追加window.onload函数

function addLoadEvent(func) {
    var old_onload = window.onload;
    if (typeof window.onload != 'function') {
        window.onload = func;
    }
    else{
        old_onload();
        func();
    }
}

// addLoadEvent(A);
// addLoadEvent(B);
// addLoadEvent(C);  //...

function A(){}
function B(){}
fuction C(){}