I know that CoerceValueCallback is used to correct a value and that ValidateValueCallback will return true or false. But my question is why we need ValidatevalueCallback? We can simply use CoerceValueCallback to validate (using if condition) and correct the value. Can you give some practical example of when to use coercion vs. validation?
Here are the rules I follow for when to use coercion vs. validation.
Use CoerceValueCallback If...
You can safely correct a value to be valid without needing to throw an error.
Your property depends on one or more other dependency properties.
You need to provide instance-level validation as opposed to class-level validation.
You allow others to override your validation logic.
Use ValidateValueCallback If...
You cannot correct a value to be valid.
You must throw an error if an invalid value is provided.
You do not want others to override your validation logic.
So, it primarily depends on whether or not your property depends on other dependency properties or if you want others to be able to override your validation logic.
Since the ValidateValueCallback is not part of the PropertyMetadata, inheritors cannot modify the callback through the DependencyProperty.OverrideMetadata function.
Also, since the ValidateValueCallback does not provide your DependencyObject as a parameter, you cannot perform advanced validation that depends on other dependency properties.
Example 1
Suppose you have Minimum, Maximum, & Value properties. When any of these change, a CoerceValueCallback shoud be used to ensure the other properties are consistent.That is, Minmum <= Value <= Maximum.
However, assuming these values are doubles, then there are some values that would never make sense, namely Double.NaN, Double.PositiveInfinity, and Double.NegativeInfinity. Therefore, a ValidateValueCallback should be used to verify that the double values are normal, numeric values.
In fact, this is exactly how RangeBase works!
Example 2
Suppose you have a RegexTextBox control which takes a string containing a regular expression (call it RegexString). If a bad regular expression is provided, then what should be used instead? It might make sense to coerce it to be a null/empty value, rendering it useless; however, I suggest that this property be validated with a ValidateValueCallback. This is because any error is now thrown at compile-time when designing via the WPF designer.
For this property, there shouldn't be a CoerceValueCallback at all.
There is a whole lot of information describing how to use these callbacks.
I'd suggest taking a look at the MSDN article, Dependency Property Callbacks and Validation, for more in-depth knowledge.
Value coercion is basically to change the value, if the the new value is not as system expected. A best example is Slider control. A Slider has both Minimum and Maximum properties. Clearly, it would be a problem if the Maximum value were allowed to fall below the Minimum value. Value coercion is used to prevent this invalid state from occuring.
Validate value, is something that system will only check whether the given input is valid or not. It will throw Argument Exception if value is invalid (if we returned false for such value). For example, we have Age property, and it should be in range of 0 to 120. In case the new value is 500, the system may warn the user instead coercing it to some hardcoded value.
Any way both callbacks are optional and can be used based on the requirement.
Related
Using CakePHP 3.7.
I have added, at the bottom of config/bootstrap.php:
Inflector::rules('irregular', ['thesis' => 'theses']);
and actually, I've tried
Inflector::rules('irregular', ['theses' => 'thesis']);
just in case I had it backwards.
And in a cell I am trying to use:
use Cake\Utility\Inflector;
$singular_and_plural = [Inflector::singularize($base_name), $base_name];
The result for singularizing the word "thesis" is "thesiss".
Can anyone point out what's wrong, here?
The first form is the correct one, the key is the singular value, and the value the plural value.
That being said, what you're showing here is incorrect/problematic usage of Inflector::singularize(), as you're passing a value to it that already is singular, doing that often gives you unexpected/wrong results. You could open an issue ticket in such cases, sometimes this can be fixed in the core, but often times it's simply not possible as it would conflict with existing, required rules.
It should also be noted that CakePHP can handle thesis/theses out of the box already, it has singular/plural rules that match that. Make sure that you are passing in the expected values, and that you don't have additional custom rules that may interfer with what you're trying to inflect.
I have a class that has property like this
public string Foo
{
get { return _foo; }
set
{
if (!string.Equals(_foo, value))
{
_foo= value;
OnPropertyChanged();
}
}
}
When I create object with _fixture = new Fixture {OmitAutoProperties = true};, I expect it has value but it's null and setter never hit. Do I miss something?
This is by design. As the documentation states:
Gets or sets if writable properties should generally be assigned a value when generating an anonymous object.
In other words, in AutoFixture, the term auto-property refers the the feature of AutoFixture that automatically populates writable properties. Perhaps a better word would have been DoNotAutomaticallyPopulateProperties.
I can understand the confusion, as in C#, auto-property can also be interpreted as meaning Auto-Implemented Properties.
