有一天，我們寫了關于如何在localStorage中保存圖像和文件的文章，它是關于我們今天可用的實用主義。 然而，localStorage有一些性能影響 - 我們將在稍后的博客中討論這個問題 - 并且未來期望的方法是使用IndexedDB。 在這里，我將向您介紹如何在IndexedDB中存儲圖像和文件，然后通過ObjectURL呈現(xiàn)它們。
本文是翻譯過來的，原文在這里 Storing images and files in IndexedDB
關于作者： Robert Nyman [Editor emeritus]
Technical Evangelist & Editor of Mozilla Hacks. Gives talks & blogs about HTML5, JavaScript & the Open Web. Robert is a strong believer in HTML5 and the Open Web and has been working since 1999 with Front End development for the web - in Sweden and in New York City. He regularly also blogs atrobertnyman.com and loves to travel and meet people.
使用IndexDB存儲圖像和文件的常規(guī)步驟
首先，我們來談談我們將創(chuàng)建一個IndexedDB數(shù)據(jù)庫，將文件保存到其中然后將其讀出并顯示在頁面中的步驟：
1、創(chuàng)建或打開數(shù)據(jù)庫
2、創(chuàng)建一個objectStore
3、將圖像文件檢索為blob
4、初始化一個數(shù)據(jù)庫事物
5、保存圖像blob到數(shù)據(jù)庫中去
6、讀出保存的文件并從中創(chuàng)建ObjectURL并將其設置為頁面中圖像元素的src
1、創(chuàng)建或打開數(shù)據(jù)庫。
// IndexedDB
window.indexedDB = window.indexedDB || window.webkitIndexedDB || window.mozIndexedDB || window.OIndexedDB || window.msIndexedDB,
IDBTransaction = window.IDBTransaction || window.webkitIDBTransaction || window.OIDBTransaction || window.msIDBTransaction,
dbVersion = 1;
/*
Note: The recommended way to do this is assigning it to window.indexedDB,
to avoid potential issues in the global scope when web browsers start
removing prefixes in their implementations.
You can assign it to a varible, like var indexedDB… but then you have
to make sure that the code is contained within a function.
*/
// Create/open database
var request = indexedDB.open("elephantFiles", dbVersion);
request.onsuccess = function (event) {
console.log("Success creating/accessing IndexedDB database");
db = request.result;
db.onerror = function (event) {
console.log("Error creating/accessing IndexedDB database");
};
// Interim solution for Google Chrome to create an objectStore. Will be deprecated
if (db.setVersion) {
if (db.version != dbVersion) {
var setVersion = db.setVersion(dbVersion);
setVersion.onsuccess = function () {
createObjectStore(db);
getImageFile();
};
}
else {
getImageFile();
}
}
else {
getImageFile();
}
}
// For future use. Currently only in latest Firefox versions
request.onupgradeneeded = function (event) {
createObjectStore(event.target.result);
};
使用它的預期方法是在創(chuàng)建數(shù)據(jù)庫時觸發(fā)onupgradeneeded事件或獲取更高版本號。 目前僅在Firefox中支持此功能，但很快將在其他Web瀏覽器中支持。 如果Web瀏覽器不支持此事件，則可以使用已棄用的setVersion方法并連接到其onsuccess事件。
2、創(chuàng)建一個objectStore（如果它尚不存在）
// Create an objectStore
console.log("Creating objectStore")
dataBase.createObjectStore("elephants");
在這里，您創(chuàng)建一個ObjectStore，您將存儲數(shù)據(jù) - 或者在我們的例子中，文件 - 并且一旦創(chuàng)建，您不需要重新創(chuàng)建它，只需更新其內(nèi)容即可。
3、將圖像文件檢索為blob
// Create XHR
var xhr = new XMLHttpRequest(),
blob;
xhr.open("GET", "elephant.png", true);
// Set the responseType to blob
xhr.responseType = "blob";
xhr.addEventListener("load", function () {
if (xhr.status === 200) {
console.log("Image retrieved");
// File as response
blob = xhr.response;
// Put the received blob into IndexedDB
putElephantInDb(blob);
}
}, false);
// Send XHR
xhr.send();
此代碼直接將文件的內(nèi)容作為blob獲取。目前只支持Firefox。 收到整個文件后，將blob發(fā)送到函數(shù)以將其存儲在數(shù)據(jù)庫中。
4、初始化一個數(shù)據(jù)庫事物
// Open a transaction to the database
var transaction = db.transaction(["elephants"], IDBTransaction.READ_WRITE);
要開始向數(shù)據(jù)庫寫入內(nèi)容，您需要使用objectStore名稱和要執(zhí)行的操作類型（在本例中為read和write）啟動事務。
5、保存圖像blob到數(shù)據(jù)庫中去
// Put the blob into the dabase
