返回在匿名函数中不起作用

Question

function mapHasStops(callback) {
    app.map.mapObject.getVisibleRegion(function (bounds) {
        for (var i = 0; i < app.map.mapContent.length; i++) {
            (function (i) {
                app.map.mapContent[i].getPosition(function (position) {
                    if (bounds.contains(position)) {
                        callback(true);
                        return;
                    }
                });
            })(i);
        }
    });
}

function updateScreenContent() {
    mapHasStops(function (bool) {
        alert(bool);
    });
}

Despite returning immediately after a successful match, the alert is called multiple times (depending on how many matches it finds) when the updateScreenContents function is only called once. Any ideas why?

Answer 1

Just fixing the code would be:

function mapHasStops(callback) {
    app.map.mapObject.getVisibleRegion(function (bounds) {
        var found = false;
        for (var i = 0; i < app.map.mapContent.length && !found; i++) {
            (function (i) {
                app.map.mapContent[i].getPosition(function (position) {
                    if (bounds.contains(position)) {
                        found = true;
                        callback(true);
                        return;
                    }
                });
            })(i);
        }
    });
}

function updateScreenContent() {
    mapHasStops(function (bool) {
        alert(bool);
    });
}

But there is probably a better way to write that function...

Answer 2

A return statement only exits from its own execution context, not any outer context. In this case, the value is returned and used by getPosition :

    // Enclosing context
    (function (i) {
                                          // Function expression is passed to *getPosition*
                                          // whose return value is not assigned
        app.map.mapContent[i].getPosition(function (position) {
            if (bounds.contains(position)) {
                callback(true);
                return;
            }
        });
    })(i);

So the function expression starting function (position) is passed to the getPosition method, which presumably calls it. It's value is returned there and proceeds to do whatever. It doesn't return any further than that.

Answer 3

Does "function (position)" pass as a callback function? If yes the for loop won't wait for it to return thus "function (position)" was called multiple times.