InvocationMapper, which basically lets you capture and/or modify the results of an interface-mapped call, but it's probably easier just to be explicit about it and wrap the whole thing in a utility function.
For example:
public interface MyLibrary extends Library {
MyLibrary INSTANCE = (MyLibrary)Native.loadLibrary();
interface MyCallback extends Callback {
void invoke();
}
void myFunction(MyCallback callback);
}
Then you provide a utility wrapper:
public void myFunction(final MyCallback callback) {
final List<Exception> exceptions = new List<Exception>();
MyLibrary.INSTANCE.myFunction(new MyCallback() {
public void invoke() {
try {
callback.invoke();
} catch(Exception e) {
exceptions.add(e);
}
}
});
if (exceptions.size() > 0) {
// ...
}
}
I can use reflection to access and call private methods of a class outside of my library. Is this a bug or desired behaviour? If it's desired, how can I make it impossible for external code to access private members/methods?
library left;
class Thing {
void _priv(String s) {
print(s);
}
}
library right;
void main() {
var t = new Thing();
var mirror = reflect(t);
mirror.type.declarations.values
.where( (d) => d.isPrivate && d is MethodMirror )
.forEach( (d) {
print(d.simpleName == #_priv); // prints false
mirror.getField(d.simpleName).reflectee("Hello World"); // prints Hello World
});
}
This privacy is not a security feature, is's only to communicate to users of your API that such a method is intended for internal usage only. Access using mirrors can't be prevented.
Disallowing it in mirrors wouldn't prevent access because the VM and dart2js just mangle or prefix private method names to prevent name collisions with public methods. These names can be predicted or found using brute force and then be called.
Calling private methods are mostly useful in writing the DSL(Domain Specific Language)s.
While writing library documentation I need to be able to reference (i.e. link to) methods from other classes (in the same library) but with the same method name (i.e. reference the delegating method from the docs of the one that is doing the work).
I have tried ClassName.method (does not work) and directly using the method (references the same class method).
Any ideas?
Thanks.
/// [B.someMethod] ..
/// [someMethod] ..
class A {
void someMethod() {
}
}
/// [A.someMethod]
/// [someMethod]
class B {
void someMethod() {
}
}
/// [A.someMethod]
void main() {
new A().someMethod();
}
All references in the doc comments work for me in this example, but sometimes DartEditor shows them only as links after a delay or after some other edits.
Its a docgen issue/bug - can be monitored here: https://code.google.com/p/dart/issues/detail?id=22144
In Dart, is it possible for a function to have a prototype associated with it?
Example Javascript code:
doStuff.prototype.isDefined = true; //is there anything like Javascript's function prototypes in Dart?
function doStuff(){
console.log("The function doStuff was called!");
}
Is it possible to do the equivalent of this in Dart (i.e., create a list of properties for each function?)
Two things to address here:
First, Dart doesn't have prototypes or prototypal inheritance, and instead uses classical inheritance. Rather than a prototype, objects have a class, and instead of a prototype chain, objects have superclasses.
Second, for your specific case, I think we'd have to see more of what you need to do to figure out the idiomatic way to do it in Dart. It should soon be possible to emulate functions with objects so that you can invoke an object and still have state and other methods associated with it.
See this article for more: http://www.dartlang.org/articles/emulating-functions/
When that capability lands you'll be able to do this:
class DoStuff {
bool isDefined = true;
call() => print("The function doStuff was called!");
}
var doStuff = new DoStuff();
main() => doStuff();
Which works if you have a fixed set of metadata about your function that you need to keep track of. It's slightly different from JavaScript because each instance of the function in Dart will have its own state for isDefined. I'm not sure if it's possible or easy to get multiple instances of the function in JavasScript, but you might need to make isDefined static so that the value is shared across all instances.
Dart does not allow you to add or remove member variables from an instance of a class at runtime. Rewriting your example in Dart it might look something like this:
class doStuff {
bool isDefined;
doStuff() {
isDefined = true;
}
void stuff() {
print('The function stuff was called!');
}
}
main() {
new doStuff().stuff();
}
If you wanted to add a property bag to a class in Dart you would write:
class PropertyObject {
Map<String, Dynamic> properties;
PropertyObject() {
properties = new Map<String, Dynamic>();
}
Dynamic operator[](String K) => properties[K];
void operator[]=(String K, Dynamic V) => properties[K] = V;
}
main() {
PropertyObject bag = new PropertyObject();
bag['foo'] = 'world';
print('Hello ${bag['foo']}');
}
Note that you can't access map properties using the '.' operator.