transaction.objectStore("elephants").put(blob, "image");
一旦事務到位，您將獲得對所需objectStore的引用，然后將您的blob放入其中并為其提供密鑰。
6、讀出保存的文件并從中創(chuàng)建ObjectURL并將其設置為頁面中圖像元素的src
// Retrieve the file that was just stored
transaction.objectStore("elephants").get("image").onsuccess = function (event) {
var imgFile = event.target.result;
console.log("Got elephant!" + imgFile);
// Get window.URL object
var URL = window.URL || window.webkitURL;
// Create and revoke ObjectURL
var imgURL = URL.createObjectURL(imgFile);
// Set img src to ObjectURL
var imgElephant = document.getElementById("elephant");
imgElephant.setAttribute("src", imgURL);
// Revoking ObjectURL
URL.revokeObjectURL(imgURL);
};
使用相同的事務來獲取剛剛存儲的圖像文件，然后創(chuàng)建一個objectURL并將其設置為頁面中圖像的src。 例如，這也可以是一個附加到腳本元素的JavaScript文件，然后它將解析JavaScript。
最后完整代碼
(function () {
// IndexedDB
var indexedDB = window.indexedDB || window.webkitIndexedDB || window.mozIndexedDB || window.OIndexedDB || window.msIndexedDB,
IDBTransaction = window.IDBTransaction || window.webkitIDBTransaction || window.OIDBTransaction || window.msIDBTransaction,
dbVersion = 1.0;
// Create/open database
var request = indexedDB.open("elephantFiles", dbVersion),
db,
createObjectStore = function (dataBase) {
// Create an objectStore
console.log("Creating objectStore")
dataBase.createObjectStore("elephants");
},
getImageFile = function () {
// Create XHR
var xhr = new XMLHttpRequest(),
blob;
xhr.open("GET", "elephant.png", true);
// Set the responseType to blob
xhr.responseType = "blob";
xhr.addEventListener("load", function () {
if (xhr.status === 200) {
console.log("Image retrieved");
// Blob as response
blob = xhr.response;
console.log("Blob:" + blob);
// Put the received blob into IndexedDB
putElephantInDb(blob);
}
}, false);
// Send XHR
xhr.send();
},
putElephantInDb = function (blob) {
console.log("Putting elephants in IndexedDB");
// Open a transaction to the database
var transaction = db.transaction(["elephants"], IDBTransaction.READ_WRITE);
// Put the blob into the dabase
var put = transaction.objectStore("elephants").put(blob, "image");
// Retrieve the file that was just stored
transaction.objectStore("elephants").get("image").onsuccess = function (event) {
var imgFile = event.target.result;
console.log("Got elephant!" + imgFile);
// Get window.URL object
var URL = window.URL || window.webkitURL;
// Create and revoke ObjectURL
var imgURL = URL.createObjectURL(imgFile);
// Set img src to ObjectURL
var imgElephant = document.getElementById("elephant");
imgElephant.setAttribute("src", imgURL);
// Revoking ObjectURL
URL.revokeObjectURL(imgURL);
};
};
request.onerror = function (event) {
console.log("Error creating/accessing IndexedDB database");
};
request.onsuccess = function (event) {
console.log("Success creating/accessing IndexedDB database");
db = request.result;
db.onerror = function (event) {
console.log("Error creating/accessing IndexedDB database");
};
// Interim solution for Google Chrome to create an objectStore. Will be deprecated
if (db.setVersion) {
if (db.version != dbVersion) {
var setVersion = db.setVersion(dbVersion);
setVersion.onsuccess = function () {
createObjectStore(db);
getImageFile();
};
}
else {
getImageFile();
}
}
else {
getImageFile();
}
}
// For future use. Currently only in latest Firefox versions
request.onupgradeneeded = function (event) {
createObjectStore(event.target.result);
};
})();
瀏覽器支持
URL API支持性
indexDb
Github源碼
以上就是本文的全部內(nèi)容，希望對大家的學習有所幫助，也希望大家多多支持腳本之家。
