TA貢獻1765條經驗 獲得超5個贊
這是一個從數組中預加載圖像并在最后一個完成后調用回調的函數：
function preloadImages(srcs, imgs, callback) {
var img;
var remaining = srcs.length;
for (var i = 0; i < srcs.length; i++) {
img = new Image();
img.onload = function() {
--remaining;
if (remaining <= 0) {
callback();
}
};
img.src = srcs[i];
imgs.push(img);
}
}
// then to call it, you would use this
var imageSrcs = ["src1", "src2", "src3", "src4"];
var images = [];
preloadImages(imageSrcs, images, myFunction);
而且由于我們現在正處于使用promises進行異步操作的時代，這里是上面的一個版本，它使用promises并通過ES6標準承諾通知調用者：
function preloadImages(srcs) {
function loadImage(src) {
return new Promise(function(resolve, reject) {
var img = new Image();
img.onload = function() {
resolve(img);
};
img.onerror = img.onabort = function() {
reject(src);
};
img.src = src;
});
}
var promises = [];
for (var i = 0; i < srcs.length; i++) {
promises.push(loadImage(srcs[i]));
}
return Promise.all(promises);
}
preloadImages(["src1", "src2", "src3", "src4"]).then(function(imgs) {
// all images are loaded now and in the array imgs
}, function(errImg) {
// at least one image failed to load
});
而且，這是一個使用2015 jQuery承諾的版本：
function preloadImages(srcs) {
function loadImage(src) {
return new $.Deferred(function(def) {
var img = new Image();
img.onload = function() {
def.resolve(img);
};
img.onerror = img.onabort = function() {
def.reject(src);
};
img.src = src;
}).promise();
}
var promises = [];
for (var i = 0; i < srcs.length; i++) {
promises.push(loadImage(srcs[i]));
}
return $.when.apply($, promises).then(function() {
// return results as a simple array rather than as separate arguments
return Array.prototype.slice.call(arguments);
});
}
preloadImages(["src1", "src2", "src3", "src4"]).then(function(imgs) {
// all images are loaded now and in the array imgs
}, function(errImg) {
// at least one image failed to load
});
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