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Grand Central Dispatch works on the concept of "Dispatch Queues". A dispatch queue executes tasks you designate in the order which they are passed. There are three types of dispatch queues: Serial Dispatch Queues (aka private dispatch queues) execute one task at a time, in order. They a...

3.0 To run tasks on a dispatch queue, use the sync, async, and after methods. To dispatch a task to a queue asynchronously: let queue = DispatchQueue(label: "myQueueName") queue.async { //do something DispatchQueue.main.async { //this will be called in main t...

A common question is how to juxtapose (combine) physically separate geographical regions on the same map, such as in the case of a choropleth describing all 50 American states (The mainland with Alaska and Hawaii juxtaposed). Creating an attractive 50 state map is simple when leveraging Google Maps...

Java doesn't provide a direct method in java.util.Arrays to remove an element from an array. To perform it, you can either copy the original array to a new one without the element to remove or convert your array to another structure allowing the removal. Using ArrayList You can convert the array t...

The + symbol marks a type parameter as covariant - here we say that "Producer is covariant on A": trait Producer[+A] { def produce: A } A covariant type parameter can be thought of as an "output" type. Marking A as covariant asserts that Producer[X] <: Producer[Y] prov...

By default all type parameters are invariant - given trait A[B], we say that "A is invariant on B". This means that given two parametrizations A[Cat] and A[Animal], we assert no sub/superclass relationship between these two types - it does not hold that A[Cat] <: A[Animal] nor that A[Ca...

The - symbol marks a type parameter as contravariant - here we say that "Handler is contravariant on A": trait Handler[-A] { def handle(a: A): Unit } A contravariant type parameter can be thought of as an "input" type. Marking A as contravariant asserts that Handler[X] &l...

Because collections are typically covariant in their element type*, a collection of a subtype may be passed where a super type is expected: trait Animal { def name: String } case class Dog(name: String) extends Animal object Animal { def printAnimalNames(animals: Seq[Animal]) = { anima...

Django uses special database settings when testing so that tests can use the database normally but by default run on an empty database. Database changes in one test will not be seen by another. For example, both of the following tests will pass: from django.test import TestCase from myapp.models...

Suppose you have a simple Customer model: class Customer(models.Model): first_name = models.CharField(max_length=255) last_name = models.CharField(max_length=255) is_premium = models.BooleanField(default=False) You register it in the Django admin and add search field by first_name...

To perform actions in Django using commandline or other services (where the user/request is not used), you can use the management commands. Django modules can be imported as needed. For each command a separate file needs to be created: myapp/management/commands/my_command.py (The management and...

Module Pattern The Module pattern is a creational and structural design pattern which provides a way of encapsulating private members while producing a public API. This is accomplished by creating an IIFE which allows us to define variables only available in its scope (through closure) while return...

You can specify different application IDs or package names for each buildType or productFlavor using the applicationIdSuffix configuration attribute: Example of suffixing the applicationId for each buildType: defaultConfig { applicationId "com.package.android" minSdkVersion 17 ...

Parameters to a function can be marked as mandatory function Get-Greeting{ param ( [Parameter(Mandatory=$true)]$name ) "Hello World $name" } If the function is invoked without a value, the command line will prompt for the value: $greeting = Get-Greeting ...

This example listens for input coming in over the serial connection, then repeats it back out the same connection. byte incomingBytes; void setup() { Serial.begin(9600); // Opens serial port, sets data rate to 9600 bps. } void loop() { // Send data only when you receive...

Class constants provide a mechanism for holding fixed values in a program. That is, they provide a way of giving a name (and associated compile-time checking) to a value like 3.14 or "Apple". Class constants can only be defined with the const keyword - the define function cannot be used in...

Glide includes two default transformations, fit center and center crop. Fit center: Glide.with(context) .load(yourUrl) .fitCenter() .into(yourView); Fit center performs the same transformation as Android's ScaleType.FIT_CENTER. Center crop: Glide.with(context) .load(yourUr...

A major problem with parallelization is the used of RNG as seeds. Random numbers by the number are iterated by the number of operations from either the start of the session or the most recent set.seed(). Since parallel processes arise from the same function, it can use the same seed, possibly causin...

If MsgBox("Click OK") = vbOK Then can be used in place of If MsgBox("Click OK") = 1 Then in order to improve readability. Use Object Browser to find available VB constants. View → Object Browser or F2 from VB Editor. Enter class to search View members available ...

Good error handling prevents end users from seeing VBA runtime errors and helps the developer easily diagnose and correct errors. There are three main methods of Error Handling in VBA, two of which should be avoided for distributed programs unless specifically required in the code. On Error GoTo 0...