[Kotlin] Let, also, run, apply and with are used to distinguish

In the following example, we call the return value of the prefix code T, and the braces are called block. Take the pop-up desktop program selection box as an example:

var intent = Intent(Intent.ACTION_MAIN)
intent.addCategory(Intent.CATEGORY_HOME)
intent.setClassName("android", "com.android.internal.app.ResolverActivity")
startActivity(intent)

to sum up

• let and also pass T as it to the block, the difference is that the returned result is the result of the last line of the block and T itself
• run and apply pass T as this to the block, the difference is that the returned results are the result of the last line of the block and T itself
• with also passes T as this to the block and returns the result of the last line of the block, the difference is that the wording is with(T)
  

1、let – block: (T) -> R

• Calls the specified function [block] with ‘this’ value as its argument and returns its result.
• Baidu Translation: Call the specified function [block] with this value as a parameter, and return the result.
• Personal understanding: Pass T as it to block and return the result (T is intent, block is {} and content after let)

var intent = Intent(Intent.ACTION_MAIN)
intent.let {
    
    //it:Intent
    it.addCategory(Intent.CATEGORY_HOME)
    it.setClassName("android", "com.android.internal.app.ResolverActivity")
    startActivity(it)//这里无返回值，所以整体let也是无返回值的
}.let{
    
    //it:Unit
}

2、also – block: (T) -> Unit

• Calls the specified function [block] with ‘this’ value as its argument and returns this value.
• Baidu Translation: Call the specified function [block] with the this value as a parameter, and return the this value.
• Personal understanding: pass T as it to block and return T

intent.also {
    
    //it:Intent
    it.addCategory(Intent.CATEGORY_HOME)
    it.setClassName("android", "com.android.internal.app.ResolverActivity")
    startActivity(it)
}.let{
    
    //it:Intent
}

3、run – block: T.() -> R

• Calls the specified function [block] with this value as its receiver and returns its result.
• Baidu Translation: Call the specified function [block] with this value as its receiver, and return its result.
• Personal understanding: Pass T as this to block and return the result of the last line of block, this can be omitted

intent.run {
    
    //this:Intent
    addCategory(Intent.CATEGORY_HOME)
    setClassName("android", "com.android.internal.app.ResolverActivity")
    startActivity(this)
}.let{
    
    //this:Unit
}

4、apply – block: T.() -> Unit

• Calls the specified function [block] with this value as its receiver and returns this value.
• Baidu Translation: Call the specified function [block] with this value as its receiver, and return this value.
• Personal understanding: pass T as this to block and return T

intent.apply {
    
    //this:Intent
	addCategory(Intent.CATEGORY_HOME)
    setClassName("android", "com.android.internal.app.ResolverActivity")
    startActivity(this)
}.let{
    
    //this:Intent
}
//可以这么写，因为apply返回值还是intent，执行顺序也是先apply再startActivity
startActivity(Intent(Intent.ACTION_MAIN).apply {
    
    
    addCategory(Intent.CATEGORY_HOME)
    setClassName("android", "com.android.internal.app.ResolverActivity")
    activity?.finish()
})

5、with – receiver: T, block: T.() -> R

• Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
• Baidu Translation: Call the specified function [block] with the given [receiver] as its receiver, and return the result.
• Personal understanding: Pass the receiving part (T) as this to the block and return the result of the last line of the block

with(Intent(Intent.ACTION_MAIN)){
    
    //this:Intent
    addCategory(Intent.CATEGORY_HOME)
    setClassName("android", "com.android.internal.app.ResolverActivity")
    startActivity(this)
}.let{
    
    //this:Unit
}