Lightning -- How to invoke the apex class from client-side controller

How to invoke the apex class from client-side controller

    1. If you want to invoke any apex class in the in the lightning component 
       then the class should have @AuraEnabled methods or variables in it.
    2. How to define a apex class 
    	public with sharing  class ClassName{
    		@AuraEnabled methods 
    		@AuraEnabled variables 
    	}
    
    3. if you want to invoke any apex method in the lightning component then  
    		a.Method should be annotated  with @AuraEnabled
    		b.Method should be public /global
    		c.Method should be static 
    		d.Method can return any type of value 
    		e.Method can have any type of parameters and any no of parameters 
    		f. syntax :
    			@AuraEnabled
    			public static Integer callMe(){
    				return 10;
    			}
    
    			@AuraEnabled
    			public static Account callMe(String key){
    				return [select Name,Phone from Account where name:key limit 1];
    			}
   
    			@AuraEnabled
    			public static List<Account> callMe(String key){
    				return [select Name,Phone from Account where name:key ];
    			}
    4. if you want to invoke any variable directly in the component 
    		a. variable should be public 
    		b. varaible should have @Auraenabled 
    		c. It can have any datatype 
    		Example :
    			@AuraEnabled public String name {set;get;}
    			@AuraEnabled public Integer age {set;get;}
    
    5. How to invoke the Apex class in lightning component 
    	<!-- 
			<aura:component controller="namespace:className" >  : if we have any custom namespace 
			<aura:component controller="ApexclassName" >		: by default it takes as "c"
		
		-->
    
    6.  We can define only apex class as controller in lightning component 
    
    7. If we want to define more than one apex class in the lightning component how we define
    	Example 1: 
    		public class Example extends Demo{
    
    		}
		<!-- <aura:component controller="Example" >	-->

		Example 2; 
			<!-- 	
				one.cmp
				<aura:component controller="Demo" >
				</aura:component>

				two.cmp
				<aura:component extends="c:one" controller="Example">
				</aura:component>
		-->   
    
     8. How to invoke the apex class from client-side controller 
    
    	Step 1: 
    			var abc =component.get("c.apexMethodName");
    			a. This will invoke the method from the apex class and run the operation 
    			b. once the server-side operation is completed then it will call callbackaction 
    	Step 2: callback action 
    			 abc.setCallback(this,function(response){
    								logic
    							});	
    			 a. response : what ever the value returened by the apex class this will stored as response
    				1. getState() : This will return the state of the action 
    				   a. SUCCESS 
    				   b. ERROR 
    				   c. NEW 
    				   4. INCOMPLETE 
    				   e. ABORTED
    				   f. RUNNING
    
    				2. getReturnValue() : 			
    				   a. if the apex class method has any return type ,value returened by the method 
    					  will given by this
    
    	Step 3:  Enqueue Action
    
				 $A.enqueue(abc);
         
         
         
         Apex class: 
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         public class Example_One {
            @AuraEnabled 
              public static String callMe(){
                  return 'Call back from Apex call';
              }
          }
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          Component
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          <aura:component controller="Example_One" >
              <aura:attribute name="result" type="String" />
              <lightning:button label="Submit" onclick="{!c.show}" />
              {!v.result}
          </aura:component>
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          Controller
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          ({
              show : function(component, event, helper) {
                    // invoke the apex method 
                var abc =component.get("c.callMe");
                    // create Callback action : This will return the result 
                    abc.setCallback(this,function(response){
                        // check the status of the response 
                        var state=response.getState();
                        // if the response is success
                        if(state==='SUCCESS'){
                            console.log('Apex Invoked');
                            // fetch the value returned by the apex method
                            var name=response.getReturnValue();
                            console.log('Result:'+name);
                            component.set("v.result",name);
                        }else{
                            console.log('Method invoke failed');
                        }
                    });
                    //enqueue action
                    $A.enqueueAction(abc);
              }
            })
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