Frankly, AutoFixture's terminology should, perhaps, have been chosen with greater care, but in all these years, I don't think this has ever been brought to my attention before.
Specifically in the OP Foo is a writable property, and when you disable auto-properties, the setter is never invoked.
The OmitAutoProperties setting determines if a writable property should be set or not:
Gets or sets if writable properties should generally be assigned a value when generating an anonymous object.
So, if it's true, AutoFixture does not try to set any property values and that is by design.
I'd like to be able to run a query like: MyModel.query(MyModel.some_property == None) and get results. I know that if I don't put a default=<some default> in a property, I won't be able to query for it, but if I set default=None will it index it?
Similarly, does setting values to None cause properties to be indexed in ndb.Model? What if you pass some_keyword_arg=None to the constructor?
I know that doing something like: ndb.StringProperty(default='') means you can query on it, just not clear on the semantics of using None.
Explicitly setting a property to None is defining a value, and yes defaults work and the property will be indexed. This assumes None is a valid value for a particular property type.
Some issues will arise, as you pointed out, often you use None as a sentinal value, so how do you tell between no Value provided and an explicit None?
So I have seen this error show up, just sometimes, but it is not helpful in describing the actual error which has occured. Nor does it give any clues as to what might cause it to display.
Cannot use modParams with indexes that do not exist.
Can anyone explain more verbosly what this error means, what it relates to (such as a behaviour, component, controller, etc), the most common causes and how to fix it?
To kickstart the investigation, you can find the error here.
https://github.com/cakephp/cakephp/blob/master/lib/Cake/Utility/ObjectCollection.php#L128
Layman's Terms
CakePHP is being told to apply an array of parameters to a collection of objects, such that each particular object can modify the parameters sent on to the next object. There is an error in how CakePHP is being told to do this.
In Depth
Generically, this rises from the CakePHP event publication mechanism. Somewhere in your code is an instance of ObjectCollection, which is being triggered with certain parameters. That is, a method is being called on every object in that collection.
Each callback method is given parameters. Originally the parameters are passed into trigger(). In normal cases (where modParams is false), every callback gets the same parameters. But when modParams is not strictly false, the result of each callback overwrites the parameter indicated by modParams.
So if there are two objects in the collection, modParams is 1, and the params[1] is 'a' initially, then the callback is given the first object with params[1] == a. That callback returns 'b', so when the next callback is called, the second object gets params[1] == b.
The exception raises when the modParams value given does not exist in the originally given params. Eg, if modParams is 2 and params is array (0 => 'a', 1 => 'b'), you'll get this exception.
In your case
Specifically, debugging this has to be done at a low-level because it's a method on a generic class. The backtrace from the Exception should get you bubbled up to a trigger() call on a particular concrete class. That call is being given non-false modParams and a params that doesn't have the given modParams. It could be a code bug in a concrete class extending ObjectCollection, or it could simply be a generic message arising from a method not being given expected arguments.
Have you tried reading the documentation?
/*
* - `modParams` Allows each object the callback gets called on to modify the parameters to the next object.
* Setting modParams to an integer value will allow you to modify the parameter with that index.
* Any non-null value will modify the parameter index indicated.
* Defaults to false.
*/
You did not paste any code, so I guess your 3rd arg of the method contains something wrong.
According to Apple's documentation,
setAlwaysUsesMultipleValuesMarker:
Sets whether the receiver always returns the multiple values marker when multiple objects are selected, even if they have the same value.
- (void)setAlwaysUsesMultipleValuesMarker:(BOOL)flag
Discussion:
Setting flag to YES can increase performance if your application doesn’t allow editing multiple values. The default is NO.
However, I have trouble understanding what this all means even after reading the documentation. Can anybody offer a simpler explanation with examples?
Found the answer to this question deep inside apple docs on Cocoa Binding Guide.
NSMultipleValuesMarker
The NSMultipleValuesMarker indicates that more than one object is selected in the controller and the values for the requested key aren’t the same.
By default controllers return the NSMultipleValuesMarker only when the values for the requested key differ. For example, if the value for selection.name returns an array containing three strings—”Tony”, “Tony”, “Tony”—the string “Tony” is returned instead of the NSMultipleValuesMarker.
A collection controller can be configured—either programmatically using the method setAlwaysUsesMultipleValuesMarker: or by checking the Always uses multiple values marker checkbox in Interface Builder—such that it always returns NSMultipleValuesMarker when multiple items are selected, even if the values are equal.