[解決済み] iframeが読み込まれているかどうかを検出するにはどうすればよいですか？

$(function(){
    $('#MainPopupIframe').load(function(){
        $(this).show();
        console.log('iframe loaded successfully')
    });
        
    $('#click').on('click', function(){
        $('#MainPopupIframe').attr('src', 'https://heera.it');    
    });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<button id='click'>click me</button>

<iframe style="display:none" id='MainPopupIframe' src='' /></iframe>

window.onload = function(){
    var ifr = document.getElementById('MainPopupIframe');
    ifr.onload=function(){
        this.style.display='block';
        console.log('laod the iframe')
    };

    var btn = document.getElementById('click');    
    btn.onclick=function(){
        ifr.src='https://heera.it';    
    };
};

<button id='click'>click me</button>

<iframe style="display:none" id='MainPopupIframe' src='' /></iframe>

$(function(){
    $('#click').on('click', function(){
        var ifr = $('<iframe/>', {
            id:'MainPopupIframe',
            src:'https://heera.it',
            style:'display:none;width:320px;height:400px',
            load:function(){
                $(this).show();
                alert('iframe loaded !');
            }
        });
        $('body').append(ifr);    
    });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<button id='click'>click me</button><br />