20 Jun 2018
Setup
buildscript {
// …
ext.kotlin_version = ‘'
dependencies {
classpath "org.jetbrains.kotlin" +
"kotlin-gradle-plugin:$kotlin_version"
} }
apply plugin: ‘kotlin’
dependencies {
compile “org.jetbrains.kotlin:kotlin-stdlib-jre8”
}
Utility Functions
Favor Kotlin top-level extension functions over the typical Java utility classes. And for easier consumption within Java code, use @file:JvmName to specify the name of the Java class which would get generated by the Kotlin compiler.
// Use this annotation so you can call it from Java code
@file:JvmName(“StringUtil”)
fun String.lengthIsEven(): Boolean = length % 2 == 0
val lengthIsEven = “someString”.lengthIsEven()
Data classes
They are classes that have the specific purpose of holding data:
data class Dog(val name: String, val age: Int)
The data class will have the following function out of the box:
toString of the form “Dog(name=Mark, age=12)” equals() and hashCode() copy()
It will also become a subject for destructuring declarations:
val markTurtle = Dog(“Joe”, 50)
val (name, age) = markTurtle
println(name)
Inline functions
A lambda expression in Kotlin is translated to Java anonymous classes in Java 6 or 7, that is an overhead. Lambda calls are affecting the call stack which has a performance impact.
inline functions can be used to flat out calls instead of invoking another method call and adding that to the call stack. So it makes sense to use inline functions when we pass in the lambdas.
inline fun callBlock(block: () -> Unit) {
println(“Before calling block”)
block()
println(“After calling block”)
}
When we call callBlock it gets translated to something like the following:
callBlock { println(“The block operation”) }
String var1 = “Before calling block”;
System.out.println(var1)
String var2 = “The block operation”;
System.out.println(var2);
var1 = “After calling block”;
System.out.println(var1);
vs. the following if the function was not marked as inline
callBlock { println(“The block operation”) }
callBlock((Functinos0)null.INSTANCE);
You have to be careful with inline functions though because it literary copies the method content where it is called and if the body of the functions is too large you really do not want to do this.
Knowing that, the following obviously will not make any sense because it has zero effect.
inline fun foo(noinline block: () -> Unit) {// Single lambda marked as noinline
inline fun foo() { // No lambdas
Checking for null in conditions
First, you can explicitly check if b is null, and handle the two options separately:
val l = if (b != null) b.length else -1
The compiler tracks the information about the check you performed, and allows the call to length inside the if. More complex conditions are supported as well:
if (b != null && b.length > 0)
print(“String of length ${b.length}”)
else
print(“Empty string”)
Note that this only works where b is immutable (i.e. a local variable which is not modified between the check and the usage or a member val which has a backing field and is not overridable), because otherwise it might happen that b changes to null after the check.
Safe Calls
Your second option is the safe call operator, written ?.:
b?.length
This returns b.length if b is not null, and null otherwise. The type of this expression is Int?.
Safe calls are useful in chains. For example, if Bob, an Employee, may be assigned to a Department (or not), that in turn may have another Employee as a department head, then to obtain the name of Bob’s department head, if any), we write the following:
bob?.department?.head?.name
Such a chain returns null if any of the properties in it is null.
Elvis Operator
When we have a nullable reference r, we can say “if r is not null, use it, otherwise use some non-null value x”:
val l: Int = if (b != null) b.length else -1
Along with the complete if-expression, this can be expressed with the Elvis operator, written ?::
val l = b?.length ?: -1
If the expression to the left of ?: is not null, the elvis operator returns it, otherwise it returns the expression to the right. Note that the right-hand side expression is evaluated only if the left-hand side is null.
Note that, since throw and return are expressions in Kotlin, they can also be used on the right hand side of the elvis operator. This can be very handy, for example, for checking function arguments:
fun foo(node: Node): String? {
val parent = node.getParent() ?: return null
val name = node.getName() ?: throw IllegalArgumentException(“name expected”)
}
The !! Operator
We can write b!!, and this will return a non-null value of b (e.g., a String in our example) or throw an NPE if b is null:
val l = b!!.length()
Thus, if you want an NPE, you can have it, but you have to ask for it explicitly, and it does not appear out of the blue.
Safe Casts
Regular casts may result into a ClassCastException if the object is not of the target type. Another option is to use safe casts that return null if the attempt was not successful:
val aInt: Int? = a as? Int
04 Mar 2018
A profissional app handles offline/online connectivity in order to interact with the user according to his phone state.
