Javascript 'Promise' 如何进行同步

Question

我正在使用 Ionic2/Typescript。

在继续之前，我想完成 2 个 Promise(即同步)。因此，我将对这两个函数的调用放在 Promise.all(...) 中，期望它们在调用 resolve 之前完成。

我有以下代码:

public openDatabase(): Promise<Array<Message>> {
    let promise: Promise<Array<Message>> = new Promise<Array<Message>>(resolve => {
        if (false && this.database && this.database != null) {
            Promise.all([this.refreshChats(this.database), this.refreshMessages(this.database)]).then(() => {
                console.log('openDatabase1: resolve');
                resolve(this.messages);
            });
        } else {
            this.database = new SQLite();
            this.database.openDatabase({
                name: "data.db",
                location: "default"
            }).then(() => {
                Promise.all([this.refreshChats(this.database), this.refreshMessages(this.database)]).then(() => {
                    console.log('openDatabase2: resolve');
                    resolve(this.messages);
                });
            }, (error) => {
                console.log("OPEN ERROR: ", error);
            });
        }
    });
    return promise;
}

public refreshChats(db: any): Promise<any> {
    console.log('refreshChats ');
    return db.executeSql("SELECT * FROM chats", [])
        .then((chatData) => {
            let promises: Array<any> = [];
            this.chats = [];
            if (chatData.rows.length > 0) {
                for (var i = 0; i < chatData.rows.length; i++) {
                    promises.push(this.populateChat(db, chatData.rows.item(i)));
                }
            }
            Promise.all(promises).then(() => {
                console.log('refreshChats return this.chats.length = ' + this.chats.length);
                return this.chats;
            });
        })
        .catch(error => {
            console.log("ERROR REFRESHING CHATS: " + JSON.stringify(error));
            console.log(error);
        });
}

从我的控制台输出中，您可以看到在聊天完成刷新之前调用了 resolve:

refreshChats
populateChat  Object {...}
openDatabase2: resolve
refreshChats return this.chats.length = 1

任何关于我应该如何构建我的 promise 的建议都将受到赞赏。

Answer 1

您需要从内部 promise 回调返回新的 promise :

public openDatabase(): Promise<Array<Message>> {
    let promise: Promise<Array<Message>> = new Promise<Array<Message>>(resolve => {
        if (false && this.database && this.database != null) {
            return Promise.all([this.refreshChats(this.database), this.refreshMessages(this.database)]).then(() => {
                console.log('openDatabase1: resolve');
                resolve(this.messages);
            });
        } else {
            this.database = new SQLite();
            return this.database.openDatabase({
                name: "data.db",
                location: "default"
            }).then(() => {
                return Promise.all([this.refreshChats(this.database), this.refreshMessages(this.database)]).then(() => {
                    console.log('openDatabase2: resolve');
                    resolve(this.messages);
                });
            }, (error) => {
                console.log("OPEN ERROR: ", error);
            });
        }
    });
    return promise;
}

public refreshChats(db: any): Promise<any> {
    console.log('refreshChats ');
    return db.executeSql("SELECT * FROM chats", [])
        .then((chatData) => {
            let promises: Array<any> = [];
            this.chats = [];
            if (chatData.rows.length > 0) {
                for (var i = 0; i < chatData.rows.length; i++) {
                    promises.push(this.populateChat(db, chatData.rows.item(i)));
                }
            }
            return Promise.all(promises).then(() => {
                console.log('refreshChats return this.chats.length = ' + this.chats.length);
                return this.chats;
            });
        })
        .catch(error => {
            console.log("ERROR REFRESHING CHATS: " + JSON.stringify(error));
            console.log(error);
        });
}

注意，返回 Promise.all 并返回 this.database.openDatabase。