A Flutter could not be different and in this post I will you an amazing library which will facilitate our development.
The flutter_offline developed by jogboms is an utility library for handling offline/online connectivity.
Installation and Usage
The process is really straightforward. First of all, you need to add the library’s dependency in your project:
dependencies:
flutter_offline: "^0.2.1"
Next step is import the package that you included in you project and start using the library as the following example shows:
20 Feb 2018
In Ethereum, it is possible to interact with contracts via calls (aka message calls) or via transactions.
The official documentation says:
Transaction
A piece of data, signed by an External Actor. It represents either a Message or a new Autonomous Object. Transactions are recorded into each block of the blockchain.
Message Call
The act of passing a message from one Account to another. If the destination account is associated with non-empty EVM Code, then the VM will be started with the state of said Object and the Message acted upon. If the message sender is an Autonomous Object, then the Call passes any data returned from the VM operation.
In other words, a transaction is an asynchronous operation which is broadcasted to the network and processed by miners. A transaction consumes Ether and modify the blockchain. A message call is a read-only and not Ether consumer operation of a contract function. The results of a ‘call’ will not be published or broadcasted on the blockchain.
Modifiers and Visibility
Solidity Modifiers
Since Solidity 0.4.17, it provides the following functions modifiers on its framework: view and pure.
View – should be used in functions that did not change any state values of a contract;
Pure – should be used if it does not even read any state information. Pure functions can be used for tasks like permission-control, calculations and typecasting.
Visibility
public – are part of the contract interface and can be either called internally or via messages. For public state variables, an automatic getter function (see below) is generated;
private – and state variables are only visible for the contract they are defined in and not in derived contracts;
internal – and state variables can only be accessed internally (i.e. from within the current contract or contracts deriving from it), without using this;
external – are part of the contract interface, which means they can be called from other contracts and via transactions. An external function f cannot be called internally (i.e. f() does not work, but this.f() works). External functions are sometimes more efficient when they receive large arrays of data;
24 Jan 2018
gas – is the name for a special unit used in Ethereum
gas price – a value set by the creator of the transaction; it’s a single gas unit’s price;
fee – the result of gas * gas price
Transactions costs x Ethereum system
-
Have in mind if the gas price is too low, no one will process your transaction;
-
The fees are paid in ether, though, which is different from gas;
-
The transaction fails (still goes into the blockchain) in case of gas price is fine but the gas cost runs “over budget”. You don’t get the money back for the work that the miners did.
-
Transaction fee = Amount of work that goes into something + storage space your code will take;
Optimize gas usage of your smart contract
Gas is necessary for the execution of smart contracts, but you shouldn’t specify too low or high price. Consider optimizing your smart contract to minimize the amount of gas required.
What should I take into account for optimized contracts?
Global Variables usage – storage: Global variables are stored in a contract’s state on the blockchain, so you are going to be charged if you use global variables. Just use when it is necessary, this is a practice in several languages, but it is especially crucial in ethereum development;
Contract Size: The size of your contract will influence the transaction cost for all interactions with it. So how big is your contract, more expansive your contract will be to processed in the Ethereum’s network. If you are able to break the Solidity contract up into smaller separate contracts, this will decrease user’s gas costs;
Further Details
Take a look in this documentation from ConsenSys for Ethereum Smart Contract Security Best Practices and this Medium post about Optimizing gas usage.
15 Jan 2018
Solidity provides special variables and functions which always exist in the global namespace and are mainly used to provide information about the blockchain or are general-use utility functions. The following is a list of properties related to the block and transactions:
• block.blockhash (uint blockNumber) returns (bytes32): Hash of the given block, works for only the 256 most recent blocks
• block.coinbase (address): Current block miner’s address
• block.difficult (uint): Current block difficulty
• block.gaslimit (uint): Current block gas limit
• block.number (uint): Current block number
• block.timestamp (uint): Current block timestamp
• msg.data (bytes): Complete call data
• msg.gas (uint): Remaining gas
• msg.sender (address): Sender of the message (current call)
• msg.sig (bytes4): First 4 bytes of the call data (function identifier)
• msg.value (uint): Number of wei sent with the message
• now (uint): Current block timestamp (alias for block.timestamp)
• tx.gasprice (uint): Gas price of the transaction
• tx.origin (address): Sender of the transaction (full call chain)
